Hydrological, Chemical, and Biological 

Characteristics of a Prairie Pothole Wetland 

Complex Under Highly Variable Climate Conditions- The Cottonwood Lake Area, East-Central North Dakota 

Edited by Thomas C. Winter 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1675 
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

The use of firm, trade, and brand names in this report is for 

identifcation purposes only and does not constitute 

endorsement by the U.S. Government 

Library of Congress Cataloging in Publications Data: 

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225 

ISBN 0-607-89431-8 
FOREWORD 

The Cottonwood Lake area is a prairie pothole wetland complex on the 

Missouri Coteau in east-central North Dakota. Studies to investigate the relation 

between waterfowl and prairie wetlands in the area were initiated in 1967 and 

continue to the present (2003). The studies complemented concurrent investigations of the feeding ecology and renesting characteristics ofbreeding waterfowl. 

The studies were expanded in 1979 to include investigations of the hydrology of 

the wetland complex and the effects of hydrology on wetland water chemistry and 

on plant and animal communities. Studies of the soils in part of the area were 

done from 1991 to 1993. Further expansion of the biological work in 1992 

involved detailed monitoring of macroinvertebrates, macrophytes, herptiles, and 

birds. 

This report presents selected results of the research conducted from 1967 to 

1998 at the Cottonwood Lake area. Information on the area is supplemented in 

Chapters 4 and 5 by information from Eisenlohr and others (1972) that dates back 

to 1961. The timing of the studies was fortuitous because during the last 2 

decades of the 20th century, the area experienced a major climate cycle that 

included the second worst drought of the 20th century and the greatest amount of 

precipitation in more than a century. At the outset of the studies in 1967, most of 

the semipermanent wetlands in the area were open-water lakes. At present, those 

wetlands are again open-water lakes. However, in the intervening years, all of the 

wetlands dried up at least once, and some dried up more than once. 

The dynamic climate conditions in the Cottonwood Lake area caused 

equally dynamic hydrological, chemical, and biological responses from plant and 

animal communities. The effects of the climate and hydrological conditions on 

plants and animals, and the implications for wetland management, which was the 

primary impetus for this report, are presented in Chapter 4. Background information on the geology and hydrology of the area is presented in Chapter 1, information on the climate is presented in Chapter 2, and information on the water 

chemistry and biology is presented in Chapter 3. Results of mathematical models 

of the vegetation changes in response to water-level changes for the present as 

well as for a condition of climate change are presented in Chapter 5. 

III 

CONTENTS 

Chapter! ...................................................................................... . 

Geohydrologic Setting of The Cottonwood Lake Area ........... . 

Abstract ........................................................................................ . 

Introduction ................................................................................... . 

Geologic Origin and Physiographic Features................................ 1 

Characteristics of Geologic Deposits............................................. 2 

Distribution and Physical Characteristics of 

Geologic Deposits............................................................... 3 

Mineralogy of Glacial Deposits................................................ 7 

Hydrologic Characteristics of Wetlands ......... .... .............. ....... .. .... 7 

Interaction of Wetlands And Atmospheric Water.................... 7 

Wetland Water Levels............................................................... 10 

Interaction of Wetlands and Ground Water.............................. 11 

References . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . 24 

Chapter 2....................................................................................... 25 

Climate of the Cottonwood Lake Area . . . . . .. . . . . . . . . . .. . .. . . . . . . . . . .. . . . . .. 25 

Abstract . . . . . . ... . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . .. ... . . . . . . . . . ... ... . . . . . . .. .. . . . . . . . . . . .......... 25 

Introduction.................................................................................... 25 

Methods.......................................................................................... 26 

Regional Climate of Central North Dakota.................................... 26 

Local Climate of the Cottonwood Lake Area ... . . ........... .. . ........ ..... 27 

Comparison of Evaporation and Precipitation at the 

Cottonwood Lake Area....................................................... 31 

Acknowledgments.......................................................................... 34 

References . . . . .. . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . ... . . . . . . . . . . ........... .. . . . . . . . . . . . . .. . . . . . . 34 

Chapter 3....................................................................................... 35 

Spatial and Temporal Variabilityin Specific Conductance and 

Chemical Characteristics of Wetland Water and in Water 

Column Biota in the Wetlands in the Cottonwood 

Lake Area .. . ........... ........... .. . ............ .............. ........... ... ......... ... . 35 

Abstract.......................................................................................... 35 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5 

Methods.......................................................................................... 36 

Specific Conductance And Chemical Characteristics Of Wetland 

Water.......................................................................................... 37 

Biological Characteristics Of Wetlands......................................... 44 

Discussion . . . . . . . . .. . . . . . . . . . .. . .. . . . . . . . . . .. . . . . . . . . .............. .. . . . . . . . . . . . . .. . . . . . ........ 49 

References...................................................................................... 52 

Chapter 4....................................................................................... 55 

Dynamics of a Prairie Pothole Wetland Complex: Implications 

for Wetland Management........................................................ 55 

Abstract.......................................................................................... 55 

Introduction.................................................................................... 55 

Methods.......................................................................................... 57 

Response of Wetlands in and Near the Cottonwood Lake Area to 

Climate Variability fom 1961 to 1998 ....................................... 57 

Key Wetlands . . ......... .............. .. . ... . . . . .. .. . . . .. . . . . . . . . . . . . . . . ....... .. . . . . . .. ... . . . 63 

Wetland T8 . . . . . . . ... .. . . . . . . . .. .. .. . . . . . . . . ..... .. .. . . . . . . . . . . . . . . . . . ... . .. . .. . .. ... . . . . . 63 

v 
VI CONTENTS 

Wetland T3 . . . . . . .... ..... . . . . ..... . . . .. . ............. .......... ................ ..... . . . . . 63 

Wetland Pl................................................................................ 67 

Wetland P8................................................................................ 74 

Wetland Pll.............................................................................. 76 

Hydrological, Chemical, and Biological Interactions in a Wetland 

Complex and the Broader Prairie Pothole Region..................... 78 

Spatial Relations....................................................................... 78 

Temporal Relations -- Response To Climate Extremes............ 79 

Implications for Wetland Management.......................................... 82 

Biodiversity and Productivity................................................... 82 

Wetland Preservation and Restoration...................................... 83 

Waterfowl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 

Conclusions . . . . .. .. .. . . . . . .. . .. .. .. . . . .. .. .. . . . . . .. .. .. .. .. . . . .. .. .. .. .. . . . .. .. .... .. . .. .. . .. .. 86 

Acknowledgments.......................................................................... 86 

References . . . .. .... .. . . . . .. .. .. .. .. . . . . .. .. .. . . . . . .. .. . . .. .. . . . .. . .. .. .. . . . .. .. . . .. .. . . .. .. . .. . . 86 

Chapter 5....................................................................................... 95 

Simulation of Hydrology and Vegetation Dynamics of Prairie 

Wetlands in the Cottonwood Lake Area................................ 95 

Abstract.......................................................................................... 95 

Introduction.................................................................................... 95 

Spatial Simulation Model............................................................... 96 

Structure And Development..................................................... 96 

Validation And Testing............................................................. 98 

Hydrologic Submodel............................................................... 99 

Vegetation Submodel... .... .. .. . .. . .. .. . . . . . .. .. . .. . . . . .. . .. . .. .. . . . . .. .... . . . . . .. .. 99 

Overall Model........................................................................... 99 

Climate Change Studies .. .. .. .. .. .. .... .. .. .. .. .. .... .. .. .. .. .... .. .. .. .. .. .... .. .. .. .. . 99 

Goddard Institute For Space Studies (GISS) Simulation......... 100 

Sensitivity Simulations............................................................. 101 

Seasonal Simulations ................................................................ 101 

Management and Restoration Applications .. .. .. ............ .... .... .. ....... 103 

Future Needs And Directions......................................................... 1 07 

Acknowledgments.......................................................................... 107 

References . . . .. . .. . .. . . . . .. .. .. . . . . . . .. .. .. . . . . .. ... . .. .. . . . . .. .. .. . . . . .. .. .. .. .. . . . .. .... . . . . . . 1 07 
CONTENTS 

FIGURES 

1. Map showing Physiographic regions of North Dakota, location of the Cottonwood Lake area, 

locations of National Weather Service stations, and location of geohydrologic section shown in 

figure 2 ....................................................................................................................................................................... 2 

2. Hydrogeologic section showing the Cottonwood Lake area with respect to regional geology and numerical 

simulation of intermediate and regional ground-water flow systems. Line of section shown in figure 1. 

(Modified from Winter and Carr, 1980) .................................................................................................................... 3 

3. Map showing locations of wetlands and locations of test holes and ground-water wells in the Cottonwood 

Lake area .................................................................................................................................................................... 4 

4. Hypsometric map of the Cottonwood Lake area and the regional topographic low occupied by Wetland Pll 

and Eric Lake ............................................................................................................................................................. 5 

5 Map showing distribution of buried sand deposits in the Cottonwood Lake area and locations of geologic 

sections shown in figure 6 ......................................................................................................................................... 6 

6. Geological sections of the Cottonwood Lake area. Lines of sections shown in figure 5 ......................................... 8 

7. Graphs showing wetland water levels, 1979-98. Wetlands PI and T3 (A), Wetlands PI, P6, and P7 

(B), Wetlands P2 and P4 (C), Wetland Pll (D), Wetlands P3, P8, and T2 (E), Wetland T6 (F), WetlandsT8 

VII 

and T9 (G), Wetlands T5 and T7 (H) ......................................................................................................................... 12 

8. Map showing extent of open water in wetlands in the Cottonwood Lake area in the fall of 1996 following 4 years 

of unusually wet climate conditions. All wetlands are larger than shown in figure 3, and a number of wetlands 

merged to form single water bodies. (From Winter and Rosenberry, 1998) ............................................................ 13 

9. Hydrologic section through the Cottonwood Lake area showing the relation of wetlands to the water-table 

profile. (Modified from Winter and Carr, 1980) ........................................................................................................ 14 

10. Graph showing water level of Wetland PI and water levels in selected ground-water wells in the uplands, 

1979-98 ...................................................................................................................................................................... 15 

11. Graph showing water level of Wetland P 1 and water levels in selected ground-water wells in the lowlands near 

the wetland, 1979-98 .................................................................................................................................................. 16 

12. Map showing configuration of the water table in the Cottonwood Lake area, May 31, 1987, indicating that ground 

water was discharging to Wetland P 1 around its entire perimeter. (From Winter and Rosenberry, 1995) .............. 17 

13. Map showing configuration of the water table in the Cottonwood Lake area, August 31, 1989, showing the cones 

of depression caused by transpiration directly from ground water around much of the perimeter of Wetland Pl. 

(From Winter and Rosenberry, 1995) ........................................................................................................................ 18 

14. Sketches showing direction of flow between Wetland PI and nearby ground-water wells for the periods of high 

and low water levels of each year from 1981 to 1990. (From Winter and Rosenberry, 1995) ................................. 19 

15. Hydrologic sections showing water-table profiles between Wetlands T3 and PI indicating the formation of 

water-table mounds in response to early-summer rainfall (A-E) and in response to late-summer rainfall (F-J). 

(From Rosenberry and Winter, 1997) ........................................................................................................................ 21 

16. Map showing configuration of the water table in the Cottonwood Lake area, May 25, 1990, indicating one of 

the few times during the study when a ground-water divide was not present between Wetlands PI and P8 ............ 22 

17. Sketch showing numerical simulation of ground-water flow in the vicinity of a highly conductive aquifer of 

limited extent within a porous media having lower hydraulic conductivity. (Modified from Winter, 1976) ........... 23 

18. Hydrologic section showing intermediate ground-water flow systems passing at depth beneath local ground-water 

flow systems associated with selected wetlands. Note the local and intermediate flow systems discharging into Wetland P 11. 

The intermediate flow system is also shown in figure 2 where ground water moves from the Cottonwood Lake area to 

Eric Lake. Wetland names are shown along top of figure. U indicates unnamed. Line of section shown in figure 4 ............... 23 

19. Map showing generalized distribution of soils in the Cottonwood Lake area. (Map courtesy of Dr. James Arndt, 

then ofNorth Dakota State University) ..................................................................................................................... 24 

20. Graph showing normal (1961-90) monthly precipitation and temperature for the Cottonwood Lake area based on 

data from three nearby National Weather Service stations ........................................................................................ 27 

21. Graph showing summer monthly average air temperatures measured at the Cottonwood Lake area and monthly 

average air temperatures calculated as weighted means of data from the Carrington, Jamestown airport, and 

Pettibone National Weather Service stations, 1982-85 .............................................................................................. 28 
VIII CONTENTS 

22. Graph showing monthly mean hours of occurrence for daily maximum and minimum air temperatures, and 

one standard deviation from the monthly mean, for the Cottonwood Lake area, 1982-85 ....................................... 28 

23. Graph showing measured daily total incident shortwave solar radiation at the Cottonwood Lake area from 

1982 to 1987 and clear-sky shortwave solar radiation at 47 degrees north latitude ................................................. 29 

24. Graph showing monthly average percent of clear-sky shortwave solar radiation for the Cottonwood Lake area, 

1982-87 ...................................................................................................................................................................... 30 

25. Graph showing monthly average windspeed, and one standard deviation from the monthly average, for the 

Cottonwood Lake area, 1982-91 ............................................................................................................................... 30 

26. Graph showing monthly average temperatures of air, surface water, bottom water, and sediment at 0.5 meter 

and 1.0 meter below the sediment-water interface at the center of Wetland Pl, 1982-85 ....................................... 31 

27. Graph showing cumulative departure from normal monthly average temperature and cumulative departure 

from normal monthly total precipitation for the Cottonwood Lake area, 1978-98 ................................................... 32 

28. Graph showing cumulative departure from normal monthly total precipitation and water level of Wetland Pl, 

1978-98 ...................................................................................................................................................................... 33 

29. Graph showing Palmer Drought Severity Index (PDSI) for climate region 5, North Dakota, 1900-98 ................... 33 

30. Graph showing monthly total evaporation and precipitation for Wetland Pl during the open-water season, 

1982-85, and 1987 ..................................................................................................................................................... 34 

31. Graph showing monthly average evaporation from Wetland Pl (1982-87) and normal (1961-90) monthly 

precipitation for May-October ................................................................................................................................... 34 

32. Graph showing summary of specific-conductance data for five wetlands for 1984 to 1987 .................................... 37 

33. Graph showing specific conductance of water in Wetland Pl for the months when the wetland was ice free 

during 1984 to 1987. Gaps between lines are periods when the wetlands were ice covered. Lines connect median 

values. Points represent data from each of the three transects from which water samples were collected for 

chemical analysis. Many points overlie one another for individual dates ............................................................... 38 

34. Graphs showing specific conductance of water in semipermanent Wetlands Pl (A), P8 (B), and Pll (C), for 

the months when the wetlands were ice free during 1979 to 1998. Gaps between lines are periods when the 

wetlands were ice covered ......................................................................................................................................... 38 

35. Graph showing summary of pH data for five wetlands for 1984 to 1987 ................................................................. 40 

36. Graph showing pH of water in Wetland P 11 for the months when the wetland was ice free during 1984 to 1987. 

Gaps between lines are periods when the wetlands were ice covered. Lines connect median values. Points 

represent data from each of the three transects from which water samples were collected for chemical analysis. 

Some points overlie one another for individual dates ............................................................................................... 40 

3 7. Graphs showing modified Schoeller diagram of the abundance of major ions in water within five wetlands 

(A) and se"lected ground-water wells (B) .................................................................................................................. 42 

38. Graph showing summary of total phosphorus data for five wetlands for 1984 to 1987 ........................................... 42 

39. Graph showing summary of orthophosphate phosphorus data for five wetlands for 1984 to 1987 ......................... 43 

40. Graph showing summary of total nitrogen data for five wetlands for 1984 to 1987 ................................................ 43 

41. Graph showing summary of chlorophyll a data for five wetlands for 1984 to 1987 ................................................ 44 

42. Graph showing chlorophyll a concentrations for Wetland Pll for the months when the wetland was ice free 

during 1984 to 1987. Gaps between lines are periods when the wetlands were ice covered. Lines connect median 

values. Points represent data from each of the three transects from which water samples were collected for 

chemical analysis. Some points overlie one another for individual dates ................................................................. 44 

43. Graphs showing summary, based on biovolume data from 1985 to 1987, of the relative abundance of taxa in 

the algal communities ofWetland Pl (A), Wetland P8 (B), Wetland Pll (C), Wetland T8 (D) and 

Wetland T3 (E) ......... "~""' .......................................................................................................................................... 48 

44. Graphs showing seasonal changes in the relative abundance of taxa in the algal community of the open-water 

zone of Wetland Pl during 1986 ............................................................................................................................... 49 

45. Graph showing summary of the abundance ofmicroinvertebrates for five wetlands during 1984 to 1987 ............. 51 

46. Graph showing seasonal changes in the abundance of microinvertebrates in Wetland T8 during the ice-free 

period of 1987 ........................................................................................................................................................... 51 

47. Graphs showing seasonal changes in the relative abundance of the microinvertebrate community of Wetland Pl 

during the ice-free period of 1986 ............................................................................................................................. 52 
CONTENTS 

48. Photograph showing aerial view of the Cottonwood Lake area looking west on July 1, 1981. The tip of the 

cultivated right triangle in the lower left side of the photograph intersects the south end of Wetland P7. 

The east-west road bends around the north side of Wetland P11located in the background .................................... 56 

49. Map showing upland vegetation in the Cottonwood Lake area, 1989 ....................................................................... 58 

50. Photographs showing Wetland P6 on October 1, 1983, dominated by deep-marsh vegetation established during 

the drought of 1976-77 (A); Wetland P6 on October 28, 1997, dominated by open water following the deluge of 

1993-98 (B); Wetland P7 on October 5, 1996, dominated by open water that flooded a 33-year old aspen stand 

located on the east side of the wetland (C) ................................................................................................................ 60 

51. Photographs showing aerial view during May, 1970, of Wetland P11 (A), wetlands in the Cottonwood Lake area 

(B) and wetlands studied by Eisenlohr and others (1972) (C) ................................................................................... 61 

52. Photographs showing Wetland P4 on July 1, 1973, during a period of drawdown. A circular stand of hybrid cattail 

that survived flooding during 1962-66 is shown in the center (A); Wetland P4 on July 10, 1974, during a second 

year of drawdown (B); aerial view of Wetland P4 on October 30, 1975, showing three circular stands of hybrid 

cattail that survived flooding during 1962-66 and persisted as distinct units for 10 years (C) ................................. 64 

53. Photographs showing Wetland P4 on October 30, 1975, dominated by recently germinated cattail. (The 

persistent clump of hybrid cattail located in the center dominates the mixed-species stand of recently germinated 

cattail) (A); Wetland P4 on July 20, 1979, dominated by a monotypic stand of cattail that persisted for 18 years 

(B); Wetland P4 on October 28, 1997, during the deluge of 1993-98 that integrated Wetlands P2, P4, and 

P5 (C) ......................................................................................................................................................................... 65 

54. Photographs showing Wetland T8 on May 5, 1975, following flooding (A); Wetland T8 on August 1, 1975, 

dominated by smartweed and sloughsedge following drawdown (B); Wetland T8 on July 23, 1991, dominated 

by Canada thistle (Cirsium arvense) during the drought of 1988-92 (C) .................................................................. 66 

55. Photographs showing Wetland T3 on July 23, 1991, dominated by Canada thistle during the drought of 

1988-92 (A); Wetland T3 on July 27, 1994, showing a small stand of cattail (B); Wetland T3 integrated with 

Wetland Pl on October 28, 1997, during the deluge of 1993-98 (C) ........................................................................ 68 

56. Photographs showing Wetland P1 on June 1, 1967, following a rise in water levels during 1962-1966 

(A); Wetland P1 on October 29, 1992, dominated by deep-marsh vegetation following the drought of 1988-92 

(B); Wetland P1 on September 25, 1999, dominated by open water following a rise in water level initiated by the 

deluge of 1993-98 (C) ................................................................................................................................................ 70 

57. Photographs showing Wetland P1 on September 12, 1977, following drawdown (A); Wetland Pl on 

September 9, 1978, dominated by shallow-marsh vegetation that germinated on the mudflat inside the 

deep-marsh zone (B); Wetland P 1 on July 1, 1984, following flooding that eliminated shallow-marsh vegetation 

in the center of the wetland (C). Shallow-marsh vegetation persisted in the seasonally flooded zone shown in 

the background ........................................................................................................................................................... 71 

58. Photographs showing aerial view looking south toward the Cottonwood Lake area on July 19, 1989, showing 

Wetlands P4 (lower left), P2 (lower right), and open water in the central zones of Wetlands P1 (center) and 

P8 (right edge) during the early stage of the drought of 1988-92 (A); Wetland T1 on October 28, 1997, looking 

east toward Wetland P1 into Wetland T3 located in the background (B); aerial view of the Cottonwood Lake 

area on October 1, 1976, showing the persistence of open water in Wetland P8located in the lower 

left comer (C) ............................................................................................................................................................. 73 

59. Photographs showing Wetland P8 on July 1, 1981, dominated by an open-water zone surrounded by a mat of 

floating cattail (A); Wetland P8 on October 29, 1992, dominated by deep-marsh vegetation following drawdown 

(B); Wetland P8 on October 28, 1997, dominated by deep-marsh vegetation persisting in the central zone 

following the deluge of 1993-98 (C) ......................................................................................................................... 75 

60. Photographs showing Wetland P11 on October 11, 1985, visited by tundra swan that feed on sago pondweed 

tubers (A); Wetland Pll on October 1, 1990, during drawdown that exposed salt crystals (B); Wetland P11 on 

June 10, 1991, during drawdown (C). Windrows of salt crystals that were blown from the basin and deposited 

in the upland southeast of the basin are shown .......................................................................................................... 77 

61. Graph showing observed water volume in Wetland P 1 compared to calculated water volume obtained using 

WETSIM 2.0 model, 1961-92 ................................................................................................................................... 99 

62. Graph showing model output of water-level changes in Wetland P1 for an 11-year simulation under current 

climate and enhanced greenhouse gas climate conditions. Temperature and precipitation adjustments for the 

enhanced greenhouse gas climate were from the GISS general circulation model. The entire wetland was dry 

IX 

at approximately 557.7 meters above sea level. (From Poiani and Johnson, 1993b) ............................................... 101 
X CONTENTS 

63A. Map showing model output of emergent vegetation cover and open-water distributions for years I, 4, 7, and II 

of an II-year simulation for semipermanent Wetland PI under current climate. Simulations were on August I 

of each year using unrestricted seed germination. (From Poiani and Johnson, I99I) ............................................. I 02 

63B. Map showing model output of emergent vegetation cover and open-water distributions for years I, 4, 7, and 11 

of an II-year simulation for semipermanent Wetland PI under an enhanced greenhouse gas climate as projected 

by the GISS general circulation model. Simulations were on August I of each year using unrestricted seed 

germination. (From Poiani and Johnson, I99I) ........................................................................................................ I03 

64. Maps showing model output of emergent vegetation cover and open-water distributions for years I, 4, 7, 8, I 0, 

and II of an II-year simulation for semipermanent Wetland PI under an enhanced greenhouse gas climate as 

projected by the GISS general circulation model. Simulations were on August I of each year. Seed germination 

was limited according to the timing of the draw down .............................................................................................. I 04 

65 Maps showing model output of emergent vegetation cover and open-water distributions for eight 5-year 

water-depth simulations for Wetland P4. Results are shown for October of year 5 ................................................ I06 
CONTENTS XI 

TABLES 

1. Summary of pH and major ion data for five wetlands of the Cottonwood Lake area. Values presented are medians 

for 1984 to 1987 ......................................................................................................................................................... 39 

2. Summary statistics of pH and major-ion data for 21 ground-water wells and specific data for selected wells in the 

Cottonwood Lake area. Values presented for summary statistics are for 1984 to 1987 and represent a total of 

36 water samples ........................................................................................................................................................ 41 

3. Median value of the coefficient of variation for each date on which water samples were collected for chemical 

characteristics of five wetlands in the Cottonwood Lake area, 1984-87 ................................................................... 45 

4. Crustacean microinvertebrate species found in the water column of selected wetlands in the Cottonwood Lake 

area, 1984-87 ............................................................................................................................................................. 46 

5. Species composition, occurrence (percent), and average canopy cover for vegetation in mesic upland sites in 

the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent 

- 4 percent, 3 = 5 percent - 24 percent, 4 = 25 percent - 49 percent, 5 = 50 percent - 7 4 percent, 6 = 7 5 percent 

- 94 percent, and 7 = 95 percent - 100 percent. ......................................................................................................... 89 

6. Species composition, occurrence (percent), and average canopy cover for vegetation in xeric upland sites in the 

Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent 

- 4 percent, 3 = 5 percent- 24 percent, 4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent 

- 94 percent, and 7 = 95 percent - 100 percent. ......................................................................................................... 90 

7. Species composition, occurrence (percent), and average canopy cover for vegetation in north aspect upland sites 

in the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = < 1 percent, 2 = 1 percent 

- 4 percent, 3 = 5 percent- 24 percent, 4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent 

- 94 percent, and 7 = 95 percent- 100 percent. ......................................................................................................... 91 

8. Species composition, occurrence (percent), and average canopy cover for vegetation in south aspect upland sites 

in the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent 

- 4 percent, 3 = 5 percent- 24 percent, 4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent 

- 94 percent, and 7 = 95 percent - 100 percent. ......................................................................................................... 92 

9. Species composition, occurrence (percent), and average canopy cover for vegetation in east aspect upland sites 

in the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent 

- 4 percent, 3 = 5 percent- 24 percent, 4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent 

94 percent, and 7 = 95 percent - 100 percent.. ........................................................................................................... 93 

10. Species composition, occurrence (percent), and average canopy cover for vegetation in west aspect upland sites 

in the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent 

- 4 percent, 3 = 5 percent- 24 percent, 4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent 

- 94 percent, and 7 = 95 percent- 100 percent. ......................................................................................................... 94 

11. Rules for changes in vegetation submodel. COV =vegetation type, WDC = water-depth category, MON =month, 

LENWDC =time period in water-depth category (months), LOC =cell location. Water-depth categories: (1) <-55 em, 

(2) -55 to -10 em, (3) -9 to 3 em, (4) 4 to 19 em, (5) 20 to 55 em, and (6) >55 em. OW= open water, DM =deep-marsh 

emergents, MSM =wet-meadow/shallow-marsh emergents, UPL =upland vegetation, SEEDL =seedlings, MIXPLT = 

mixed young plants, MIXEM =mixed species of shallow- and deep-marsh emergents, EXPSOIL = exposed soil. ................. 98 

12. Adjustments to daily temperature and precipitation data as projected by the GISS general circulation model for 

Jamestown, North Dakota. Data from Takle and Zhong (1990). Details in Poiani and Johnson (1993b) ................. 100 

13. Percentage of deep-marsh emergents and open water and area of wet-meadow/shallow-marsh emergents. 

Values are for end of 5-year management scenarios for Wetland P4 ......................................................................... 105 
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acre-foot per year (acre-ft/yr) 

foot per second (ftfs) 

foot per minute (ftfmin) 

foot per hour (ft/hr) 

foot per day (ftfd) 

By 

Length 

2.54 

25.4 

0.3048 

1.609 

1.852 

0.9144 

Area 

4,047 

0.4047 

0.4047 

0.004047 

929.0 

0.09290 

6.452 

259.0 

259.0 

2.590 

Volume 

0.1590 

0.02957 

0.4732 

0.9464 

3.785 

0.003785 

3.785 

3,785 

16.39 

0.01639 

0.01639 

28.32 

0.02832 

0.7646 

4.168 

1,233 

0.001233 

Flow rate 

0.01427 

1,233 

0.001233 

0.3048 

0.3048 

0.3048 

0.3048 

To obtain 

centimeter 

millimeter 

meter 

kilometer 

kilometer 

meter 

square meter 

hectare 

square hectometer 

square kilometer 

square centimeter 

square meter 

square centimeter 

square hectometer 

hectare 

square kilometer 

cubic meter 

liter 

liter 

liter 

liter 

cubic meter 

cubic decimeter 

cubic meter 

cubic centimeter 

cubic decimeter 

liter 

cubic decimeter 

cubic meter 

cubic meter 

cubic kilometer 

cubic meter 

cubic hectometer 

cubic meter per second 

cubic meter per year 

cubic hectometer per year 

meter per second 

meter per minute 

meter per hour 

meter per day 

XII 
CHAPTER 1 

GEOHYDROLOGIC SETTING OF THE 

COTTONWOOD LAKE AREA 

Thomas C. Winter, U.S. Geological Survey, Denver, Colorado 

ABSTRACT 

Geologic deposits in the Cottonwood Lake 

area consist largely of silty, clayey glacial till that 

contains numerous fractures and small, randomly 

distributed sand and gravel deposits. The sand 

deposits can have a substantial effect on groundwater flow between wetlands in the area and can 

cause some to drain while others have relatively 

stable inflow. Direct precipitation and runoff 

from snowmelt are the primary sources of water 

to the wetlands and evaporation accounts for the 

largest loss of water from the wetlands. The 

wetlands in the study area have a range of 

functions with respect to their interaction with 

ground water. Some of the seasonal wetlands 

recharge ground water and others recharge ground 

water and receive ground-water discharge. The 

semipermanent wetlands receive ground-water 

discharge much of the time, but some have 

reversals of flow between them and the groundwater system nearly every year. Ground-water 

flow toward the wetlands is caused by recharge in 

the uplands and by focused recharge near the 

wetland perimeters. Flow from the semipermanent wetlands to the ground-water system occurs 

when the wetland water levels are higher than the 

contiguous water table, resulting in bank storage, 

and when evapotranspiration directly from the 

ground-water system causes seepage around the 

wetland perimeters. Substantial climate 

variability during the study period caused the 

wetlands to range from being completely dry to 

having such high water levels that some of the 

wetlands merged to become large lakes. 

INTRODUCTION 

A geohydrologic framework is the foundation 

of aquatic ecosystems. Landforms, geologic deposits, 

and the movement of water over and through a 

geologic landscape determine the types and positions 

of aquatic features that are present in the landscape. 

Geochemical interactions of rocks and water 

determine the general chemical characteristics of the 

aquatic features and plants and animals respond to the 

presence (or absence) of water and its chemical 

characteristics. The purpose of this chapter is to 

describe the geohydrologic framework of the 

Cottonwood Lake area to set the stage for discussion 

of the chemical and biological responses that have 

been observed in the area for more than 30 years, from 

the mid-1960s to the late 1990s. 

GEOLOGIC ORIGIN AND PHYSIOGRAPHIC FEATURES 

The Cottonwood Lake area is located near the 

eastern edge of the Missouri Coteau in Stutsman 

County, east-central North Dakota (fig. 1). The 

Missouri Coteau is a large glacial stagnation moraine 

that transects North Dakota from the northwest comer 

to the south-central part of the State. It lies north and 

east of the Missouri River. During the Pleistocene, 

continental glaciers moved south from Canada, 

overriding and incorporating bedrock consisting of 

Precambrian igneous and metamorphic rocks, early 

Paleozoic sandstones, limestones, and dolomites, and 

Cretaceous shales and siltstones (Carlson, 1969). The 

Pierre Formation was exposed at the surface in a large 
2 CHAPTER I 

__ CANAD,.!> ________ _ 

50 100 150 KILOMETERS 

---~------ \ 

I 

I 

I 

\ ~ ~ \ 2:. 

L \ ~ ~ 

~ \ 

'6':s:. 

~':z 

o \ rn ~~~ 

5!' 

I 

I 

\ 

Figure 1. Physiographic regions ofNorth Dakota, location of the Cottonwood Lake area, locations of National Weather Service 

stations, and location of geohydrologic section shown in figure 2. 

part of the area over which the glaciers moved before 

reaching the Cottonwood Lake area, and incorporation 

of the fine-grained bedrock resulted in the very silty 

and clayey glacial till in the area. 

The Missouri Coteau rises more than l 00 m 

above the surrounding terrain along much of its length. 

The Cottonwood Lake area, situated on one of the 

higher parts of the eastern edge of the Missouri 

Coteau, lies about 120m higher than the James River 

lowland to the east and about 30 m higher than a small 

lowland within the Missouri Coteau about 4 km to the 

west (location of Eric Lake in figure 2). Woodworth, 

about 16 km to the northwest, is situated on the highest 

part of the Missouri Coteau in the general vicinity of 

the Cottonwood Lake area. 

Local relief within the 80 ha that constitute the 

Cottonwood Lake area is about 33m (fig. 3). The 

area is very hummocky and has many closed depressions, most of which contain wetlands. The wetlands 

range in altitude from about 577 m for seasonal 

Wetland T8 to about 551 m for semipermanent 

Wetland P9. Wetland Pll lies about 4 km west of the 

Cottonwood Lake area (near Eric Lake) (fig. 4) at an 

altitude of about 542 m. Because ofthe silty and 

clayey characteristics of the till, land slopes are as 

much as 80 percent in many places in the Cottonwood 

Lake area. 

CHARACTERISTICS OF GEOLOGIC 

DEPOSITS 

Depositional processes characteristic of terminal 

and stagnation moraines, such as the Missouri Coteau, 

result in highly heterogeneous geologic deposits 

commonly referred to as drift. In the Missouri Coteau, 

most of the drift consists of till, a mixture of clay 

through boulder-size rocks, that was deposited directly 

by glaciers. However, meltwater streams flowing 

from the glaciers also resulted in the deposition of 

discrete fluvial deposits of well-sorted clay, silt, sand, 

and gravel that then were buried by minor advances of 

the glaciers. Therefore, fluvial deposits within the till 

are common, have limited extent, are highly variable 

in size and shape, and are randomly distributed. 
CHAPTER I 

Q) 

0 J:: 

'<l" t ""' "' CD 0 -' E 

!:::: 0 ;: -o- -o 0 co Q) 0 00 0 co 

""' ~ 0 M ;: ,...._ 

"' N 0 ~ ::;: c~ -' ~ c-

"' Q) t::"' c c e 

·a u "' ~ 

~ 

Q) 

> 

a: 

w 

650 

(j) 600 a::....~ 

ww 1-> Ww 

:2:-' 550 

z<t: -W w·(J) 

ow 500 :::>> 

t-0 

-CD 

~<( 

<( 450 

Shale bedrock 

400 

10 KILOMETERS VERTICAL EXAGGERATION X 1 00 

EXPLANATION 

Upper surface of section is the water table 

D 

D 

Intermediate ground-water 

flow systems 

Regional ground-water 

flow system 

-24o.a - Line of equal hydraulic head 

• 

Line separating flow systems 

of different magnitude 

Stagnation point 

~ Direction of ground-water flow 

(6,030) Dissolved solids, in milligrams 

per liter 

Figure 2. Hydrogeologic section showing the Cottonwood Lake area with respect to regional geology and numerical simulation of 

intermediate and regional ground-water flow systems. Line of section shown in figure 1. (Modified from Winter and Carr, 1980). 

DISTRIBUTION AND PHYSICAL 

CHARACTERISTICS OF GEOLOGIC 

DEPOSITS 

About 55 test holes were drilled in the Cottonwood Lake area (fig. 3). Most of the test holes were 

drilled using a power auger, but one (near test hole 15) 

was drilled to bedrock, a depth of 133m, by the mudrotary method. The test drilling indicated that most of 

the drift in the Cottonwood Lake area is silty, clayey 

till containing a considerable number of rocks as large 

as boulders. However, fluvial sand and silt deposits 

were encountered in a number of the test holes. 

Although the fluvial deposits in the drift have complex 

shapes and are somewhat random in size and distribution, mapping of the sand intervals on the basis of their 

upper-surface altitudes (Winter, 1975) indicated that 

one or more continuous buried sand deposits are 

present in the area (fig. 5). To aid in the visualization 

of the three-dimensional distribution of the sand 

deposits, a number of cross-sections are shown in 

figure 6. 

Many of the test holes drilled in the western part 

of the study area encountered sand intervals that have 

upper-surface altitudes of 556 m to 559 m. (See test 

holes 16, 41, 42, 43, 44, and 46 in figs. 5 and 6, 

sections A-A', B-B', and C-C'.) Recognizing the 

uncertainty of correlating subsurface fluvial deposits 

in the drift, it nevertheless is conceivable that the sand 

intervals encountered in the test holes drilled in the 

western and southern part of the study area are a single 

continuous deposit. Sand intervals that have similar 

upper-surface altitudes were encountered in test holes 

1, 14, 19, and 24 in the southern part of the study area. 

These sand intervals could be isolated deposits or they 

could be an extension of the same deposit that occurs 

at the surface near Wetland T2 in the western part of 

the study area. If they are an extension of the same 

deposit, they would need to be part of a long sinuous 

deposit that extends to the east through the vicinity of 

3 
4 CHAPTER 1 

test holes 14 and 19 and then continues to the 

southeast to the vicinity of test holes 1 and 24. That 

the sand deposit would need to be sinuous is dictated 

by the number of test holes in the southern part of the 

study area, such as test holes 17, 18, 20, and 21 , and in 

the southeast part of the study area, such as test holes 

2, 3, 7, and 30 (figs 5 and 6, sections A-A' and B-B'), 

that penetrated deeper than 556 min altitude but did 

not penetrate sand. The test holes that did not 

penetrate sand indicate that, if the sand intervals 

encountered in test holes 1, 14, 19, and 24 are an 

extension of the deposit that occurs in the western part 

ofthe study area, the deposit would need to bifurcate 

Base from U.S. Geological Survey 

northeast of Wetland P6, with one arm extending to 

the vicinity of test hole 1 and the other extending to 

the vicinity of test hole 24. The deposit may extend 

beyond test hole 24, but the extent could not be 

defined because test holes 4, 5, 6, and 54 were not 

drilled deeply enough. Much of the sand in the deposit 

in the southern part of the study area is fine to medium 

grained, and the deposit contains fine gravel in some 

locations. As will be discussed later in this chapter, 

the continuity of the sand deposit has important 

implications for the hydrologic function of some of the 

wetlands in the Cottonwood Lake area. 

75 150 METERS 

CONTOUR INTERVAL APPROXIMATELY 3 METERS 

DATUM IS SEA LEVEL 

EXPLANATION 

e Semipermanent wetland 

~ Seasonal wetland 

• 21 Test hole and well 

Figure 3. Locations of wetlands and locations of test holes and ground-water wells in the Cottonwood Lake area. 
47" 

06' 

Base modified from U.S. Geological Survey 

CHAPTER 1 

EXPlANATION 

Meters above sea level 

>580 

570-580 

560-570 

550-560 

<550 

1 KILOMETER 

D Lakes and wetlands 

A A' 

1-----------j Line of section shown 

in Figure 18 

Figure 4. Hypsometric map of the Cottonwood Lake area and the regional topographic low occupied by Wetland 

P 11 and Eric Lake. 

5 
6 CHAPTER 1 

Another smaller buried sand deposit is present 

at a higher altitude in the eastern part of the study area. 

Test holes 6, 30, and 34 penetrated sand intervals in 

that area that have upper-surface altitudes of 566 m to 

569 m (figs. 5 and 6, section D-D'). Much ofthe sand 

in the deposit in the eastern part of the study area is 

very-fine to fine grained and is quite silty. Other sand 

intervals in the study area, particularly north of 

Wetland P 1, appear to be thin isolated deposits (figs. 5 

Base from U S. Geological Survey 0 

and 6, section A-E). Sand also is present at the surface 

in the vicinity of test holes 29 and 36. 

Relatively thick silt deposits are present at the 

surface in two small valleys within the watershed of 

Wetland P 1. [n the valley southwest of Wetland P 1, 

silt extends to depths of 1 mat test hole 17 and 4.9 m 

at test hole 18. In the valley northwest of Wetland Pl, 

silt extends to depths of 1.2 mat test hole 26 and 4.3 m 

at test hole 27. 

75 150 METERS 

CONTOUR INTERVAL APPROXIMATELY 3 METERS 

DATUM IS SEA LEVEL 

EXPLANATION 

Buried sand deposit-Upper-surface 

altitude is between 556 and 559 

meters above sea level 

Buried sand deposit- Upper-surface 

altitude is between 566 and 569 

meters above sea level 

Semipermanent wetland 

~ Seasonal wetland 

Test hole and observation well 

0 26 Test holes that penetrated sands 

that appear to be isolated 

e 2l Test holes that penetrated deeper 

than 556 meters in altitude, 

but did not penetrate sand 

A-A' Line of section 

Figure 5. Distribution of buried sand deposits in the Cottonwood Lake area and locations of geolog ic sections shown in 

figure 6. 
CHAPTER! 7 

Fractures are common in the till in the study 

area. The large clay content of the till in the Missouri 

Coteau causes the till to crack upon drying, and results 

in the numerous fractures (Meyboom and others, 

1966; Grisak and Cherry, 1975; Keller and others, 

1986). To determine the presence and characteristics 

of fractures in the till in the Cottonwood Lake area, 

soil pits 3 m x 1 m in size were dug to a depth of 2.2 m 

at the top, side, and base of the hills lope northwest of 

Wetland P1 (Swanson, 1990). Fractures observed in 

the till in the soil pits have a spacing of about 10 em 

and openings of various sizes; some are hairline 

fractures, but others have openings as great as 1 to 

2 mm (Swanson, 1990). Many of the fractures are 

lined with gypsum crystals that are 1 to 2 mm in 

diameter, and some of the weathering rinds are as 

much as 2 em wide. The depth to which the fractures 

extend in the study area is not known. 

Single-well slug tests to determine the hydraulic 

properties of the till also indicated that fractures are 

common in the till in the study area. The fractured till 

proved to be relatively permeable, and hydraulic 

conductivity values were in the 1 o-4 to 1 o-7 em s - 1 

range. These values are considerably greater than the 

hydraulic conductivity values that would be expected 

from a solid block of till; Hendry (1988) reported 

values to be about 1 o-8 em s - 1 for a similar till in 

Canada. 

The middle soil pit confirmed the presence of 

the thick silt deposit in the vicinity of test hole 27, and 

soil au gering in the floor of the pit revealed the 

presence of saturated white marl containing abundant 

gastropod shells from 35 to 60 em below the pit 

bottom [about 2.5 m below land surface (Swanson, 

1990)]. These data indicate that the water level of 

Wetland P 1 probably was about 2 m higher and that 

the wetland covered a larger area at some time in the 

past compared to the time the pits were dug and 

mapped in 1990. 

MINERALOGY OF GLACIAL DEPOSITS 

The mineralogy of the drift in the Cottonwood 

Lake area reflects the mineralogy of the bedrock over 

which the glaciers moved. Igneous and metamorphic 

rocks derived from the Canadian Shield make up a 

large percentage of the rocks in the drift. Many of 

these rocks have large amounts of quartz and feldspar. 

Calcite and dolomite derived from the lower Paleozoic 

rocks north of the area are common. Shale particles 

derived from the Pierre Formation, and lignite, are 

abundant. Minor amounts of pyrite are also present in 

the drift. Smectite is the most common clay mineral 

(Swanson, 1990). 

Gypsum is very abundant as a secondary 

mineral, both in fracture fillings, as mentioned 

previously, and disseminated in the drift. Crystals as 

large as 2 em long and 1 em in diameter were 

recovered from drill cuttings of the silt deposit 

southwest of Wetland P 1. Calcite, largely in the form 

of fine crystals, also occurs as a secondary mineral 

near the wetland perimeters (Swanson, 1990; Arndt 

and Richardson, 1993). Iron oxide staining is 

common, particularly near the wetland perimeters, and 

iron oxide concretions were recovered from some drill 

cuttings of the till. 

HYDROLOGIC CHARACTERISTICS OF WETLANDS 

Although biological studies in the Cottonwood 

Lake area began in the 1960s, hydrological studies did 

not begin until 1979. In that year, the first test holes 

were drilled, and deep piezometers and water-table 

wells were constructed. The water-table wells were 

constructed by inserting well screen and casing into 

the test holes discussed in the previous section of this 

chapter. Therefore the number designations for the 

wells are the same as for the test holes. In addition, 

staff gages were installed in the wetlands, a recording 

stage gage was placed in Wetland Pl, and a climate 

station was established near Wetland Pl. Climate, 

surface-water, and ground-water data have been 

collected at these hydrologic installations since 1979, 

but hydrologic information for only the 20-year period 

from 1979 to 1998 is presented in this report. 

INTERACTION OF WETLANDS AND 

ATMOSPHERIC WATER 

Precipitation is the primary source of water to 

wetlands in the study area. During the winter, the 

precipitation is stored on the land surface as snow. 

The snow commonly is deepest in the wetlands 

because the wetlands are more protected from the 
8 CHAPTER I 

A A' 

585.2 

579.1 

573.0 

566.9 10 

15 Wetland 12 11T4 

560.8 I 13 T3 T1 P1 

554.7 

548.6 

...J 

w B B' 

> w 585.2 ...J 6 30 

<( 

w 579.1 24 (/) 

UJ 

> 0 573.0 co 40 <( 

(/) 566.9 a:: 19 14 w 

IUJ 

~ 560.8 

~ 

u.i 554.7 

0 

::::> 

I- 548.6 ~ <( A E 

585.2 

579.1 

573.0 

566.9 53 

16 36 

27 

560.8 

554.7 

548.6 

Figure 6. Geological sections ofthe Cottonwood Lake area. Lines of sections shown in figure 5. 
c 

585.2 

579.1 

573.0 

...J 566.9 w 

> w 

...J 560.8 

<( 

w 

en 

w 554.7 

> 0 

OJ 

<( 548.6 

en 

a: 

UJ D 

r 585.2 UJ 

~ 

~ 579.1 24 

u.i' 

Cl 

=> r 573.0 

~ <( 566.9 

560.8 

554.7 

548.6 

CHAPTER! 

44 

D' 

6 30 

34 

EXPLANATION 

Till 

Sand 

Silt 

P4 Wetland 

36 Testhole 

75 150 METERS 

Datum is sea level 

Vertical Exaggeration, X9 

C' 

Figure 6. Geological sections of the Cottonwood Lake area. Lines of sections shown in 

figure 5 (continued). 

9 
10 CHAPTER I 

wind than the uplands and the tall wetland plants trap 

more snow than the shorter upland plants. Because 

soil frost impedes infiltration during the winter, 

snowmelt from the uplands usually enters the wetlands 

as surface runoff during a relatively short time in the 

spring while the soils are still frozen. The flux of 

water, including meltwater from the snow in the 

wetlands, results in a sharp rise in wetland water 

levels, and the water levels generally reach the highest 

levels of the year in late April and May. Spring 

rainfall also commonly contributes to the high water 

levels at that time of year. During the growing season, 

the only precipitation that contributes water to the 

wetlands generally is that which falls directly on the 

water surface. Most of the water that falls on the 

uplands infiltrates into the soil zone and is transpired. 

Only during prolonged or intense summer rainstorms 

does precipitation that falls on the uplands infiltrate 

past the root zone to recharge ground water. Precipitation following killing frosts in the fall can infiltrate 

past the root zone to recharge ground water, but this 

precipitation usually results in only moderate rises in 

ground-water levels. In some years, water from latefall rain is stored over the winter in the soil zone as ice, 

and the water contributes to ground-water recharge or 

wetland water the following spring (Winter and 

Rosenberry, 1995). Further information on the characteristics of precipitation in the study area is given by 

Rosenberry (Chapter 2 of this report). 

Evaporation from wetlands in the study area is 

driven by radiation energy. When this study began, 

the loss of water by evaporation from the wetlands in 

the prairie environment had not been studied 

sufficiently well to estitnate evaporation from network 

data, such as from National Weather Service stations. 

Therefore, instruments were placed at Wetland P 1 to 

determine evaporation by the energy-budget method, 

which generally is considered to be one of the most 

accurate methods for determining evaporation 

(Harbeck and others, 1958; Gunaji, 1968). The instruments included sensors to measure the incoming solar 

radiation, atmospheric radiation, air temperature, 

vapor pressure, windspeed, and temperature of the 

wetland water surface. A further purpose of the 

energy-budget studies was to provide evaporation 

values against which empirical methods of 

determining evaporation could be evaluated so that an 

acceptable simpler and cheaper method could be used 

for long-term monitoring of evaporation. 

Results of the energy-budget studies, which 

were conducted for 6 years (1982-87), indicated that 

evaporation varied considerably from season to 

season, as expected, and also from year to year 

(Parkhurst and others., 1998). For exatnple, the 

highest midsmntner high rate of evaporation was 

7.0 mm d-1 in 1987, and the lowest midsummer high 

rate was 4.4 mm d-1 in 1982. Other midsummer high 

rates of evaporation were 5.3 mm d-1 in 1983, 

4.9 mm d-1 in 1984, and 5.6 mm d-1 in 1985. These 

evaporation rates apply only to the open water of 

Wetland P 1; the difference between these rates and 

transpiration by aquatic plants in the vegetated part of 

the wetland has yet to be determined. Most of the 

semipermanent wetlands in the Cottonwood Lake area 

are as shallow as Wetland P 1; therefore, it is assumed 

that evaporation rates for the other semipermanent 

wetlands in the area are similar to the rates for 

Wetland Pl. 

WETLAND WATER LEVELS 

Wetland water levels usually followed a 

similar annual pattern and were highest in the spring 

following snowmelt and spring rainfall (fig. 7). 

During the remainder of the open-water period, the 

water levels gradually declined because evaporation 

exceeded rainfall and ground water contributed a 

relatively small amount of water to the wetlands. 

Heavy rains during the open-water periods caused the 

water levels to rise temporarily, but the water levels 

usually resumed their downward trend soon after the 

rainy period. 

Since this study began, the Cottonwood Lake 

area has experienced highly variable to extreme 

climate conditions (Rosenberry, Chapter 2 of this 

report). Therefore, the interannual and long-term 

fluctuations of the wetland water levels followed a 

much more unpredictable pattern than the annual 

pattern. Although precipitation was below normal 

during the first 9 years of the study (1979-87), the 

wetland water levels had relatively minor interannual 

variations (fig.7, A-D). Relatively high water levels in 

1979 were followed by 2 years of low levels and a 

return to high levels in 1982. The water levels then 

increased for the next 2 years, 1983 and 1984, and 

then declined substantially in 1985. The water levels 

increased again for the next 2 years, and, in 1987, 

reached the highest levels for the period of record at 
CHAPTERl 11 

that time. The water levels declined precipitously in 

1988, and some of the semipermanent wetlands 

became dry. The dry conditions that began in 1988 

continued for the next 4 years, and 1988-92 became 

the second worst drought of the 20th century for this 

part of North Dakota (Winter and Rosenberry, 1998). 

During those 5 years, Wetland P 1 dried up every 

summer (fig. 7 A), Wetland P11 dried up for the first 

time in 1990 and again in 1991 and 1992 (fig. 7D), and 

Wetland P8 dried up for the first time in 1991 and 

again in 1992 (fig. 7E). Wet conditions returned 

during the spring of 1993 and all of the wetlands 

contained water, but the great deluge that began in July 

1993 caused the water levels to rise to unprecedented 

levels. The wet period continued to 1998 and caused 

water levels to rise to the point where some of the 

wetlands merged to become single lakes (fig. 8). For 

example, Wetland P1 merged with Wetland T3 in 1995 

(fig. 7A), and Wetland P2 merged with Wetland P4 in 

1997 (fig. 7C). The time at which the wetlands 

merged to become single lakes is shown in figure 7 

where hydrographs for the individual wetlands begin 

to plot on top ofhydrographs for the other wetlands. 

All of the hydrographs shown in figure 7, B-D, 

are for wetlands that are enclosed by surface 

watersheds that are considerably higher than the 

normal wetland water levels. The hydrographs for 

those wetlands show the interannual variability of 

water levels even during the wet years from 1993 to 

1998. Other wetlands in the Cottonwood Lake area 

have low points on their surface watersheds that 

allowed the wetlands to spill to adjacent wetlands only 

when the water levels become high enough. Wetlands 

that spill when their water levels are only slightly 

higher than normal will spill to adjacent wetlands 

fairly frequently, usually following spring snowmelt 

and (or) periods of abundant rainfall. Hydrographs for 

those wetlands (fig. 7, A, E, F, and H) show a characteristic plateau at which a spill is likely to occur. 

Wetland T3 frequently spilled to Wetland P 1 prior to 

the drought that began in 1988 and before the two 

wetlands merged in 1995 (fig. 7A). Wetland P8 

frequently spilled to Wetland P9 except during the 

driest years (fig. 7E), and Wetland P3 spilled to 

Wetland P9 only during the wet years that began in 

1993 (fig. 7E). Some seasonal wetlands, such as 

Wetland T6 (fig. 7F) and Wetlands T5 and T7 (fig. 

7H), spilled most years except during the drought 

when they seldom contained water. Other seasonal 

wetlands, such as Wetlands T8 and T9, rarely spilled 

even during the wettest years (fig. 7G). Wetlands T8 

and T9 apparently have a greater elevation difference 

between their normal water levels and the water level 

at which a spill occurs because those wetlands rarely 

overflowed. 

INTERACTION OF WETLANDS AND 

GROUND WATER 

The high topographic position of the Cottonwood Lake area precludes the wetlands in the area 

from receiving ground-water inflow from regional 

ground-water flow systems, therefore, the wetlands are 

associated largely with local ground-water flow 

systems. Although some of the recharge in the study 

area contributes to the recharge of regional flow 

systems, most of the recharge is to the local flow 

systems. Winter and Carr (1980) indicated that 

wetlands in the study area serve the full range of 

functions with respect to ground water; some recharge 

ground water, some receive ground-water discharge 

but also recharge ground water (termed flow-through), 

and some receive ground-water discharge (fig. 9). For 

example, Wetlands T8 and T5 do not receive groundwater discharge, but they do recharge ground water 

during the brief period of the year that they hold water. 

Water-table gradients are always away from those two 

wetlands, and the water table declines as much as 2 m 

below the bottoms of the wetlands during dry times of 

the year. 

Seasonal wetlands that are topographically 

lower and at an intermediate altitude in the study area, 

such as Wetlands T3 and T4, are flow-through 

wetlands most of the time. Semipermanent wetlands 

that are at an intermediate altitude, such as Wetland 

P1, receive ground-water discharge most of the time. 

However, reversals of flow between the wetlands and 

the ground water around the wetland perimeters are 

common. The flow reversals are caused by focused 

recharge and transpiration directly from ground water, 

as discussed later. The topographically lowest 

wetlands, such as Wetland P8, nearly always receive 

ground-water discharge, but they also may have 

transpiration-induced outflow to ground water. 

The 20-year records of wetland water levels and 

ground-water levels indicate that the annual and longterm interannual fluctuation patterns of the groundwater levels were similar to those of the wetland water 

levels (figs. 10 and 11). The annual pattern showed 

high levels in the spring and an overall decline for the 
12 

561 

560 

559 

558 

557 

563 

562 

~ 

w' 561 0 

:::::> 

~ 

~ <( 560 

559 

\.A \ ' \ 

~\f\\\1, '\~1 

'V 

1 ~ 1 

f 

CHAPTERl 

,4.. ,:•, 

i\ 

558~-L~~~~~-L-L~~~~~_L_L~ 

546,-,,-,-,-~-.-.-.,-.-,-~-.-.-.~ 

545 

544 

543 

542 ;" " I 

558 

557 

f-v\ \~ 

555~-L~~~~~-L-L~~~~~-L-L~ 

566~~,-.-,-~~-,~,-.-,-~~-,-.~ 

565 

564 

563 

562~-L~~~~~-L-L~~~~~-L-L~ 

579~~,-.-,-~~-,-.,-.-,-~~-,-.~ 

578 

577 

576 

575~-L~~~~~_L_L~~~~~_L_L~ 

571 .. -.,-.-,-~-.-.-.,-.-,-~-.-.-.~ 

570 

569 

568 

CAL EN DAR YEARS 

Figure 7. Wetland water levels, 1979-98. Wetlands Pl and T3 (A), Wetlands Pl, P6, and P7 (B), Wetlands P2 and 

P4 (C), Wetland Pll (D), Wetlands P3, P8, and T2 (E), Wetland T6 (F), WetlandsT8 and T9 (G), Wetlands T5 and 

T7 (H). 
CHAPTER I 13 

99'06' 

75 150 METERS 

CONTOUR INTERVAL APPROXIMATELY 3 METERS 

DATUM IS SEA LEVEL 

EXPlANATION 

e Semipermanent wetland 

~ Seasonal wetland 

• 21 Test hole and well 

Figure 8. Extent of open water in wetlands in the Cottonwood Lake area in the fall of 1996 following 4 years of 

unusually wet climate conditions. All wetlands are larger than shown in figure 3, and a number of wetlands merged to 

form single water bodies. (From Winter and Rosenberry, 1998) 

remainder of the year. The long-term interannual 

pattern showed minor variations during the first 9 

years (1979-87), an overall decline during the drought 

(1988-92), and a substantial rise during the deluge 

(1993-98). 

Although the general patterns of highs and lows 

were similar, the magnitudes of the fluctuations in the 

ground-water levels were much larger than those in the 

wetland water levels, and the magnitudes differed 

between wells. A given amount of water added to a 

porous media will result in a greater increase of 

hydraulic head than the same amount of water added 

to an open-water surface. Because the water fills only 

the pore spaces between the solid material, 1 em of 

water added to earth materials having an effective 

porosity of 20 percent would result in a water-table 

rise of 5 em; whereas, 1 em of water added to an openwater surface would result in a water-level rise of 1 

em. Where water tables are shallow, such as near 

surface-water bodies that are connected to the groundwater system, infiltrated water that recharges ground 

water causes transient water-table mounds to form 

directly adjacent to the surface water. This process is 

referred to as focused recharge (Winter, 1983). The 

transient water-table mounds generally have hydraulic 

heads greater than the hydraulic head of the surface 
14 CHAPTERl 

water. However, in the same areas of shallow water 

tables, plant roots can reach the capillary fringe above 

the water table and transpire water from the groundwater system. Consequently, the plants act as pumps, 

lowering the water table and causing cones of depression near the wetland perimeters (Meyboom, 1967; 

Winter and Rosenberry, 1995). 

Given the transient relations between the 

ground-water and surface-water hydraulic heads, a 

typical annual interaction between a wetland and 

contiguous ground water is as follows. When the 

water table near the wetland is higher than the wetland 

water level, it is usually because a given amount of 

ground-water recharge resulted in higher water levels 

than the equal amount of water added to the openwater surface. Wetland water levels are sometimes 

higher than ground-water levels for a brief time in the 

early spring when water from snowmelt and (or) 

spring rains on the uplands runs into the wetland over 

frozen soil. The runoff results in less water being 

available for ground-water recharge in the uplands and 

more water being available for focused recharge in the 

lowlands and for the wetland itself. When the soil 

thaws, the ground-water levels generally rise higher 

than the wetland water levels because of focused 

recharge. As the season progresses, evapotranspiration directly from the ground-water system by plants 

that have roots deep enough to reach the capillary 

fringe lowers the water table. The resulting gradient 

causes seepage from the wetland, but the seepage is 

transpired by plants and does not recharge the groundwater system. At the same time, ground water 

continues to move toward the wetland from higher 

parts of the ground-water flow system, but much of 

w 585 

~ 6 ....J 575 , ID W 

~ <{ ~ 565 16 

T1 P1 

/ Testhole and well 15 ::JCI)....J 

I- a: <{ 555 -+( 

that water also is transpired before it reaches the 

wetland. 

The highly variable climate conditions that 

occurred during the last 2 decades of the 20th century 

provided an opportunity to evaluate the effect of 

changing climate conditions on the generally expected 

relation between the wetlands and ground water in the 

Cottonwood Lake area. The following discussion 

focuses primarily on the interaction of ground water 

with Wetland P 1 because that wetland was selected for 

the most detailed study of water and chemical budgets 

and has the most wells within its watershed. 

The hydro graphs of water levels in upland wells 

within the watershed of Wetland P 1 were always 

higher than the wetland water level (fig. 1 0), 

indicating that ground-water gradients are toward the 

wetland. This relation held true for the relatively 

normal climate conditions during the first 9 years of 

the study as well as during the drought and deluge. 

Using data from only the upland wells would indicate 

that Wetland Pl is a ground-water discharge wetland 

under all climate conditions. However, the relation 

between the wetland water levels and the water levels 

in the lowland wells near the wetland was more 

complex. 

For most years, the water levels in the wells 

nearest the wetland were higher than the wetland 

water levels in spring and early summer and lower 

than the wetland water levels in late summer and fall 

(fig. 11 ). However, during the drought, the groundwater levels declined further each year to where spring 

recharge did not always raise the levels to the bottom 

of the wetland. In contrast, during the first 2 years of 

the deluge, the ground-water levels were higher than 

T3 T4 T5 T8 

~ww \ 

~ ~ cn 545 Direction of ground-water flow 

~ 535~----------------------------------------------~ VERTICAL EXAGGERATION X 5 

150 METERS 

Figure 9. Hydrologic section through the Cottonwood Lake area showing the relation of wetlands to the 

water-table profile. (Modified from Winter and Carr, 1980) 
CHAPTER! 15 

566 ....J 

w 

> w 

....J 

<( 

w 564 (/) 

w 

> 0 

a:l 

<( 

(/) 562 c: 

w 

1-

w 

~ 

z 

~ 560 

C'l 

:::::> 

1-

~ <( 

558 

CALENDAR YEARS 

Figure 10. Water level ofWetland Pl and water levels in selected ground-water wells in the uplands, 1979-98. 

the wetland water levels throughout the year. By 

1995, the wetland expanded to the vicinity ofwell25, 

and the wetland water levels and the water level in 

well 25 coincided in subsequent years. As the high 

water levels persisted, the seasonal reversals of flow 

became reestablished, as indicated by the hydrographs 

for the wetland and well 20 (fig. 11 ). These data 

indicate that reversals of flow between the wetland 

and ground water occur even when the wetland 

expands beyond the zones of wetland plants and the 

wetland perimeter is in direct contact with upland 

plants. 

Hydrographs of the water levels in Wetland P 1 

and of the water table in the vicinity of wells 20 and 25 

provide a time-series view of the relation between the 

wetland and only two of the wells near the wetland. 

To provide an areal view of the relation between the 

wetland and to the ground water in the wetland 

watershed, water-table maps were prepared for several 

dates to indicate the range of conditions that occurred 

during the study period. The configuration of the 

water table during a wet period is shown in figure 12. 

Ground-water levels in all wells were greater on that 

date than the wetland water level, indicating that 

ground water was discharging to the wetland around 

its entire perimeter. In contrast, during the drought, 

transpiration from ground water caused cones of 

depression to form around much of the wetland 

perimeter (fig. 13), and seepage from the wetland 

moved toward the depression. This seepage then was 

transpired along with the ground water that moved 

toward the wetland from higher in the flow system. 

The configuration of the water table in and around 

Wetland P1 has many variations between the two 

extremes. For example, cones of depression formed 

nearly every summer in some areas around the 

wetland perimeter, but the depressions commonly 

were not as continuous as shown in figure 13 and did 

not always form in the same place. The directions of 

flow, either toward the wetland or away from the 

wetland toward the cones of depression, are shown in 

figure 14 for the periods of high and low water levels 

for each year from 1981 to 1990. 
16 CHAPTER I 

....J 

w 

> w 

561 

....J 560 

<( 

w 

(/') I, 

w :: 6 '• 

r59 ~·~~,:~. w 558 Well25 ~ ~ ' ' 

: \ 

u] 

g 557 

1-

!S 

<( 

556 

555 

' ' : ' 

Weii20/ 

a> 0 I"'- 00 co ~ ~ ~ 

N (\') 

00 00 

~ ~ 

... 

... '~ 

... 1

' 1 '' 1 I 

1 1 1 I 

It I I, 

~ ~ : 

. ,·~,.' 1 I 1 I 

: : ~ : 1• I • I I '\ ~! :j 

..;t L!) co I"'- 00 00 00 00 

~ ~ ~ ~ 

00 a> 00 00 

~ ~ 

'\_~ ., 

·n·,..:i : 

I 0 o; N (\') ..;t L!) co I"'- 00 a> a> a> a> a> a> a> a> 

~ ~ ~ ~ ~ ~ ~ ~ ~ 

CALENDAR YEARS 

Figure 11. Water level of Wetland PI and water levels in selected ground-water wells in the lowlands near the 

wetland, 1979-98. 

To gain an increased understanding of the 

transient flow processes near the wetland perimeters, a 

number of water-table wells were installed along a 

transect from Wetland T3 to Wetland Pl. The wells 

were instrumented with continuous recorders to 

document the small-scale time and space effects of 

focused recharge and transpiration from ground water 

on the configuration of the water table and to assess 

the effect of those processes on the flow of ground 

water between the wetlands. Data were collected for a 

5-year period from 1989 tol993. As a result, data for 

4 of the 5 years were collected during the drought, data 

collection was terminated when the wells became 

flooded during the deluge that began in 1993. 

The data indicate that when Wetland T3 

contained water, seepage from the wetland moved 

through the ground-water system and discharged to 

Wetland Pl. During those few times that Wetland T3 

contained water over the four dry years of the 5-year 

period (fig. 7A), infiltration of precipitation resulted in 

the formation of transient water-table mounds between 

the two wetlands. The mounds interrupted the groundwater flow from Wetland T3 to Wetland P 1, but the 

mounds persisted for only a few days (Rosenberry and 

Winter, 1997). 

When Wetland T3 was dry, ground-water 

gradients commonly were away from Wetland Pl 

toward transpiration-induced cones of depression. 

However, infiltration of snowmelt and precipitation 

still caused the formation of transient water-table 

mounds, but they generally were below the bottom of 

Wetland T3. The magnitude and duration of the 

mounds depended on the timing and amount of the 

rainfall. Rainfall in spring or early summer usually 

created large water-table mounds that were located 

near the center of the upland between the two 

wetlands, and rainfall in late summer or early fall more 

often created small water-table mounds that were 

located adjacent to the lower wetland. On two 

occasions, the late-season water-table mounds formed 

adjacent to the lower wetland and then migrated away 

from the wetland as the height of the mounds 

dissipated. Although water-table mounds formed 

frequently following rainfall during the 5-year period 
CHAPTER I 17 

99"06' 

Base from U.S. Geological Survey 75 150 METERS 

CONTOUR INTERVAL APPROXIMATELY 3 METERS 

DATUM IS SEA LEVEL 

EXPLANATION 

- 59.3- Water-table contour-Number is in 

meters greater than 500 meters 

above sea level. Interval is variable 

~ Sernipennanentwedand 

~ Seasonal wedand 

+ Ground-water divide • 

21 Test hole and well 

Direction of ground-water flow 

Figure 12. Configuration of the water table in the Cottonwood Lake area, May 31, 1987, indicating that ground water 

was discharging to Wetland PI around its entire perimeter. (From Winter and Rosenberry, 1995) 

of data collection, two examples, one for early 

summer and one for late summer, are shown in figure 

15. The early-summer series (fig. 15, A-E) shows 

water-table configurations before and after rainfall of 

51 mm on June 14 and 30 mm on June 28-30, 1991. 

The late-summer series (fig. 15, F-J) shows watertable configurations before and after rainfall of 52 mm 

on August 18 and 43 mm on August 31-September 3, 

1989. Before the recharge event in both cases (fig. 15, 

A and F), a cone of depression was present adjacent to 

the wetland. Because the antecedent water table was 

higher in early summer than in late summer, the 

equivalent amount of rainfall resulted in higher and 

longer lasting mounds in early summer. For example, 

the early-summer mound was as much as 1.3 m higher 

than the water level of Wetland Pl (fig. 15B), and the 

mound persisted for about 12 days before a trough 

reformed near well 51 on June 28 (fig. 15C). The 

highest late-summer mound formed following the 

second period of late-summer rainfall and was 0.5 m 

higher than the wetland water level (fig. 151). That 

mound persisted for only about 6 days (Rosenberry 

and Winter, 1997). 

Data from the long-term records of wetland 

water levels and ground-water levels throughout the 

Cottonwood Lake area and from the shorter-term 

instrumented record of water levels between Wetlands 

T3 and Pl indicate that several generalizations can be 
18 CHAPTERl 

99"06' 

Base from U.S. Geological Survey 0 75 150 METERS 

CONTOUR INTERVAL APPROXIMATELY 3 METERS 

DATUM IS SEA LEVEL 

EXPLANATION 

- 59.3- Water-table contour-Number is in 

meters greater than 500 meters 

above sea level. Interval is variable 

~ Sernipennanentwetland 

QD Seasonal wetland 

+ Ground-water divide Test hole and well 

Direction of ground-water 

flow 

Figure 13. Configuration ofthe water table in the Cottonwood Lake area, August 31, 1989, showing the cones of 

depression caused by transpiration directly from ground water around much of the perimeter of Wetland P 1. (From 

Winter and Rosenberry, 1995) 

made with respect to the interaction of the wetlands 

with ground water: ( 1) Flow reversals between the 

wetlands and ground water are common; (2) during 

high-water conditions, ground-water gradients are 

toward the wetlands most of the time; and (3) during 

low-water conditions, ground-water gradients are 

away from the wetlands toward transpiration-induced 

cones of depression much of the time, particularly 

from midsummer through the fall. 

Data for the Cottonwood Lake area also provide 

insight into several other features of the geohydrologic 

setting of the wetlands. These features are: ( 1) the 

transient characteristics of the major ground-water 

divides separating the wetlands and the rates of 

ground-water flow from wetland to wetland, (2) the 

effect of highly permeable ground-water drains on 

ground-water flow near the wetlands, (3) the superposition of ground-water flow systems, and ( 4) the 

characteristics of the wetland soils. 

With respect to the first feature, a ground-water 

divide separates the ground-water basins of Wetlands 

P 1 and P8 most of the time. The ground-water basin 

ofWetland PI extends from the vicinity ofwell16 

(figs. 12 and 13) to Wetland T8 and is bounded to the 

north and south by water-table highs underlying the 

land-surface ridges contiguous to Wetland P 1. The 

only part of the divide that is occasionally lower than 

Wetland P 1, especially during drought conditions, is 

near wells 15 and 16. When the divide is not present, 

ground water can move from the basin of Wetland PI 
1981 

• 20 

16);),

26 25 

13 5-26 

17. •14 

1982 

• 20 

26 25 

• • 

,.·~" 

17.M. 1983 

20. 

16.·1;)26 .25 ·" 

5-10 

• 17. 14 

1984 

• 20 

26 25 

• • 

16•1:).13 

5-31 

• 17. 14 

1985 

• 20 

CHAPTERl 

16·fj26 25 • " 

11-2 

17. •14 

• 20 

26 25 

• • 

16. B2-15 .13 

• • 17 14 

• 20 

26 25 

• • 

16·D-19 .13 

• • 17 14 

• 20 

16 •• B2. .25 _,, 

9-23 

• 17. 14 

• 20 

EXPLANATION 

Direction of 

ground-water 

flow 

26 25 

16.·B· • 13 

4-9 

• 17. 14 

1986 

26 25 

16··o· .13 

8-14 

• 17. 14 

• • 20 20 

26 25 

16··B· • 13 

5-31 

• 17. 14 

1987 

26 25 

,.·~" 17.Q,. 

• 20 • 20 

26 25 

16. ·u· • 13 

17 14 

1988 

26 25 

• • 

,.·~" 17.v,. 

• 20 • 20 

16 •• B2. • 25 • , 

5-13 

• 17. 14 

1989 

16··£;,26 .25 _,, 

8-31 

• 1/ 14 

• • 20 20 

26 25 

• • 

26 25 

• • 

,.·~" 

17° ~. 1990 

16•1).

13 

10-2 

• 1/ 14 

• 20 20 • 

Figure 14. Direction of flow between Wetland PI and nearby ground-water wells for the periods of high and low water 

levels of each year from 1981 to 1990. (From Winter and Rosenberry, 1995) 

19 
20 CHAPTERl 

to that of Wetland P8 (fig. 16). However, the low 

hydraulic conductivity of the till allows only a small 

amount of ground water to transfer from one groundwater basin to the other. Using 0.016 for the hydraulic 

gradient of the water table between the two wetlands 

and an effective porosity of 0.2, the average linear 

pore-water velocity calculated for the movement of 

ground water between the two wetlands ranges from 

0.025 m yr-1 if the hydraulic conductivity is 

10-6 em s-1 to 2.5 m yr-1 if the hydraulic conductivity 

is 10-4 em s-1. These rates of ground-water flow 

through the till probably are typical of rates 

throughout the Cottonwood Lake area. 

With respect to the role of ground-water drains, 

a cone of depression is present between Wetlands T5 

and T8 and between Wetlands T4 and T5 (fig. 9) for 

all climate conditions. Theoretical numerical 

modeling studies of ground-water flow have indicated 

that zones of high permeability in the subsurface can 

cause increased downward gradients over the 

up gradient parts of the zones (Freeze and Witherspoon, 1967; Winter, 1976). For example, figure 17 

shows that increased hydraulic gradients toward the 

highly permeable zone on the side toward the groundwater divide cause flow to converge into the 

permeable zone. Conversely, on the side of the 

permeable zone toward the ground-water discharge 

area, gradients are the opposite and flow diverges 

away from the permeable zone. The upgradient side 

of the permeable zone can be thought of as a drain. 

The mechanism of enhanced flow toward a groundwater drain could be responsible for the persistent 

cone of depression between Wetlands T5 and T8 and 

Wetlands T4 and T5. If the subsurface sand deposit in 

the southwest part of the study area extends as a 

continuous deposit as far east as wells 1 and 24, as 

shown in figure 5, the deposit could cause the 

enhanced downward gradients that essentially would 

drain the ground-water system between the two 

wetlands. 

With respect to the superposition of groundwater flow systems, the Cottonwood Lake area 

provides a field example of the effect of superposition 

on the ecological characteristics of prairie pothole 

wetland complexes (See Swanson and others, Chapter 

4 of this report). The configuration of the water table 

in the area is highly irregular. As a result, some of the 

wetlands are enclosed by local ground-water flow 

systems where water cannot move from wetland to 

wetland by way of the ground-water system. 

However, other wetlands recharge ground water, and 

some of that water moves through the ground-water 

system and discharges to wetlands at a lower altitude. 

For example, because of the known ground-water 

divides between some of the wetlands, it is unrealistic 

to view the ground-water flow field from Wetland T8 

to Wetland P11 as a continuous flow system where 

water flows from one wetland to the next lower 

wetland through the ground-water system. A more 

realistic flow field would show that ground water 

discharging to Wetland P 11 would be receiving ground 

water that was recharged within its own local flow 

system as well as from ground water that was 

recharged in the highest part of the Cottonwood Lake 

area. Some of the ground water recharged in the 

higher part of the Cottonwood Lake area would flow 

as an intermediate ground-water flow system, passing 

at depth beneath the local flow systems associated 

with Wetlands P1, P8, P10, and several unnamed 

wetlands before discharging into Wetland P 11 

(fig. 18). 

With respect to wetland soils, Arndt and 

Richardson (1993) indicated that evapotranspiration 

from ground water is a key part of the process that 

concentrates salts in the subsurface around wetland 

perimeters. Stolte and others (1992) also discussed the 

role of evapotranspiration from ground water as part 

of the process that concentrated salts in soils around 

the perimeter of a ground-water-recharge wetland in 

Manitoba. Evapotranspiration from ground water 

results in wetland water and solutes being drawn into 

the subsurface during dry times of the year. The 

annual flushing of the subsurface in the spring 

apparently does not completely dissolve and remove 

the salts. The process that concentrates salts in the 

subsurface around wetland perimeters, repeated over 

thousands of years, has resulted in the presence of 

large amounts of gypsum and calcite crystals along 

parts of the perimeter of Wetland P1 (Swanson, 1990; 

Arndt and Richardson, 1993 ). The storage of salts in 

the subsurface is probably the reason that Wetland P1 

is not more saline (LaBaugh and Swanson, Chapter 3 

of this report). As indicated earlier, the ground-water 

basin ofWetland P1 is a closed local flow system most 

of the time, and except for the outseepage caused by 

evapotranspiration, the basin would have outseepage 

only during the times that the ground-water divide 

near well 16 is absent. As indicated earlier, the loss of 

water and solutes from the basin of Wetland P1 to 

Wetland P8 by way of ground water is minimal and 

could not account for a large loss of solutes from 

Wetland Pl. 
CHAPTER I 

p,_.-Wetland 64 50 49 48 47 T3 

A we/ 51 Wetland1 

560 

559 

559 

558 

....I 

w 557 > w 

....I 

<( 

w c en 560 w 

6 

co 

<( 559 

en 

c:: 

LU 

1- 558 w 

::2E 

~ 557 

ui 

0 

~ 

1-

6 

<( 

559 

558 

557 

560 

559 

558 

557 

0 25 50 75 100 125 150 

METERS 

P1 

F 

H 

J 

0 25 

50 49 48 47 

51 

T3 

Sept 4, 1989 

50 75 100 125 150 

METERS 

Figure 15. Hydrologic sections showing water-table profiles between Wetlands T3 and Pl 

indicating the formation of water-table mounds in response to early-summer rainfall (A-E) and in 

response to late-summer rainfall (F-J). (From Rosenberry and Winter, 1997) 

21 
22 CHAPTER! 

99"06' 

75 150 METERS 

CONTOUR INTERVAL APPROXIMATELY 3 METERS 

DATUM IS SEA LEVEL 

EXPLANATION 

- 59.3- Water-table contour-Number is in 

meters greater than 500 meters 

above sea level. Interval is variable 

\C) Semipermanent wetland 

QD Seasonal wetland 

Direction of ground-water 

flow 

Test hole and well 

Figure 16. Configuration of the water table in the Cottonwood Lake area, May 25, 1990, indicating one of the few 

times during the study when a ground-water divide was not present between Wetlands PI and P8. 

Mapping of the wetland soils in the Cottonwood 

Lake area also has provided useful information for the 

general delineation of wetlands. In mapping the soils 

throughout the watershed of Wetland P1, James Arndt 

(oral and written commun., 1993) found wetland soils 

on the hillsides that were about 2 m above the level of 

Wetland P1 prior to the deluge that began in 1993. 

The outer limit of the calciaquoll wetland soils 

(fig. 19) is similar to the present area of open water in 

Wetland P1 (fig. 8). The area of open water in the 

wetland is at least as extensive at present (2002) as it 

was 150 years, and perhaps 500, years ago (Winter and 

Rosenberry, 1998). Thus, the soils indicated that 

Wetland P 1 had been as large in the past as it is at 

present, and probably was that large many times in the 

past. This information indicates that mapping of 

wetland soils is perhaps the most useful way to 

delineate wetlands because the soils integrate highly 

variable climate and hydrologic conditions over many 

millenia. 
A 

590 

580 

CJ) 

a:....J 570 ww f-> Ww 560 

:::ii:....J 

z<l: 550 

-W uJCJ) 540 ow ::J> 530 f--0 -CO 

~<I: 520 

<1: 

510 

500 

CHAPTER I 

Scale is arbitrary 

Line of 

equal 

hydraulic 

head 

water 

flow line 

Figure 17. Numerical simulation of ground-water flow in the vicinity of 

a highly conductive aquifer of limited extent within a porous media 

having lower hydraulic conductivity. (Modified from Winter, 1976) 

u P10 u PB T1 P1 T3 T5 TB T9 

'----""'C""/ ).~ /1 

""',, ', 

.............. ..... 

Y) I 

/ I 

/ 

/ /// / 

/' 

A' 

1 KILOMETER VERTICAL EXAGGERATION X 20 

EXPLANATION 

-+-- Based on field data } Direction of ground-water flow 

---<E - - Inferred 

Figure 18. Hydrologic section showing intermediate ground-water flow systems passing at depth beneath local groundwater flow systems associated with selected wetlands. Note the local and intermediate flow systems discharging into 

Wetland PI I. The intermediate flow system is also shown in figure 2 where ground water moves from the Cottonwood Lake 

area to Eric Lake. Wetland names are shown along top of figure . U indicates unnamed. Line of section shown in figure 4. 

23 
24 CHAPTER! 

Soils in the Wetland P1 Area 

0 150 METERS 

EXPLANATION 

USDA Soil Classification 

Argiaquolls and 

Argialbolls 

Endoaquolls 

Calciaquolls 

Figure 19. Generalized distribution of soils in the Cottonwood 

Lake area. (Map courtesy of Dr. James Arndt, then ofNorth 

Dakota State University) 

REFERENCES 

Arndt, J.L., and Richardson, J.L., 1993, Temporal variations 

in the salinity of shallow ground water from the 

periphery of some North Dakota wetlands (USA): 

Journal ofHydrology, v. 141, p. 75-105. 

Carlson, C.G., 1969, Bedrock geologic map ofNorth 

Dakota: North Dakota Geological Survey 

Miscellaneous Map 10. 

Freeze, R.A., and Witherspoon, P.A., 1967, Theoretical 

analysis of regional ground water flow. 2, The effect of 

water-table configuration and subsurface permeability 

variation: Water Resources Research, v. 3, p. 623-634. 

Grisak, G.E., and Cherry, J.A., 1975, Hydrologic characteristics and response of fractured till and clay confining a 

shallow aquifer: Canadian Geotechnical Journal, v. 12, 

p. 23-43. 

Gunaji, N.N., 1968, Evaporation investigations at Elephant 

Butte Reservoir in New Mexico: International Association of Scientific Hydrology Publication No. 78, 

p. 308-325. 

Harbeck, G.E., Jr., and others, 1958, Water-loss 

investigations. Lake Mead studies: U.S. Geological 

Survey Professional Paper 298, 100 p. 

Hendry, M .J ., 1988, Hydrogeology of clay till in a prairie 

region of Canada: Ground Water, v. 26, p. 607-614. 

Keller, C.K., Vander Kamp, G., and Cherry, J.A., 1986, 

Fracture permeability and ground water flow in clayey 

till near Saskatoon, Saskatchewan: Canadian 

Geotechnical Journal, v. 23, p. 229-240. 

Meyboom, P., 1967, Groundwater studies in the Assiniboine 

River drainage basin, Part II, Hydrologic characteristics of phreatophytic vegetation in south-central 

Saskatchewan: Geological Survey of Canada 

Bulletin 139, 64 p. 

Meyboom, P., van Everdingen, R.O., and Freeze, R.A., 

1966, Patterns of ground water flow in seven discharge 

areas in Saskatchewan and Manitoba: Canadian 

Geological Survey Bulletin 147, 57 p. 

Parkhurst, R.S., Winter, T.C., Rosenberry, D.O., and 

Sturrock, A.M., 1998, Evaporation from a small prairie 

wetland in the Cottonwood Lake area, North Dakotaan energy budget study: Wetlands, v. 18, no. 2, 

p. 272-287. 

Rosenberry, D.O., and Winter, T.C., 1997, Dynamics of 

water-table fluctuations in an upland between two 

prairie-pothole wetlands in North Dakota: Journal of 

Hydrology, v. 191, p. 266-289. 

Stolte, W.J., Barbour, S.L., and Eilers, R.J., 1992, A simulation of the mechanisms influencing salinity development around prairie sloughs, In Robards, R.D., and 

Bothwell, M.L., eds., Aquatic Ecosystems in SemiArid Regions, Implications for Resource Management: 

The National Hydrology Research Institute 

Symposium Series No.7, Environment Canada, 

Saskatoon, Saskatchewan, Canada, p. 165-177. 

Swanson, K.D., 1990, Chemical evolution of ground water 

in clay till in a prairie wetlands setting in the Cottonwood Lake area, Stutsman County, North Dakota: MS 

Thesis, University of Wisconsin, Madison, 229 p. 

Winter, T.C., 1975, Delineation of buried glacial-drift 

aquifers: U.S. Geological Survey Journal of Research, 

v. 3, no. 2, p. 137-148. 

Winter, T.C., 1976, Numerical simulation analysis ofthe 

interaction oflakes and ground water: U.S. Geological 

Survey Professional Paper 1001,45 p. 

Winter, T.C., I 983, The interaction oflakes with variably 

saturated porous media: Water Resources Research, 

v.l9,no.5,p.l203-1218. 

Winter, T.C., and Carr, M.R., 1980, Hydrologic setting of 

wetlands in the Cottonwood Lake area, Stutsman 

County, North Dakota: U.S. Geological Survey WaterResources Investigations Report 80-99, 42 p. 

Winter, T.C., and Rosenberry, D.O., 1995, The interaction 

of ground water with prairie pothole wetlands in the 

Cottonwood Lake area, east-central North Dakota, 

1979-1990: Wetlands,v.l5,no.3,p.l93-211. 

__ 1998, Hydrology of prairie pothole wetlands during 

drought and deluge: A 17-year study of the Cottonwood Lake wetland complex in North Dakota in the 

perspective of longer term measured and proxy 

hydrological records: Climatic Change, v. 40, no. 2, 

p. 189-209. 
CHAPTER2 

CLIMATE OF THE COTTONWOOD LAKE AREA 

Donald 0. Rosenberry, U.S. Geological Survey, Denver, Colorado 

ABSTRACT 

The climate of central North Dakota is 

continental, with typically long, cold, dry winters and 

short, mild, variably wet summers. The average 

annual temperature is 4 °C, and average annual precipitation is 440 mm. Central North Dakota receives an 

average annual 59 percent of possible sunshine. 

Annual windspeed averages 4.5 m s-1

, and prevailing 

winds are from the northwest. The number of daylight 

hours in a 24-hour period ranges from less than 9 

hours in December to more than 16 hours in June. 

This large range contributes to the large seasonal 

temperature changes; extreme temperatures range 

from -47 to 48°C. Normally, 17 days each year have 

temperatures that exceed 32°C, 56 days each year have 

temperatures below -18°C, and 195 days have temperatures below 0°C. November through February are the 

driest months, and average precipitation during those 

months ranges from 12 to 14 mm per month. Average 

annual snowfall is 865 mm, and average maximum 

snow depth is 300 mm. Snow normally covers the 

area from December 4 to March 24. June is normally 

the wettest month of the year and has an average 91 

mm of rainfall. Thunderstorms produce most of the 

rainfall during summer. Average annual lake evaporation is 81 0 mm per year, almost twice the average 

annual precipitation. Spring runoff, overland flow, 

and ground-water seepage make up the difference 

between evaporation and precipitation. Wetland water 

levels respond markedly to seasonal and annual 

climatic perturbations. 

Conditions at the Cottonwood Lake area 

were warmer and drier than normal for the first 14 

years of the study period and were especially dry 

during a drought that lasted from 1988 through 

1992. The cumulative precipitation deficit, 

relative to normal, during the 14-year period was 

635 mm. Conditions were much wetter than 

normal from 1993 through 1998; during that time, 

the cumulative precipitation excess, relative to 

normal, was 885 mm. This wet period was the 

wettest of the century for central North Dakota. 

INTRODUCTION 

The hydrology, geochemistry, and biology of the 

Cottonwood Lake area are being studied to improve 

the understanding of wetland processes. Knowledge 

of the climate of the Cottonwood Lake area is 

paramount to understanding the hydrology, which 

drives the geochemical and biological processes. The 

climate of the area is continental and is characterized 

by large temperature changes over daily and annual 

time scales (Jensen, 1972). The area receives light to 

moderate precipitation and nearly continuous wind. 

Air masses arrive from polar, Pacific, and Gulf of 

Mexico sources. The Pacific air masses typically are 

moderate and pleasant, but the Gulf of Mexico and 

polar air masses generally bring extreme climatic 

conditions to the area. 

Annual evaporative losses in the area are almost 

twice as large as precipitation amounts (Eisenlohr and 

others, 1972). For many of the prairie wetlands, 

surface runoff and ground-water seepage are the only 

hydrologic fluxes available to make up the difference 

between evaporation and precipitation. Therefore, 

many wetlands in the area dry up during prolonged 

periods of drought when evaporative and transpirative 

losses are not compensated for by precipitation, 

runoff, and ground-water inflow. 

Information on the normal climatic conditions 

for central North Dakota is presented in the first part 

25 
26 CHAPTER2 

of this chapter. This information is followed by a 

more detailed description of climatic conditions for 

the Cottonwood Lake area based on data collected at 

the site since 1979. 

METHODS 

Temperature and precipitation data from three 

National Weather Service stations near the Cottonwood Lake area were used to represent monthly and 

annual normal conditions for the study area. Normals 

were calculated for the 30-year period from 1961 to 

1990 using data collected at the Carrington, 

Jamestown airport, and Pettibone stations (fig. 1 ). 

These stations are located 39 km north, 39 km 

southeast, and 32 km west of the Cottonwood Lake 

area. Weighted averages were determined to represent 

normal conditions for the area. Other climatic characteristics of the area were summarized from data 

collected at National Weather Service stations at 

Bismarck, Devils Lake, and Fargo (Jensen, 1972). 

Climate data have been collected during openwater periods from a raft or platform in the center of 

Wetland Pl and at a land station adjacent to the 

wetland since 1979. Climate parameters that were 

monitored for all of the period from 1979 to 1998 

include air temperature, relative humidity, windspeed, 

and precipitation. Incident shortwave solar radiation 

and incident longwave atmospheric radiation were 

monitored for part of the period. Wetland surfacewater temperature was monitored for the entire period. 

Bottom-water temperature and sediment temperature 

at 0.5- and 1.0-m depths below the sediment-water 

interface were monitored for part of the period. Since 

1981, the climate data have been recorded by a digital 

data logger and exist as hourly and daily averages or 

totals of scans made at 1-minute intervals. During the 

open-water periods of 1979 and 1980, temperature, 

humidity, and windspeed data were collected by 

analog instruments. Rainfall during the open-water 

periods was monitored with a standard 8-in National 

Weather Service gage, monitored weekly, and an 

8-in.-diameter tipping-bucket gage with 0.01 inch 

resolution. The tipping-bucket gage was monitored 

continuously during open-water periods by a landbased data recorder. Evaporation during 1982 to 1987 

was determined using the energy-budget method 

(Parkhurst and others, 1998). 

REGIONAL CLIMATE OF CENTRAL 

NORTH DAKOTA 

The climate of central North Dakota, is 

continental, with typically long, cold, dry winters and 

short, mild, variably wet summers. The average 

annual temperature is 4 °C and average annual precipitation is 440 mm. Large seasonal differences in the 

climate are related to the number of daylight hours in a 

24-hour period. This number ranges from less than 

9 hours in December to more than 16 hours in June. 

The large range in the number of daylight hours 

contributes to the large seasonal temperature changes. 

Extreme temperatures in central North Dakota range 

from -4 7 to 48°C and are nearly as great as the 

statewide extremes of -51 and 49°C. The freeze-free 

period normally lasts 119 days from May 20 to 

September 15. 

Winter is typically cold and dry; normally, 

56 days each year have temperatures below -l8°C, 

and 195 days have temperatures below 0°C. January 

is the coldest month and has an average temperature of 

-13.5°C (fig. 20). The diurnal temperature range is 

smallest from November through March and averages 

ll.5°C. Based on data from the Jamestown airport 

station, soil frost normally reaches a depth of 1.4 m 

although depths as great as 1.8 m have been recorded. 

November through February are the driest months; 

average precipitation during those months ranges from 

12 to 14 mm per month (fig. 20). Average annual 

snowfall is 865 mm, the average maximum snow 

depth is 300 mm, and an average snowpack of at least 

25 mm covers central North Dakota from December 4 

to March 24. On average, two blizzards, usually 

originating in either Alberta, Canada or Colorado, 

occur in North Dakota each year. Blizzards that 

originate in Alberta generally pose a greater hazard 

than those that originate in Colorado because they tend 

to be faster moving and often approach with little 

advance warning. 

A maximum mean monthly temperature of 

20.5°C occurs in July, but August is nearly as warm 

(fig. 20). During a normal year, 17 days have 

temperatures that exceed 32°C. The chance of 

extended warm periods is greatest from July 19 to 

August 1. The diurnal temperature range is greatest 

during summer, and a maximum average monthly 

difference of 16.5°C is reached in August. June is 

normally the wettest month of the year and has an 
CHAPTER2 27 

average of 91 mm of rainfall. Most summer rainfall is 

generated by thunderstorms. From April to October, 

an average of 30 thunderstorms can be expected; and, 

during June through August, 7 to 8 thunderstorms 

occur each month. 

CJ) 

a: 100 30 w 

1- 90 25 w 

~ 80 20 CJ) 

::J .:J ....1 70 15 ~~ ~ 60 10 :Jw 

1-U ~ 50 5 <(CJ) 

z a:w Ww 

0 40 0 a.. a: 

i= 30 ~(!) 

~ -5 ww 

t-o 

a: 20 -10 ~ u 

w 10 -15 a: 

a.. 0 -20 z ID a: a: ~ z ....1 (!) a.. 1- > u <( w <( a.. :J :J :J w u 0 w ...., LL. ~ <( ~ ...., ...., <( CJ) 0 z 0 

Figure 20. Normal (1961-90) monthly precipitation and 

temperature for the Cottonwood Lake area based on data from 

three nearby National Weather Service stations. 

On average, 72 days each year have measurable 

precipitation. Most precipitation events are light; 

48 percent result in 3 mm of precipitation or less. 

Only 4 percent of the precipitation events result in 

more than 25 mm of precipitation during a 24-hour 

period. The chance for precipitation is greatest from 

6 to 9 a.m., the chance reaches 30 percent at 8 a.m. 

during the winter and 13 percent at 6 a.m. during the 

summer. The chance for precipitation is smallest 

during midaftemoon in the winter and during midday 

in the summer. The chance for precipitation is 

uniformly small during the fall, and averages about 

12 percent throughout the day. 

Relative humidity is generally highest near 

6 a.m. and lowest during midaftemoon and has a much 

greater diurnal range in the summer than in the winter. 

Mean monthly maximum values range from 

71 percent in January to 86 percent in July. Mean 

monthly midaftemoon minimum values range from 

67 percent in January to about 50 percent in July. 

Typically, daily-average relative humidity is lowest 

during midspring and early fall. 

On an average annual basis, central North 

Dakota receives 59 percent of total possible sunshine. 

This value is a ratio of incoming solar radiation 

measured on the ground and the amount of solar 

radiation that the earth would receive at that latitude if 

there were no clouds. Based on data from the 

Bismarck and Fargo stations, July is the sunniest 

month and receives an average of74 percent of 

possible sunshine. November is the cloudiest month 

and receives an average of only 43 percent of possible 

sunshine. Central North Dakota averages about 210 

clear or partly cloudy days each year, monthly totals 

range from an average of 24 days during July to about 

14 days during November through February. 

Wind is one of the more noticeable features of 

the climate of central North Dakota. Wind prevails 

from the northwest during all seasons at an average 

speed of 4.5 m s-1. Average monthly windspeed is 

greatest during late winter through early spring and 

reaches a maximum of 5.4 m s-1 in April. The July 

monthly windspeed averages 4.0 m s-1. Monthly 

maximum windspeeds are normally greatest during 

July and August and usually accompany thunderstorm 

activity. However, monthly maximum windspeeds 

associated with winter cold fronts often are large also. 

Whereas precipitation generally decreases from 

east to west across central North Dakota, evaporation 

increases from northeast to southwest. Average 

annual lake evaporation is 810 mm near the study area 

and ranges from 7 60 mm 80 km northeast of the study 

area to 860 mm 80 km southwest of the study area 

(Eisenlohr and others, 1972). 

LOCAL CLIMATE OF THE 

COTTONWOOD LAKE AREA 

Many characteristics of climate are highly 

variable spatially, and areally averaged data often do 

not provide accurate information for a specific site. 

Also, few wetlands are instrumented with climate 

stations, and the climate over a wetland can be 

different from the climate over an area where wetlands 

are not present. Therefore, to obtain accurate, sitespecific data for estimating evaporation using the 

energy-budget method, climate data have been 

collected for the Cottonwood Lake area since 1979 

(Sturrock and others, 1986; 1987; Parkhurst and 

others, 1994, 1998). These data also allow a more 

site-specific characterization of the climate over a 

wetland. For example, monthly average air temperatures calculated as weighted means of data from the 

Carrington, Jamestown airport, and Pettibone stations 

were compared to monthly average air temperatures 

measured at the study area for June, July and August 

1982-85 (fig. 21). The average ofthe differences 
28 CHAPTER2 

between the National Weather Service data and the 

study area data for the 12 months was only 0.1 °C, but 

the differences for the individual months ranged from 

0.3°C during June 1982 to 2.7°C during August 1985. 

These differences can cause error when network data 

are used to estimate evaporation for a specific site. 

Also, most network climate stations provide only air 

temperature and precipitation data, and daily-average 

air temperature data often are determined by averaging 

the daily maximum and minimum values. However, 

for the Cottonwood Lake area, data were collected 

every minute, and 1 ,440 values were averaged each 

day to provide a more accurate daily-average air 

temperature. 

22t---- ----------- --------·:...·· --•----- ------- ----·--------- ----- ---

20 I-------~- -- -·-------. - ~-------------- _ .. -- .. :~---. ----- .& -------

• • • • 

181------------,_., __________ ---------------- --------- ------------------

~ 161------------- -----~----- - --------~----------------------------~­ ~ . . . ~ 141--·---------- ---------------------------------------- ..._ _______ ------

0:: 

~ 121----------------------------------------------- ------------------------

~ 

~ 10 I I I 

z ....1 (!) 

:::> :::> :::> """") """") <( 

z ....1 (!) 

:::> :::> :::> """") """") <( 

z ....1 (!) 

:::> :::> :::> """") """") <( 

z ....1 (!) 

:::> :::> :::> """") """") <( 

1982 1983 1984 1985 

EXPLANATION 

A Site data 

• National Weather Service data 

Figure 21. Summer monthly average air temperatures 

measured at the Cottonwood Lake area and monthly average 

air temperatures calculated as weighted means of data from 

the Carrington, Jamestown airport, and Pettibone National 

Weather Service stations, 1982-85. 

Daily maximum and minimum values for many 

of the climate variables, and the times that the values 

occurred, were collected for the study area. The 

monthly mean hours of occurrence for the maximum 

and minimum air temperatures are shown in figure 22. 

These monthly means do not include days when the 

maximum occurred during nighttime hours or when 

the minimum occurred between noon and midnight; 

the means represent typical days, when the maximum 

occurs sometime during the day and the minimum 

occurs sometime during the night or morning. The 

monthly mean hour of occurrence for the maximum 

air temperature was about 4 p.m. The maximum 

usually occurred between 2 and 6 p.m., and the time 

was fairly consistent throughout the summer. The 

hour of occurrence for the maximum air temperature 

varied the most during May, and occurred earliest 

during May and latest during July. The monthly mean 

hour of occurrence for the minimum air temperature 

was about 4:15a.m., and the minimum usually 

occurred between 2 and 6:30a.m. The hour of 

occurrence for the minimum air temperature varied 

little between May and September, and the minimum 

occurred earliest during July and latest during May. 

12 -----------------------------------------------·--------.--.-------- CI: 

::::> 10 -... -- ------.------- .. ------------------------------.. -... ----.-.-- 0 

:I: 8 ------ -·-- ----------------------- .... -- .... -.---.-------------- ------

6 

4 ----- HlH ------- -- l ---- ---Hl - - - -- --- - --T - --- t· - - --

2 --------- - -- -- - ------ --- ---- - ---- -------- ----- - --- Minimum 

MAY JUN JUL AUG SEP 

Figure 22. Monthly mean hours of occurrence for daily 

maximum and minimum air temperatures, and one standard 

deviation from the monthly mean, for the Cottonwood Lake 

area, 1982-85. 

Incident solar and atmospheric radiation for the 

Cottonwood Lake area were monitored from 1982 to 

1987. Longwave atmospheric radiation varies 

primarily with air temperature, but shortwave solar 

radiation varies primarily due to changes in cloud 

cover. The daily total incident shortwave solar 

radiation for the study area and the theoretical 

maximum total value assuming a clear sky are shown 

in figure 23. The shortwave solar radiation had 

considerable variability from year to year. For 

example, the springs of 1984 and 1986 appear to have 

been relatively cloud free based on the high values of 

solar radiation relative to the maximum possible 

values. The summers and falls of 1982 and 1983 

appear to have been more cloudy than the other years. 

The monthly average percent of clear-sky shortwave 

solar radiation for 1982 to 1987 indicates that 

radiation was greatest during May through July; the 

monthly average ranged from 68 percent in July, to 

slightly less than 50 percent during October and 
CHAPTER2 29 

November (fig. 24). This range in average monthly 

solar radiation is somewhat less than the range based 

on data from the Bismarck and Fargo stations that was 

previously reported in this chapter. 

Windspeed also was highly variable on a daily, 

seasonal, and annual basis and also varied spatially. 

During 1982 to 1991, the annual average windspeed at 

Wetland P 1, measured during May to October each 

year, varied from 2.3 m s-1 during 1991 to 3.6 m s-1 

during 1986. Monthly average values during the same 

period indicate that the windiest conditions occurred 

during May and the calmest conditions occurred 

during August (fig. 25). 

A focus on the climatic conditions over a 

wetland is essential to understanding the variability of 

the temperature of the water in the wetland. 

Therefore, to obtain water temperature data, thermistor 

thermometers were installed at the center of Wetland 

P 1 at the wetland water surface and at the wetland 

water bottom. In addition, thermistor thermometers 

900 

800 

>- <( 

Cl 

a: 700 w 

c... 

a: 

w 

1- 600 w 

~ 

i= 

z 500 UJ 

u 

w 

a: 

<( 400 :J 

d 

(f) 

a: 

w 300 ll.. 

(f) 

w 

a: 200 0 

....J 

<( 

u 

100 

0 

1982 1983 1984 

were installed in the wetland sediment at 0.5 and 1.0 m 

below the sediment-water interface. The water depth 

of Wetland PI ranged from 0.3 to 1.2 m during the 

period for which the temperature data were collected. 

The data indicate that during the open-water season 

the average temperature of the entire water column 

was greater than the average air temperature (fig. 26). 

Monthly average surface-water temperatures ranged 

from 16.6°C during September to 23. 7°C during July. 

Monthly average air temperatures were 2.6°C 

(August) to 4.6°C (May) lower than the surface-water 

temperatures, and monthly average bottom-water 

temperatures at were 1.4 °C (June) to 3.1 oc (May) 

lower than the surface-water temperatures. Wetland 

water temperatures peaked during July, but the much 

colder sediment temperatures peaked during August 

and September. Wetland sediments warmed at an 

average rate of slightly less than 0.1 oc day-1 

throughout the spring and summer. 

1985 1986 1987 

EXPLANATION 

• Measured 

-Clear-Sky 

Figure 23. Measured daily total incident shortwave solar radiation at the Cottonwood Lake area from 1982 to 

1987 and clear-sky shortwave solar radiation at 47 degrees north latitude. 
30 CHAPTER2 

Air temperature and precipitation are the most 

commonly used parameters for characterizing the 

climate of an area, and onsite collection of these data 

provides information about variations in the climate 

relative to conditions normal for the area. The 

cumulative departures from normal monthly average 

values for both air temperature and precipitation from 

1978 to 1998 for the Cottonwood Lake area are shown 

in figure 27. The plots were created by calculating 

monthly averages, determining the departures from 

normal for each monthly average, and then 

determining the running sum of the departures from 

the normal monthly averages. Onsite data were used 

when available; otherwise, the temperature data were 

the weighted averages for the Carrington, Jamestown 

airport and Pettibone stations; precipitation data were 

from the Woodworth station. Normal monthly values 

are those shown in figure 20. In figure 27, segments 

of the curves that have a positive slope represent 

periods of above-normal temperature or precipitation, 

and segments of the curves that have a negative slope 

represent periods of below-normal temperature or 

precipitation. If conditions averaged out to be normal 

for the study period, the lines would end near their 

respective zero points by the end of 1998. The figure 

indicates that, for the first 14 years of the study, 

conditions were warmer and drier than normal, and, 

for the last 6 years of the study, conditions were 

slightly cooler and much wetter than normal. 

z 

0 

~ 100 

<( 90 0 

<( 80 c:: 

>- 70 ~ 

en 60 

0:. 

<( 50 w 

....J 40 

u 

LL 30 

0 20 1-

z 

w 10 

u 

a: 0 LlJ MAY JUN JUL AUG SEP OCT NOV a.. 

Figure 24. Monthly average percent of clear-sky shortwave 

solar radiation for the Cottonwood Lake area, 1982-87. 

The air temperature averaged out to be about 

normal for the first 9 years shown in figure 27. From 

1986 through 1992 the air temperature was much 

greater than normal (with the exception of 1989 when 

the air temperature was about normal for the year). 

Much of the 6-year period of above-normal air 

temperatures coincided with a severe drought that 

lasted from 1988 through 1992. From 1993 through 

1998, the air temperature was below normal for 

3 years. 

5~--~----~----~----.-----.----, 

0 

z 

8 4 w 

en 

c:: 

LlJ 

a.. 

en 

ffi 3 

1-

w 

~ 

MAY 

Standard deviation 

JUN JUL AUG SEP OCT 

Figure 25. Monthly average windspeed, and one standard 

deviation from the monthly average, for the Cottonwood Lake 

area, 1982-91. 

Precipitation was below normal for 9 of the first 

15 years shown in figure 27; the cumulative precipitation deficit, relative to normal, was 635 mm for the 

15-year period. Precipitation was above normal for 

only 3 of those 15 years. During an extended drought 

that began in 1988 and lasted through 1992, precipitation was less than normal by 575 mm, an amount 

1.3 times the normal total annual precipitation for the 

Cottonwood Lake area. For 1993 through 1998, the 

remaining 6 years of the 21-year period shown in 

figure 27, precipitation was much greater than normal. 

The cumulative precipitation excess, relative to 

normal, was 885 mm during the extremely wet period. 

The severe drought that occurred from 1988 

through 1992 had a great impact on the wetland water 

levels in the Cottonwood Lake area. The water level 

of Wetland P 1 fluctuated slightly less than 1 m during 

the first 10 years shown in figure 28, although precipitation was below normal during most of that period. 

However, the wetland dried up during every year of 

the 5-year drought and dried up earlier each year as the 

drought persisted. The extremely wet period that 

began in 1993 had an even greater impact on the water 

level of Wetland P 1. During the 6 years of greaterthan-normal precipitation from 1993 through 1998, the 

wetland water level rose to more than 1.5 m above the 

highest water level recorded during the first 15 years 

of the study, and the wetland went from being dry 

early in 1993 to a maximum depth of2.7 min 1997. 
CHAPTER2 31 

The Palmer Drought Severity Index (PDSI) is 

an indicator of periods of above- or below-normal 

moisture available in the soil for crop production 

(Palmer, 1965). The PDSI can be used to put into 

perspective the recent drought and wet period in the 

Cottonwood Lake area relative to a long-term record 

that extends back to 1895. The PDSI is an indicator of 

meteorologic drought. Some have disputed its use as 

an indicator of hydrologic drought (Alley, 1985) and 

some have disputed the spatial comparability of its 

definitions of severe and extreme drought (Guttman 

and others, 1992). Nevertheless, the PDSI is 

appropriate for showing the severity and duration of 

wet or dry meteorologic conditions, such as the warm 

and dry conditions in the Cottonwood Lake area from 

1988 through 1992 and the extremely wet conditions 

from 1993 through 1998, relative to normal conditions 

over periods of months to years. 

EXPLANATION 

----- Air 

--+- Surface water 

---.- Bottom water 

--+- Sediment at 0.5 meter 

___.,__ Sediment at 1.0 meter 

Figure 26. Monthly average temperatures of air, surface 

water, bottom water, and sediment at 0.5 meter and 1.0 meter 

below the sediment-water interface at the center of Wetland 

PI, 1982-85. 

The PDSI data for central North Dakota for 

1900 to 1998 are shown in figure 29. Values above 

zero indicate wet periods, and values below zero 

indicate dry periods. The greater the value, the greater 

the degree of wetness or dryness. For example values 

of-3 to --4 indicate severe drought, and values of less 

than --4 indicate extreme drought. According to the 

PDSI data, central North Dakota experienced a 

drought from late 1988 to early 1993 that was matched 

in severity only during the late-1930s. Periods of 

drought also occurred more frequently during the 

latter years shown in figure 29, leading to a hypothesis 

that the long-term climate of central North Dakota 

may be becoming drier with time. Todhunter (1993) 

presented data from James town that support this 

hypothesis. Todhunter used a simple linear regression 

model to show that the air temperature at Jamestown is 

increasing at a rate of 1.3 oc per century and that 

precipitation is decreasing at a rate of 1. 7 mm per 

century. The extremely wet period from 1993 through 

1998 was the wettest of the century, according to the 

PDSI data (figure 29). Winter and Rosenberry (1998) 

cite data from Devils Lake that indicate the wet period 

was the wettest in the past 130 years and perhaps the 

wettest in the past 500 years. 

COMPARISON OF EVAPORATION AND 

PRECIPITATION AT THE COTTONWOOD 

LAKE AREA 

On average, evaporation at the Cottonwood 

Lake area is slightly greater than 800 mm/yr, or about 

1. 8 times the average annual precipitation (Eisenlohr 

and others, 1972). During a normal year, precipitation, runoff from snowmelt in the spring, overland 

flow after high-intensity rainfall events, and groundwater seepage combine to offset evaporative losses. 

During years when precipitation is below normal, or 

when below-normal snowfall results in diminished 

snowmelt, water levels can decline substantially, even 

to the point of wetlands drying up. During the warm 

and dry period from 1988 through 1992 Wetland P1 

dried up during mid to late summer every year. 

Monthly total evaporation, determined by the 

energy-budget method, and precipitation for Wetland 

P1 are shown in figure 30. Problems with instrumentation prevented evaporation estimates for 1986. The 

evaporation/precipitation (E/P) ratios ranged from 

1.0 during May 1987 to 34.0 during July 1985. The 

median ratio for the 25-month comparison was 2.3. 

Shjeflo (1968), in a 2-year study (1963-64) of a nearby 

wetland, reported an average monthly E/P ratio of 

1.92 during the May-October months. The average 

monthly evaporation rate during Shjeflo 's study was 

114 mm; the average monthly evaporation rate during 

this 5-year comparison was 108 mm. The likely 
32 CHAPTER2 

200 120 

z uJ 

0 a: 

:::> i= 100 100 1- <( <( 1- a: a: w 

u a.. 

w 0 80 ~ 

a: w a.. 1-

-I -I 

<( -100 60 <(en 

~ ~:::> a:: en a:_ 

Oa:: om 

~ ~ -200 

z....l 

40 w ~u 

0~ om a:- a::w LL:::l LLW 

w- -300 20 wa:: a:~ a::C.9 

:::>w =>z 1-0 1--

a: a::z <( -400 0 <(-

a.. a.. w w 0 0 

w w > -500 -20 > 

i= i= 

<( <( 

-I -I 

:::> -600 -40 :::> 

~ ~ 

:::> :::> 

u u 

-700 -60 co en 0 00 N ~ .._,. 1.!) <.o ..... co en 0 a; N (Y) .._,. 1.!) <.o ..... co ..... ..... co co co co co co co co en en en en en en en en 

~ ~ en ~ ~ ~ ~ en ~ ~ ~ ~ ~ ~ ~ ~ ~ en .- .- .- ~ ~ ~ 

Figure 27. Cumulative departure from normal monthly average air temperature and cumulative departure from normal monthly 

total precipitation for the Cottonwood Lake area, 1978-98. 

explanation for the greater E/P ratio for this comparison is the below-normal precipitation for the period. 

During 1982-87, with the exception of 1986, 

precipitation was only 61 percent of normal. 

Evaporative loss was normally greatest during 

July, and the greatest difference between evaporation 

and precipitation normally occurred during July 

(fig. 31). Wetlands normally lost water during that 

month, and losses from Wetland P 1 during July were 

usually large. The greatest difference between 

evaporation and precipitation recorded during this 

study occurred during July 1985 when evaporation 

was 150 mm greater than precipitation (fig. 30). 

During that time, the water level of Wetland P 1 

declined nearly 200 mm, indicating that other 

processes also were acting to remove water from the 

wetland. The energy-budget method used for 

estimating evaporation assumes an open-water body 

devoid of emergent vegetation; this was not the case 

for Wetland P 1, which contained a slowly encroaching 

perimeter of cattails and hardstem bullrush. Possibly, 

transpiration by these plants was acting to increase the 

amount of water removed from the wetland. Stannard 

and Rosenberry ( 1991) showed that for a shallow open 

lake in Nebraska, evapotranspiration from an area of 

emergent hard-stem bullrush was twice as great as 

evaporation from open water. However, Shjeflo 

(1968) indicated that evapotranspiration from 

wetlands covered with emergent vegetation was no 

greater than evaporation from open-water wetlands. 

His assessment was that as transpiration increases, 

evaporation decreases a like amount due to 

diminished windspeeds at the water surface. Shjeflo 

also reported that transpiration occurred mainly during 

June-August, while the conditions contributing to 

diminished wind speeds occurred even after the plants 

were dormant. 

Perhaps transpirationallosses at the Cottonwood Lake area are more significant than indicated by 

Shjeflo. Or perhaps ground-water seepage had a net 

effect of removing water from the wetland. Winter 

(Chapter 1, of this report) and Rosenberry and Winter 

(1997) have shown that transpiration-induced cones of 

depression commonly exist around much of the 

perimeter of Wetland P 1, inducing seepage of water 

from the wetland to the contiguous ground-water 

system and increasing the overall loss of water from 

the wetland. 
CHAPTER2 33 

200 561.5 _J 

w 

z > 

0 w _J 

i= 100 561.0 <( 

~ w CJ) 

a: w u 0 560.5 > w 0 a: a:! a.. <( _J CJ) <( -100 560.0 a: ~ w a:CJ) IOa: w 

zw ~ 

~tJ -200 559.5 ~ 0~ a:- u_:J 0:: w- -300 559.0 0 a:~ z 

=>z <( 

I-- _J 

a: -400 

I-

<( 558.5 w 

a.. $ w LL 0 0 w -500 558.0 _J 

> w i= > <( w _J _J 

::J -600 557.5 c:: 

~ UJ 

::J I-

<( u $ -700 557.0 00 0") 0 00 N ("I') -.;!" LO co r-. 00 0") 0 c;; N ("I') -.;!" LO co r-. 00 r-. r-. 00 00 00 00 00 00 00 00 00 0") 0") 0") 0") 0") 0") 0") 0") 

~ ~ 0") 0") 0") 0") 0") 0") 0") 0") ~ ~ ~ 0") 

~ 0") 0") 

~ 0") 0") 

.... .... .... .... ~ 

Figure 28. Cumulative departure from normal monthly total precipitation and water level of Wetland PI, 1978-98. 

8 

>< w 6 Cl 

~ 

~ 4 

a: 

UJ 

> 2 w CJ) 

I- 0 I 

(.!) 

::J -2 0 

a: 

0 

a: -4 w 

~ _J 

<( -6 

a.. 

-8 0 LO 0 LO 0 LO 0 LO 0 10 0 10 0 LO 0 10 0 LO 0 10 

0 0 c;; c;; N N ("I') ("I') -.;!" -.;!" LO 10 co co r-. r-. 00 00 0") 0") 

0") 0") 0") 

~ ~ ~ 0") ~ 0") 0") 

~ 0") 0") 

~ 0") 0") 

~ 0") 

.... .... .... .... .... 

Figure 29. Palmer Drought Severity Index (PDSI) for climate region 5, North Dakota, 1900-98. 
34 CHAPTER2 

200 

180 

:::c 160 1-

z 

0 140 

::;?! 

c: 120 w 

0... 

Cf) 100 c: 

w 

1- 80 w 

::;?! 60 :::::i 

....J 

~ 40 

20 

0 

MJJASOMJJASOMJJASOMJJASOMJJASO 

1982 1983 1984 1985 1987 

EXPLANATION 

---- Evaporation ---...-- Precipitation 

Figure 30. Monthly total evaporation and precipitation for 

Wetland Pl during the open-water season, 1982-85, and 1987. 

~oz 160 

<( ~ --(/) 0: Zo: 120 UJZQUJ 

~Qi=t;:j 

>-~~~ 80 

:i!~§;j 1-a_U- 40 Z<(UJ~ 

0>0: 

~UJa.. 0 

MAY JUN JUL AUG SEP OCT 

Figure 31. Monthly average evaporation from Wetland P 1 

(1982-87) and normal (1961-90) monthly precipitation for 

May-October. 

ACKNOWLEDGMENTS 

The author thanks John Enz and Chris Brenk, North 

Dakota State University, Fargo, North Dakota, for 

providing climate data collected at the Carrington, 

Jamestown airport, and Pettibone stations. 

REFERENCES 

Alley, W.M., 1985, The Palmer Drought Severity Index as a 

measure of hydrologic drought: Water Resources Bulletin, 

v. 21 no. 1, p. 105-114. 

Eisenlohr, W.S., Jr., and others, 1972, Hydrologic investigations 

of prairie potholes in North Dakota, 1959-68: 

U.S. Geological Survey Professional Paper 585-A, 102 p. 

Guttman, N.B., Wallis, J.R., and Hosking, J.R.M., 1992, Spatial 

comparability of the Palmer Drought Severity Index: Water 

Resources Bulletin, v. 26, no. 6, p. 1111-1119. 

Jensen, R.E., 1972, Climate ofNorth Dakota: North Dakota 

State Water Commission, 48 p. 

Palmer, W.C., 1965, Meteorologic drought: U.S. Weather 

Bureau Research Paper no. 45, 58 p. 

Parkhurst, R.S., Sturrock, A.M., Rosenberry, D.O., and Winter, 

T.C., 1994, Climatic and lake temperature data for Wetland 

P1, Cottonwood Lake area, Stutsman County, North 

Dakota, 1982-1987: U.S. Geological Survey Open-File 

Report 94-546, 40 p. 

Parkhurst, R.S., Winter, T.C., Rosenberry, D.O., and Sturrock, 

A.M., 1998, Evaporation from a small prairie wetland in 

the Cottonwood Lake area, North Dakota - an energy 

budget study: Wetlands, v. 18, no. 2, p. 272-287. 

Rosenberry, D.O., and Winter, T.C., 1997, Dynamics ofwatertable fluctuations in an upland between two prairie-pothole 

wetlands in North Dakota: Journal of Hydrology, v. 191, 

p. 266-289. 

Shjeflo, J.B., 1968, Evapotranspiration and the water budget of 

prairie potholes in North Dakota: U.S. Geological Survey 

Professional Paper 585-B, 49 p. 

Stannard, D.I., and Rosenberry, D.O., 1991, A comparison of 

short-term measurements of lake evaporation using eddy 

correlation and energy-budget methods: Journal of 

Hydrology, v. 122, p. 15-22. 

Sturrock, A.M., Hanson, B.A., Scarborough, J.L., and Winter, 

T.C., 1986, Climatic data for the Cottonwood Lake area, 

Stutsman County, North Dakota, 1982: U.S. Geological 

Survey Open-File Report 86-477, 24 p. 

1987, Climatic data for the Cottonwood Lake area, 

Stutsman County, North Dakota, 1983: U.S. Geological 

Survey Open-File Report 87-216, 28 p. 

Todhunter, P., 1993, Historical temperature and precipitation 

trends at Jamestown, North Dakota: Proceedings, North 

Dakota Academy of Science, v. 47, p. 5. 

Winter, T.C., and Rosenberry, D.O., 1998, Hydrology of prairie 

pothole wetlands during drought and deluge: A 17 -year 

study of the Cottonwood Lake wetland complex in North 

Dakota in the perspective of longer term measured and 

proxy hydrologic records: Climatic Change, v. 40, no. 2, 

p. 189-209. 
CHAPTER3 

SPATIAL AND TEMPORAL VARIABILITY IN SPECIFIC 

CONDUCTANCE AND CHEMICAL CHARACTERISTICS OF 

WETLAND WATER AND IN WATER COLUMN BIOTA IN THE 

WETLANDS IN THE COTTONWOOD LAKE AREA 

James W. LaBaugh, U.S. Geological Survey, Reston, Virginia 

George A. Swanson, U.S. Fish and Wildlife Service (Retired), Jamestown, North Dakota 

ABSTRACT 

Five wetlands in the Cottonwood Lake area 

were the subject of a detailed chemical and 

biological study during 1984 to 1987. The study 

was part of a larger investigation of longer 

duration to understand the relation between the 

hydrology of the wetlands and their use by 

waterfowl. Chemical characteristics of water in 

the wetlands varied considerably, particularly for 

major ions, and were related to the interaction of 

the wetlands with ground water and atmospheric 

exchange (precipitation and evaporation). Plant 

nutrients, such as phosphorus, nitrogen, and 

silica, varied more spatially within the wetlands 

than did major ions. The algal communities of the 

wetlands included numerous species and differed 

among the wetlands. Chlorophyll a, which is a 

measure of the amount of algae in the wetlands, 

varied as much spatially as phosphorus, nitrogen, 

and silica. However, no distinct seasonal pattern 

in chlorophyll a was evident for the seasonal 

wetlands. Chlorophyll a concentrations in the 

semipermanent wetlands usually followed a 

seasonal pattern of ( 1) declining concentrations 

following spring thaw, when the open-water 

zones contained little submerged vegetation, 

(2) remaining small through mid summer, when 

submerged vascular vegetation was abundant, and 

(3) increasing in the fall before ice formation, 

when submerged vascular vegetation in the openwater zones was senescent. The periods of large 

concentrations were usually coincident with the 

use of the wetlands by migrating waterfowl. The 

microinvertebrate communities of the wetlands, 

as constituted by representatives of Cladocera, 

Copepoda, and Rotifera, included some species 

common to all five wetlands. Copepoda generally 

were the most abundant and Rotifera the least 

abundant members of the microinvertebrate 

communities. Replicate samples were required to 

characterize seasonal differences in algal and 

microinvertebrate communities due to spatial 

variation. 

INTRODUCTION 

The prairie wetlands in mid North America are 

an important resource for migrating waterfowl 

(Swanson and Duebbert, 1989). The chemical 

composition of water in these wetlands (LaBaugh, 

1989), as well as vegetation (Stewart and Kantrud, 

1972; Kantrud, Millar, and van der Valk, 1989), varies 

according to differences in the hydrologic setting 

(Sloan, 1972; LaBaugh and others, 1987), hydroperiod 

(van der Valk, 1981 ), and soils (Arndt and Richardson, 

1993). Although prairie wetlands have been the focus 

of hydrological, chemical, and ecological studies by 

scientists in Canada and the United States over the past 

half-century (van der Valk, 1989), most studies have 

35 
36 CHAPTER3 

dealt with vegetation and waterfowl. Less is known 

about the microscopic plants and animals that inhabit 

the wetlands. Crumpton (1989) pointed out the lack of 

information on algae, and studies of microscopic 

invertebrates are rare (Bataille and Baldassare, 1993). 

Thus, not much is known about the spatial distribution 

of the microorganisms within or among prairie 

wetlands, and only a few studies have documented 

how some of the microorganism populations vary over 

time (Barica, 1975; Kling, 1975). 

Studies at the Cottonwood Lake area included 

the collection of algae and microinvertebrates from the 

water column of selected wetlands from 1984 to 1988. 

Collection of these organisms has coincided with 

concurrent research on hydrological and chemical 

properties of the wetlands. The main objective of the 

studies is to determine how the hydrogeologic setting 

and wetland-ground water interactions affect the 

chemical characteristics and biological communities 

of the wetlands. The purpose of this chapter is to 

present information about the chemical characteristics 

of water in the wetlands (and of relevant ground 

water) and the spatial and temporal variation in the 

algal and microinvertebrate communities of five 

selected wetlands. The data presented herein are for 

1984 to 1987, a period for which detailed analyses of 

community characteristics are available. Ancillary 

chemical data collected between 1979 and 1998 are 

used to place the 4-year period of data collection that 

included biota in perspective with long-term changes 

in the study area. 

METHODS 

A tube water-column sampler (Swanson, 1978) 

having a minimum diameter of 10 em was used to 

collect water samples from the wetlands. Each 

wetland was divided into three transects according to 

procedures described in LaBaugh, and others (1987). 

During 1984 to 1987, samples were collected from 

approximately the same location along each transect 

every month that the wetlands contained water and 

were ice free. On the rare occasions when the 

wetlands were very shallow, samples were collected 

from the middle of the wetland instead of along each 

of the three transects. The contents of the sampler 

were poured into a large polyethylene jug or large 

plastic bucket from which water was taken for 

processing. Multiple insertions were made when 

insufficient water was collected by a single insertion 

of the sampler into the water column. 

Ground-water samples were collected for 

chemical analysis by pumping or bailing a well dry 

and then collecting the water that seeped into the well 

overnight. If the well yielded water readily when 

pumped, three casing volumes of water were removed 

before sampling. 

Some of the water collected from the wetlands 

was used to determine the chlorophyll a concentration 

in the water column. A known volume of water was 

pumped through a glass-fiber filter using a handoperated vacuum pump. The filter was placed in a 

glass vial, stored in ice, and then shipped in ice to the 

U.S. Geological Survey National Water Quality 

Laboratory in Arvada, Colorado. At the laboratory, 

the chlorophyll a concentration was determined by the 

high pressure liquid chromatographic and fluorometric 

technique described in Britton and Greeson (1988). 

Water collected for algal analysis was put into a 

1-L high-density polyethylene bottle and the contents 

were preserved with 10 mL of Lugol's solution. The 

composition of the solution and the prescribed proportion of preservative to sample are described in Vollenweider (1974). Algal samples collected in 1984 were 

examined to determine the algal species and their 

abundance. Algal samples collected during 1985 to 

1987 also were examined to determine the biovolume 

represented by each species in addition to their 

abundance. Identification, enumeration, and 

biovolume determination of the algal species were 

done by Richard Dufford, Ft. Collins, Colorado, using 

standard inverted microscopic techniques. 

Some of the water collected from the wetlands 

also was used to determine species identification and 

enumerate invertebrates found in the water column of 

the wetlands. For invertebrate samples, the total 

volume of water collected from a transect was 

measured using a 1-L graduated cylinder and then 

poured through an 80-m mesh Wisconsin plankton 

bucket. The total volume of water represented by the 

sample was recorded; usually this volume was 

between 2 and 6 L. The contents of the plankton 

bucket then were placed into a 250-mL high-density 

polyethylene bottle. Prior to preservation with 10 mL 

of 40-percent formalin, the organisms in the bottle 

were narcotized with soda water. Preserved samples 

were sent to Dr. William Taylor, Michigan State 

University, for analysis. 
CHAPTER3 37 

Water collected from the wetlands also was 

analyzed for physical properties and chemical 

inorganic constituents. The samples were analyzed for 

specific conductance, pH, and dissolved calcium, 

magnesium, sodium, potassium, sulfate, chloride, 

orthophosphate phosphorus, nitrate plus nitrite 

nitrogen, iron, manganese, and silica. Total alkalinity, 

kjeldahl nitrogen, and phosphorus concentrations also 

were determined. Details of the sample processing 

and analytical techniques used are described in 

LaBaugh and Swanson (1992). 

SPECIFIC CONDUCTANCE AND 

CHEMICAL CHARACTERISTICS 

OF WETLAND WATER 

Chemical characteristics of the water in the 

wetlands were not uniform, and specific conductance 

varied among the wetlands (fig. 32). The variation in 

specific conductance during the period when the 

wetlands were ice free, usually from April through 

October, reflected differences in the hydrologic 

functions of the wetlands. Water in Wetland T8, 

which is in an area of recharge to ground water and 

receives little, if any, ground water discharge, had 

smaller specific-conductance values than water in the 

remaining selected wetlands (Note that the range in 

specific-conductance values for Wetland T8 for 1984 

to 1987 is small enough that the size of the symbols 

used to indicate values outside of the 1Oth and 90th 

percentiles in figure 32 mask the median and percentile symbols). The remaining selected wetlands 

received some ground-water discharge and water in 

those wetlands had larger specific-conductance values 

than water in Wetland T8. Among the wetlands 

receiving ground-water discharge, Wetland P8 had the 

smallest specific-conductance values, reflecting the 

fact that the wetland had an intermittent outlet by 

which water and solutes could be removed. Wetlands 

P 1 and P 11 did not have outlets during 1984 to 1987 

and Wetland T3 spilled to Wetland P1 only occasionally in spring. In addition, ground water in part of the 

basin of Wetland P8 contained fewer solutes (specific 

conductance was less than 600 ~S em -l) than typically 

found in ground water in the other wetland basins. 

14 

13 

12 

0 

z 11 

<(a: CJ)w 

::> I- 10 

ow 

:::r:~ 

~--- z!z 9 

-:w 

wu 8 

Ua: Zw :c::(a.. 7 ~(/) 

uz 

=>w 6 0~ 

zw 0- U(/) 5 

0 

Ua: 

u:::u 4 

u~ 

~ w 

a.. 3 (/) 

2 

P1 

I 

~ g 

P8 P11 T3 

WETLAND 

EXPLANATION 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

10th percentile 

• Extreme value 

T8 

Figure 32. Summary of specific-conductance data for five 

wetlands for 1984 to 1987. 

The specific conductance of water in each 

wetland also varied over time. An example of the 

seasonal changes in specific conductance for a 

semipermanent wetland is illustrated by data for 

Wetland P1 for 1984 to 1987 (fig. 33). Commonly, 

specific-conductance values were smallest in spring 

after ice melt and increased through the summer and 

into the fall as water levels declined. However, the 

general pattern of increased specific conductance from 

the spring to the fall was interrupted by episodes of 

decreased specific conductance as a result of dilution 

by rainfall. Specific-conductance values for each of 

the three transects in Wetland P 1 are shown in figure 

33. Each point in the figure represents the value for 

one transect. Therefore, three points are shown for 

each date. In most cases, values for each of the three 
38 CHAPTER3 

transects are quite similar and many points overlie one 

another. However, as indicated in the figure, large 

differences exist among the values for October 1986. 

The seasonal changes in specific conductance for the 

seasonal wetlands were similar to those for the 

semipermanent wetlands; commonly, specificconductance values were the smallest in spring after 

ice melt and largest in the summer prior to the 

complete loss of all standing water. 

8 

zen I I I 

-z 7- Wetland P1 _ u.,jw 

u~a: 

ZWw 6- -

~Cilt- uOUJ 5- - ::>a:~ 

Cl~i= 4r- I z~z -

oow 3- 'I uzu / - U<(a: u::cn:f 

~0 

-:::> 2 - r- C... I 1 - -

(f) I- • 

0 I I I 

1984 1985 1986 1987 

Figure 33. Specific conductance of water in Wetland P1 for 

the months when the wetland was ice free during 1984 to 1987. 

Gaps between lines are periods when the wetlands were ice 

covered. Lines connect median values. Points represent data 

from each of the three transects from which water samples 

were collected for chemical analysis. Many points overlie one 

another for individual dates. 

Specific conductance of water in the wetlands 

varied from year to year in both the seasonal changes 

and the range of values. While the seasonal changes 

were related to episodic rainfall punctuating declines 

in wetland water levels, annual changes in the range of 

values was related to periods of generally wetter or 

drier conditions. A period of decreased rainfall in 

1988 resulted in relatively larger specific-conductance 

values for Wetland P 1 than those observed in the 

preceding years (fig. 34A). Between 1988 and 1992, 

wetland water levels declined to the point where, in 

each year, the wetland was dry in late summer; this 

condition previously was observed in 1976. A deluge 

in July 1993, followed by years in which more of the 

annual input of water from the atmosphere occurred 

during October to April than in earlier years, resulted 

in increased water levels and relatively smaller 

maximum specific-conductance values than those 

observed since 1979 when detailed hydrological 

studies began in the Cottonwood Lake area. 

A 

7 Wetland P1 

6 

5 

4 

20 

10 

Figure 34. Specific conductance of water in semipermanent 

Wetlands P1 (A), P8 (B), and P11 (C), for the months when 

the wetlands were ice free during 1979 to 1998. Gaps between 

lines are periods when the wetlands were ice covered. 

All semipermanent wetlands in the Cottonwood 

Lake area did not respond uniformly to wetter or drier 

conditions, although the wetlands are in close 

proximity. Specific conductance of the water in 

Wetland P8 continued to increase in succeeding years 

ofthe declining water levels from 1988 through 1992, 

whereas specific conductance of the water in Wetlands 
CHAPTER3 39 

P 1 and P 11 did not increase in succeeding years during 

that period (fig. 34). During the latter part of the 

relatively drier years between 1988 and 1992, the 

relation between Wetland P1 and ground water 

changed. At times, the ground-water levels adjacent 

to the wetland were lower than the wetland water 

levels so that the direction of flow was from the 

wetland to the ground-water system rather than from 

the ground-water system to the wetland. This change 

in the direction of flow provided a means for removing 

water and solutes from Wetland P 1 that did not occur 

for Wetland P8 during the same time period. In the 

case of Wetland P 11, the smaller specific-conductance 

values in 1991 and 1992 were related to the fact that 

when Wetland P 11 became dry in 1990 and 1991, salts 

in the wetland were removed by wind action 

(deflation) in the fall and winter. When the wetland 

refilled with water in the springs of 1991 and 1992, 

fewer salts were present to dissolve so specificconductance values were less than in the previous 

years of relatively dry conditions. While the focus in 

this chapter is on the chemical and biological data for 

1984 to 1987, placing these years within the context of 

a 20-year period as shown in figure 34 points out the 

importance of recognizing that characteristics of the 

wetlands can be quite variable both spatially and 

temporally because of the relation between the 

wetlands and ground water and because of the 

response of the wetlands to changes in rainfall. 

Water in the wetlands typically was alkaline 

although in wetlands P8, T3, and T8 circumneutral 

values were recorded between 1984 and 1987. The pH 

values were larger for the semipermanent wetlands 

than for the seasonal wetlands (table 1, fig. 35). The 

smaller pH values for the seasonal wetlands may be 

due to the large amounts of rain and snow inputs 

relative to ground-water inputs, decomposition of 

organic matter, or organic acids. The pH values for the 

semipermanent wetlands commonly were smallest in 

the spring after ice melt, increased during midsummer, 

and decreased in the fall. The pH values for each of 

the three transects in Wetland P 11 are shown in figure 

36. The differences between the data points, which 

represent the pH values measured on a specific day at 

each of the three transects, indicate pH was not 

uniform within the wetland on a given date of sample 

collection. 

The wetlands differed not only in specific 

conductance and pH but also in the relative abundance 

of dissolved major ions (table 1). The most abundant 

cations were calcium or potassium in Wetland T8, 

calcium or magnesium in Wetland T3, magnesium in 

Wetlands P 1 and P8, and sodium in Wetland P 11. The 

most abundant anions were sulfate in Wetlands T3, P1, 

and Pll, bicarbonate or sulfate in Wetland P8, and 

bicarbonate in Wetland T8 (most of the alkalinity 

reported in table 1 is due to the bicarbonate anion). 

The seasonal changes in concentration commonly 

were similar to the seasonal changes in specific 

conductance, particularly for the most abundant cation 

and anion. The major-ion composition generally 

reflected the influence of ground-water input. For 

example, those wetlands receiving some ground-water 

discharge had an increased abundance of magnesium, 

sodium, and sulfate. 

Table 1. Summary of pH and major ion data for five wetlands of the Cottonwood Lake area. Values presented are medians for 1984 to 

1987. 

Wetland Number of Median pH Median concentration 

samples (standard units) (milliequivalents per liter) 

Calcium Magnesium Sodium Potassium Alkalinity Sulfate Chloride 

Pl 78 8.2 5.99 18.5 5.65 1.35 6.45 25.0 0.380 

P8 78 8.1 2.77 6.00 2.04 0.715 7.41 2.81 0.479 

Pll 78 8.7 2.24 30.4 43.5 2.56 10.4 64.5 6.21 

T8 27 7.0 0.349 0.312 0.017 0.460 0.979 0.333 0.053 

T3 36 7.7 8.48 9.05 1.70 0.511 5.710 14.0 0.128 

1 At the pH values present in the wetlands, most of the alkalinity was due to the bicarbonate ion. 
40 CHAPTER3 

9 

CJ) 

~ t: 

z 

::> 

0 

c:: 

<( 

0 8 z 

~ CJ) 

~ 

:i 

a. 

7 

P1 

. 

~ . 

~ . 

. 

P8 P11 T3 

WETLAND 

EXPLANATION 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

10th percentile 

• Extreme value 

~ 

T8 

Figure 35. Summary of pH data for five wetlands for 1984 to 

1987. 

The relation of the chemical characteristics of 

the water in the wetlands also was examined by use of 

a modified Schoeller diagram (fig. 37). Values shown 

in figure 3 7 A are for May 1986 for Wetland T8, June 

1986 for Wetland T3, and August 1986 for the 

remaining wetlands. The May and June data for the 

seasonal wetlands were the last data obtained before 

the wetlands either went dry or became too shallow for 

the collection of a sufficient amount of water for 

chemical analysis. The differences in the concentrations for Wetland T8 and the concentrations for the 

remaining wetlands reflect the fact that the wetlands 

having larger concentrations than Wetland T8 received 

some ground-water discharge. As concentrations 

increase along ground-water flow paths, magnesium 

exceeds calcium as saturation levels are reached for 

calcium and bicarbonate minerals. Along flow paths 

where ground water is in contact with the till for some 

time, ion exchange can take place, making it possible 

for sodium concentrations to exceed magnesium 

concentrations. Weathering of sulfate minerals at the 

same time that saturation levels for bicarbonate 

minerals are reached results in sulfate concentrations 

exceeding bicarbonate concentrations. Thus, the 

concentrations of those major ions are larger for 

wetlands receiving some ground-water discharge than 

for Wetland T8, and the relative abundance of those 

major ions also reflects where the wetlands lie in 

relation to the ground-water flow systems. 

10 I I I 

• Wetland P11 

CJ) 

1\ r 

1-

z •• ::> 9- ;/\- 0 if. c:: 

<( 

0 

z 

~ • • CJ) 81- -

~ 

:c' 

a. 

7~------~~--------~'--------~'------~ 

1984 1985 1986 1987 

Figure 36. pH of water in Wetland P 11 for the months when 

the wetland was ice free during 1984 to 1987. Gaps between 

lines are periods when the wetlands were ice covered. Lines 

connect median values. Points represent data from each of the 

three transects from which water samples were collected for 

chemical analysis. Some points overlie one another for 

individual dates. 

To relate the chemical characteristics of water in 

the wetlands to the chemical characteristics of the 

ground water, samples were collected from four 

ground-water wells (wells 4, 12, 13, and 16) prior to 

1983. Those wells were thought to represent different 

parts of the ground-water flow system within the 

Cottonwood Lake area. Between 1983 and 1987, 

additional samples were collected from wells 2, 3, 4, 

7, 8, 10, 11, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 25, 

26, 27, 28, and 29, to determine the spatial variability 

of the chemical characteristics of the ground water in 

the vicinity of the wetlands. (See fig. 3 for the 

locations of the wells.) A summary of the groundwater chemical characteristics determined for 1984 to 

1987, and specific data for selected wells, is presented 

in table 2. Ground water in the vicinity of the 
CHAPTER3 41 

wetlands differed in the relative abundance of major 

ions. Calcium was the most abundant cation and 

bicarbonate was the most abundant anion in ground 

water having small major-ion concentrations and 

specific-conductance values of less than 900 J.LS·cm-1 

(wells 2, 3, 7, 8, 10, 11, and 27). Data for well 11 

(table 2, fig. 37B), a representative of that type of 

ground water, are characteristic of ground water in 

areas of ground-water recharge. Calcium was the 

most abundant cation and sulfate was the most 

abundant anion in ground water having large majorion concentrations and specific-conductance values 

between 1,000 J.LS·cm-1 and 4,500 J.LS·cm-1 (Wells 4, 

14, 15, 16, 22, 28, and 29). Data for well16 (table 2, 

fig. 37B), a representative of that type of ground water, 

are characteristic of ground water between areas of 

ground-water recharge and areas of ground-water 

discharge to the wetlands. Magnesium was the most 

abundant cation and sulfate was the most abundant 

anion in ground water having specific-conductance 

values between 4,500 J.LS·cm-1 and 16,200 J.LS·cm-1 

(wells 13, 17, 18, 19, 20, 21, 23, and 25). Data for 

well17 (table 2, fig. 37B), a representative of that type 

of ground water, are characteristic of ground water in 

areas of ground-water discharge to the wetlands. 

The relation of the chemical characteristics of 

the ground water to the chemical characteristics of the 

water in the wetlands also is evident from examination 

of figure 3 7. Wetland T8 recharges ground water and, 

therefore, water in that wetland is unlike ground water 

in the relative abundance and concentration of major 

cations. Wetlands T3, P1, P8, and Pll receive groundwater discharge and, therefore, water in those wetlands 

is similar to ground water in the concentration of 

major cations. The relative abundance of major ions 

in water in Wetlands T3, P1, and P8 is similar to the 

relative abundance of major ions in the different parts 

of the ground-water flow system represented by wells 

11, 16, and 17. Ground-water samples were not 

collected in the vicinity of Wetland P 11 because no 

wells were installed in that area. However, Wetland 

P 11 contained water in which sodium was the most 

abundant cation, something not observed in the ground 

water collected elsewhere in the study area. Ion 

exchange in the ground water is a likely mechanism 

that could explain the major ion relations in Wetland 

P11. 

In contrast to the major-ion concentrations, total 

phosphorus concentrations were larger for the seasonal 

wetlands than for the semipermanent wetlands 

(fig. 38). Vegetation in the shallow-marsh zones of the 

seasonal wetlands, and leaching from standing dead 

plants in those zones, likely provided sources of 

phosphorus not always present in the open-water 

zones of the semipermanent wetlands. Large 

orthophosphate phosphorus concentrations usually 

occurred in the water in seasonal Wetland T8 (fig. 39). 

Phosphorus in the water in each wetland varied 

seasonally, but no distinct seasonal pattern was 

evident. 

Table 2. Summary statistics of pH and major-ion data for 21 ground-water wells and specific data for selected wells in the Cottonwood 

Lake area. Values presented for summary statistics are for 1984 to 1987 and represent a total of36 water samples. 

pH Concentration 

(standard units) (milliequivalents per liter) 

Calcium Magnesium Sodium Potassium Alkalinity 

Median 7.3 16.9 16.0 4.35 0.383 7.79 

Maximum 7.9 28.9 263 78.3 1.25 12.4 

Minimum 6.9 3.04 1.56 0.11 0.025 4.95 

Specific data for selected wells 

Well11a 7.8 3.04 1.56 

Well16a 7.0 26.4 16.4 

Well17b 7.1 17.9 263 

a Date of sample collection was August 26, 1986. 

b Date of sample collection was August 20, 1985. 

1.26 0.194 5.05 

5.65 0.588 5.33 

78.3 0.869 11.7 

Sulfate Chloride 

41.6 0.224 

333 8.74 

0.158 0.019 

0.158 0.070 

43.7 0.705 

333 8.741 
42 CHAPTER3 

1000.00 

100.00 

10.00 

a: 1.00 w 

1-

::J 

a: 0.10 w T8~, ',' 

a.. 

(/) 

1-

z 0.01 

w 

...J 1000.00 

~ I I I I I I 

::J 

0 100.00 loUJ 

::J 

~ 

~ 10.00 lo1.00 lo0.10 '=- ' ,~ 

~ 

0.01 I I I I I I I 

~ ~ ~ ~ w w ww 0 => => => => a: ~ ~~ u U5 0 U5 LL zz 0 _J _J w 0 (J) _J => 00 <( z (J) ~ J: (J) Ill Ill u (!) u a: a: <( 0 <(<( 

~ a... uu 

roo z 

<( 

Figure 3 7. Modified Schoeller diagram of the abundance 

of major ions in water within five wetlands (A) and selected 

ground-water wells (B). 

' 

Total nitrogen concentrations were similar for 

all wetlands (fig. 40), and total nitrogen commonly 

varied more spatially in water in a wetland than 

temporally over a season. Dissolved inorganic 

nitrogen was usually a small fraction of the nitrogen 

content of water in the wetlands. Most of the 

dissolved inorganic nitrogen in the water was 

ammonia. Nitrate and nitrite typically were below 

detection levels, particularly in the water in Wetlands 

T3 and T8. The differences in ammonia and nitrate 

plus nitrite indicated nitrification probably was 

inhibited in the wetlands. 

Chlorophyll a concentrations varied from less 

than 1 f..lg/L for all five wetlands to as much as 3 80 

J..Lg/L for Wetland T3 (fig. 41). No seasonal pattern in 

chlorophyll a was evident for the seasonal wetlands. 

For the semipermanent wetlands, chlorophyll a 

concentrations declined in the spring after ice melt, 

usually remained small through midsummer, and 

increased in September or October. Data for Wetland 

P11 provide an example of this seasonal pattern 

(fig. 42). Large chlorophyll a concentrations for the 

semipermanent wetlands usually coincided with 

periods when migrating waterfowl were present. 

a: 

w 

1-

::J 

a: 

w 30 a.. 

(/) 

w 

...J 

0 

~ 

0 

a: 

u 

~ 

~ 20 

u) 

::J 

a: 

0 

J: 

a.. 

(/) 

0 

J: 

a.. 10 ...J 

~ 

~ 

P1 P8 P11 T3 

WETLAND 

EXPLANATION 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

10th percentile 

• Extreme value 

T8 

Figure 38. Summary of total phosphorus data for five 

wetlands for 1984 to 1987. 

The collection of water samples from three 

transects within each wetland provided the 

information needed to determine the variability of the 

chemical constituents in the water in each wetland. 

One measure of this variability was the coefficient of 

variation, which was determined by dividing the 

standard deviation by the sample mean. Data from the 

transects enabled calculation of the coefficient of 

variation for each date on which the samples were 

collected during 1984 to 1987; these data are 
CHAPTER3 

summarized in table 3. The coefficients of variation 

for dissolved iron and manganese were generally 

larger than those for major ions, perhaps reflecting 

variation in oxidizing and reducing conditions brought 

about by mixing or resuspension of sediments along 

the different transects during the time of sample 

collection. Generally, the coefficients of variation for 

phosphorus and nitrogen also were larger than those 

for major ions. This indicates chemicals such as 

phosphorus and nitrogen, which are more directly 

affected by uptake or loss from the biota, varied more 

spatially in the water in a wetland on any date of 

sample collection than did major ions. The 

coefficients of variation for chlorophyll a also were 

larger than those for major ions. 

a: 

w 

I-

::i 

a: 

w 

a.. 

(/) 

w 

...J 

0 30 

~ 

0 

c::: 

~ 

~ 

~ 

c.n 

::J 

a: 20 

0 

::I: 

a.. 

(/) 

0 

::I: 

a.. 

w 

~ ::I: 

a.. 10 (/) 

0 

::I: 

a.. 

0 

::I: 

Ia: 

0 

P1 P8 P11 

WETLAND 

EXPLANATION 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

10th percentile 

• Extreme value 

T3 T8 

Figure 39. Summary of orthophosphate phosphorus data for 

five wetlands for 1984 to 1987. 

1700 

1600 

1500 

a: 

w 1400 

I-

::i 1300 a: 

w 

a.. 1200 

(/) 

w 1100 ...J 

0 

~ 1000 

0 

a: 

u 900 

~ 800 ~ 

z' 700 

w 

(!) 600 0 

a: 

I- 500 z 

~ ...J 400 

~ i 

~ 300 

~ • 

~ 200 

~ • 

100 

0 

P1 P8 P11 T3 T8 

WETLAND 

EXPLANATION 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

10th percentile 

• Extreme value 

Figure 40. Summary oftotal nitrogen data for five wetlands 

for 1984 to 1987. 

43 
44 CHAPTER3 

a: 

w 

1-

::J 

a: 300 

w 

a.. 

en 

~ <( 

a: 

(!) 

0 

a: 

u 200 ~ 

~ 

ctl~ 

__J 

__J 

>- :::c 

a.. 

0 

a: 100 0 

..J 

:::c 

u 

P1 P8 P11 T3 

WETLAND 

EXPLANATION 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

10th percentile 

• Extreme value 

T8 

Figure 41. Summary of chlorophyll a data for five wetlands 

for 1984 to 1987. 

a: 

w 

1-

::J roa: 300 

..JW 

..JO.. 

>-en 

:::c~ a..<( 200 

~a: Ot!J 

..JO 

:::Ca: 

uu 100 

~ 

~ 

0 

1984 1985 1986 1987 

Figure 42. Chlorophyll a concentrations for Wetland P 11 for 

the months when the wetland was ice free during 1984 to 1987. 

Gaps between lines are periods when the wetlands were ice 

covered. Lines connect median values. Points represent data 

from each of the three transects from which water samples were 

collected for chemical analysis. Some points overlie one 

another for individual dates. 

BIOLOGICAL CHARACTERISTICS 

OF WETLANDS 

Numerous algal species were found in the water 

in the wetlands during 1984 to 1987. Some information on the algal species is presented in LaBaugh and 

Swanson (1988). A large variety of algae were present 

in the water because the size and depth of the wetlands 

enabled mixing of communities from open-water 

areas, sediments, and vegetation zones. Biovolume 

data obtained for 1985 to 1987 provided information 

on the relative abundance of major taxa within each 

wetland. The taxa that were the most abundant 

differed among the wetlands (fig. 43). Cryptophytes 

and diatoms commonly were the most abundant taxa 

for Wetlands P 1 and P8, and diatoms and chlorophytes 

usually were the most abundant taxa for Wetland P11. 

Chrysophytes were relatively more abundant in the 

seasonal wetlands than in the semipermanent 

wetlands. Chlorophytes dominated the algal 

community of Wetland T8, but no taxon was distinctly 

the major part of the community of Wetland T3. 

The composition of the algal community of each 

wetland varied over time. Biovolume data for 

Wetland P 1 for 1986 show the seasonal changes in the 

relative abundance of different taxa in the algal 

community of the open-water zone along each of three 

transects in the wetland (fig. 44). Note that the June 

peak in diatoms in samples collected along transects A 

and C was not present in the sample collected along 

transect E. Also, the August peak in diatoms in 

samples collected along transects C and E was not 

present in the sample collected along transect A. 

Thus, because of the spatial variation within the 

wetland, more than one sample from a single point in 

the wetland was needed to characterize the seasonal 

changes in the communities. Algal community data 

from all of the wetlands also indicate differences in the 

composition of the algal community in different parts 

of each wetland--in the open-water zones of the 

semipermanent wetlands and in the shallow-marsh 

zones of the seasonal wetlands. 
CHAPTER3 45 

Table 3. Median value of the coefficient of variation for each date on which water samples were collected for chemical characteristics of 

five wetlands in the Cottonwood Lake area, 1984-87. 

Wetland 

Chemical Characteristic Pl P8 Pll T3 T8 

Median value of the coefficient of variation 

Calcium 3.5 1.9 

Magnesium 1.9 1.2 

Sodium 2.0 1.2 

Potassium 2.8 2.1 

Alkalinity 0.8 0.5 

Sulfate 2.4 3.5 

Chloride 4.9 3.4 

Silica 5.6 6.6 

Total nitrogen 6.4 10 

Nitrate plus nitrite nitrogen 50 40 

Ammonia 18 15 

Ortho phosphorus 44 12 

Total phosphorus 17 15 

Iron 15 18 

Manganese 26 24 

Chlorophyll a 38 30 

The microinvertebrate communities of the 

wetlands included some species that were common to 

all five wetlands and some that were unique to 

individual wetlands (table 4). The main difference 

among the wetlands was the fact that no calanoid 

copepods (Diaptomus species in table 4), organisms 

often associated with open-water zones, were found in 

the seasonal wetlands. The total number of microinvertebrates was larger for the seasonal wetlands than 

for the semipermanent wetlands (fig. 45), but no 

distinct seasonal trend was evident for the seasonal 

wetlands. However, in 1987, one of the few years in 

which the seasonal wetlands contained water into 

midsummer, the number of microinvertebrates in 

Wetland T8 increased from the spring until the 

wetland became dry in August (fig. 46). The amount 

of algae also increased from the spring to the summer 

that year and chlorophyll a concentrations exceeded 

100 g/L, coincident with the maximum abundance of 

microinvertebrates. 

Copepods commonly were the most abundant 

organisms in the microinvertebrate communities of the 

wetlands. Data for Wetland P1 for 1986 show the 

seasonal changes in the relative abundance of 

copepods in the micro invertebrate community of that 

wetland (fig. 47). The relative importance of the 

copepods was due mainly to the relatively large 

number of juvenile copepods in the samples. Rotifers 

5.0 6.9 14 

1.7 8.9 10 

1.9 8.9 34 

3.3 6.4 5.8 

1.3 5.7 8.3 

3.1 9.9 4.9 

3.7 12 9.2 

27 6.1 5.5 

7.6 5.1 6.2 

32 28 27 

16 8.2 37 

38 64 12 

20 27 8.9 

12 19 19 

40 45 45 

46 40 42 

commonly constituted the smallest percentage of the 

microinvertebrate community of the wetlands. 

Because of the spatial variation in the distribution of 

organisms within the wetlands, more than one sample 

was needed to characterize the community of each 

wetland. 

Other microinvertebrate and macroinvertebrate 

organisms also were found in samples collected along 

each transect. Ostracods (seed shrimps), which are 

crustaceans like the cladocerans and copepods, often 

were found but were not identified further. Hydracarina (water mites) were infrequent members of the 

invertebrate community of the wetland water column. 

Larvae of flies, such as midges, phantom midges, and 

mosquitoes, were occasionally caught in the process of 

sample collection. Other taxa, including representatives ofTrichoptera (caddisflies), Gastropoda (snails), 

Ephemeroptera (mayflies), and Hemiptera (bugs), 

were infrequently captured, but were not identified to 

the species level. 
Table 4. Crustacean rnicroinvertebrate species found in the water column of selected wetlands in the Cottonwood Lake area, 1984-87. ~ 

~ 

Wetland 

Pl PS Pll T3 TS 

Cladocera 

Alana guttata Alana guttata 

Bosmina species Bosmina species 

Chydorid species Chydorid species Chydorid species 

Chydorus sphaericus Chydorus sphaericus Chydorus sphaericus 

Ceriodaphnia species Ceriodaphnia species Ceriodaphnia species Ceriodaphnia species Ceriodaphnia species 

Ceriodaphnia 

laticaudata 

Ceriodaphnia Ceriodaphnia Ceriodaphnia Ceriodaphnia 

quadrangula quadrangula quadrangula quadrangula 

Ceriodaphnia reticulata Ceriodaphnia reticulata Ceriodaphnia reticulata Ceriodaphnia reticulata Ceriodaphnia reticulata 

Daphnia galeatae Daphnia galeatae 

Mendotae Mendotae 

Daphnia pulex Daphnia pulex Daphnia pulex Daphnia pulex Daphnia pulex 

Daphnia rosea 

Daphnia simi/is Daphnia simi/is 

Diaphanosoma n 

Luectenbergianum = > 

Kurzia latissima "'C 

>-3 

1:.'!'.1 

Lantonopsis :=c 

!.H 

occidental is 

Moina macrocopa 

Pleuroxis denticulatus 

Pleuroxis procurvatus 

Scapholoberius species 

Scapholoberius aurita Scapholoberius aurita Scapholoberius aurita 

Scapholoberius Scapholoberius mucronata 

mucro nata 

Simocephalus species Simocephalus species 

Simocephalus vetulus Simocephalus vetulus Simocephalus vetulus Simocephalus vetulus 

Copepoda 

Acanthocyclops vernalis Acanthocyclops vernalis Acanthocyclops vernalis 

Cyclops crassicaudis 
Table 4. Crustacean microinvertebrate species found in the water column in selected wetlands in the Cottonwood Lake area, 1984-87 

continued. 

Wetland 

Pl P8 Pll T3 T8 

Diacyclops icuspidatus Diacyclops bicuspidatus Diacyclops bicuspidatus Diacyclops bicuspidatus Diacyclops icuspidatus 

thomasi thomasi thomasi thomasi thomasi 

Ectocyclops phaleratus Ectocyclops phaleratus 

Eucyclops agilis Eucyclops agilis 

Microcyclops varicans Microcyclops varicans Microcyclops varicans 

Orthocyclops modestus 

Paracyclops fimbriatus P aracyclops fimbriatus Paracyclops fimbriatus 

poppei poppei poppei 

Diaptomus breweri Diaptomus breweri 

Diaptomus clavipes Diaptomus clavipes 

Diaptomus franciscanus Diaptomus franciscanus 

Diaptomus hesperus 

Diaptomus immunis Diaptomus immunis 

Diaptomus sicilis Diaptomus sicilis Diaptomus sicilis 

Harpacticoida 

Canthocamptus species 

Unidentified species Unidentified species Unidentified species Unidentified species 

("':) 

"C:j 

~ ...., 

l:.!l!'j 

:::0 

(,H 
48 

100 

A WETLAND Pl 

80 

60 

40 .· .. 

.. 

·: .. 

·: .. 

20 

• 

0 i ;;:I:; 

100 

UJ B WETLAND PB 

~ 

:::> 

_J 

80 0 

> 0 

CD 

_J 60 <2: 

(.!) 

_J 

<( 

_J 

~ 40 

0 

~ ffz 20 UJ 

u 

0 a: 

UJ 

a.. 

0 

..... 

·:.· 

....... 

100 

c WETLAND P11 

80 

60 

40 

20 

0 

TAXON 

~ . 

I 

bJ 

CHAPTER3 

~ 

D WETLANDT8 

I ' ~ . 

E WETLANDT3 

EXPLANATION 

Cyanobacteria 

Chrysophytes 

Cryptophytes 

Diatoms 

Dinoflagellates 

Euglenolds 

Chlorophytes 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

10th percentile 

• Extreme value 

Figure 43. Summary, based on biovolume data from 1985 to 1987, of the relative abundance of taxa in the algal communities of 

Wetland Pl (A), Wetland P8 (B), Wetland Pll (C), Wetland T8 (D) and Wetland T3 (E). 
100 

80 

60 

40 

20 

0 

100 

LU 

~ 

::J 

....J 80 

0 

> 0 

co 

....J 60 

~ 0 

Iu.. 40 

0 

Iz 

LU 20 u 

c:: 

LU 

a... 

0 

100 

80 

60 

40 

20 

Cryptophytes 

Euglenoids 

Cyanobacteria 

Chlorophytes 

Diatoms 

Wetland P1 

Transect A 

Wetland P1 

Transect E 

Figure 44. Seasonal changes in the relative abundance of 

taxa in the algal community of the open-water zone of 

Wetland PI during 1986. 

CHAPTER3 49 

DISCUSSION 

The variety in the abundance of major ions in 

the five selected wetlands in the Cottonwood Lake 

area is similar to the variety in wetlands throughout 

Stutsman and Kidder Counties (Swanson and others, 

1988). Furthermore, this variety in the abundance of 

major ions is typical of wetlands in the glaciated 

prairie ofNorth America (LaBaugh, 1989). The 

seasonal and annual variation in major-ions concentrations in the Cottonwood Lake area prairie wetlands 

was a function of the seasonal and annual variation in 

the balance between water inputs and losses as 

determined by rain, snow, evapotranspiration, and 

interaction with ground water as described by 

LaBaugh and Swanson (1992), and LaBaugh and 

others (1996, 1998). In addition, the wetlands varied 

in chemical characteristics, more so for nitrogen and 

phosphorus than for major ions, probably due to the 

use and release of those nutrients by biota. 

Neely and Baker (1989) provided a summary of 

the different processes that can affect nitrogen and 

phosphorus in prairie wetlands. They concluded that 

the role of emergent vegetation in removing nitrogen 

and phosphorus from the wetlands was the only 

process for which detailed information was available. 

Atmospheric exchange affects gains and losses of 

nitrogen in particular, but Neely and Baker (1989) 

found too few studies of that process were available to 

make generalizations about its importance. Also, few 

detailed studies of the water and chemical fluxes of 

wetlands in general are available (LaBaugh, 1986). 

Kadlec's (1986) field manipulation of water levels in 

experimental wetlands in Delta Marsh, Manitoba, 

included an analysis of water and chemical fluxes that 

indicated atmospheric inputs were important sources 

of nitrogen and phosphorus. More detailed analyses of 

the Delta Marsh wetlands have shown that changes in 

aquatic macrophytes in response to wet and dry 

conditions control the cycling of nitrogen and 

phosphorus in experimental wetlands (Kadlec and 

others, 2000), but the importance of microbial 

processes has not yet been adequately documented 

(Murkin and others, 2000) 
50 CHAPTER3 

In one of the few detailed studies oftemporal 

variation in plant nutrients within shallow prairie 

lakes, Barica (1975) indicated seasonal fluctuations of 

total, particulate, and dissolved nitrogen and 

phosphorus were quite variable. Lakes having marked 

growth and decline in summer algal populations had 

the most variability in the summer. Wetlands in the 

Cottonwood Lake area usually were characterized by 

small algal populations in the summer, but the 

wetlands had considerable seasonal variation in total 

and dissolved nitrogen and phosphorus. Large populations of cyanobacteria, which characterized the 

summer community in the prairie lakes in Barica's 

study, were not evident in the wetlands in the Cottonwood Lake area during 1984 to 1987. However, in 

1967, when Wetland P 1 was deeper due to wetter 

climatic conditions and had no emergent vegetation in 

the deep-marsh zone, large numbers of the cyanobacteria species Aphanizomenon were present. Wetter 

conditions returned to the Cottonwood Lake area from 

1993 through 1998 and Wetland P 1 again was deeper. 

The wetland had no emergent vegetation by 1998, and 

cyanobacteria were present in the wetland. Thus, 

changes in water level in response to variability in 

hydrological fluxes affect the algal community as well 

as vascular plant communities and zonation (Kantrud 

and others, 1989). 

Barica's (1975) study also indicated silica varied 

seasonally due to fluctuations in the diatom 

community. Diatoms were a large component of the 

algal communities of the wetlands in the Cottonwood 

Lake area, and silica was as variable seasonally and 

spatially as nitrogen and phosphorus. In addition, 

other siliceous algae, the chrysophytes, were 

important parts of the communities in some wetlands 

and likely contributed to seasonal fluctuations of silica 

as well. In the prairie lakes in Barica's (1975) study, 

silica concentrations fluctuated from less than the 

detection limit to 16 mg/L. In the wetlands in the 

Cottonwood Lake area, silica concentrations varied 

from less than the detection limit to greater than 20 

mg/L. 

Summarizing what is known about algae in 

prairie wetlands, Crumpton (1989) noted the variety of 

aquatic habitats in these wetlands would likely yield a 

diverse algal community. The lack of detailed studies 

of algae in prairie wetlands forced Crumpton to make 

inferences about the composition of algal communities 

and the relation of the communities to hydrological 

and chemical conditions. Robinson and others (2000) 

found the algal communities of experimental wetlands 

in Delta Marsh on the edge of Lake Manitoba to be 

varied; algal assemblages in the open water, attached 

to plants, attached to soft sediments, or beneath the 

submersed surface, fluctuated in response to manipulation of water levels designed to simulate wet or dry 

conditions typical of conditions experienced in the 

mid-continent prairie. Data from the Cottonwood 

Lake area indicate the algal community was diverse in 

the wetlands and that the relative composition varied 

seasonally within the wetlands. Different taxa 

dominated the community of each wetland. In the 

prairie lakes studied by Barica (197 5), some 

communities consisted primarily of diatoms, chrysophytes, and cryptomonads, and were similar to the 

communities in some of the wetlands in the Cottonwood Lake area. One factor that had an effect on the 

algal communities in the wetlands was the influence of 

migrating waterfowl. This factor was not considered 

in Crumpton's analysis. The exact nature of the birds' 

effect is not clear, but, during feeding, sediment 

resuspension could result in mixing of benthic algae 

into the water column, thereby increasing the amount 

of algae in the water column. Sediment resuspension 

also could increase sediment water exchange of 

nutrients. Increased sediment water exchange, 

coupled with deposition of waterfowl fecal material, 

could provide sufficient nutrients to stimulate the 

production of algae, thereby increasing the amount of 

algae collected in the water column. 

Another factor that could affect the amount of 

algae collected in the water column was the influence 

of submerged vascular vegetation in the open-water 

zone. After spring thaw, the open-water zones of the 

semipermanent wetlands contained little submerged 

vascular vegetation. Thus, the entire water column of 

the wetlands was well mixed and the only competition 

for nutrients and light was among algae in the water. 

In the summer, after the submerged vascular 

vegetation had grown and partly filled the open-water 

zone, circulation was restricted, resulting in increased 

competition for light and nutrients. In the fall, when 

the submerged vascular vegetation in the open-water 

zone became senescent, competition for light and 

nutrients from vascular plants decreased, and the 

entire water column of the open-water zones of the 

wetlands circulated freely. 
CHAPTER3 51 

a: 

L.U 

1-

::J 

a: 

L.U 

c.. 

(/) 

--' 

<( 

:::> 

0 

> Ci 

z 

8000 

7000 

6000 

5000 

4000 

3000 

2000 

1000 

OL--- ---- ----~----~----!---~ PB P11 T3 TB 

WETLAND 

EXPLANATION 

~ 90th percentile 

75th percentile 

Median 

25th percentile 

10th percentile 

• Extreme value 

Figure 45. Summary of the abundance ofmicroinvertebrates 

for five wetlands during 1984 to 1987. 

Few studies have been made of the invertebrate 

community of prairie wetlands. Bataille and Baldassare (1993) indicated a variety of organisms occupied 

their three study wetlands in southwestern Manitoba. 

In those wetlands, crustaceans, particularly ostracods, 

were the most abundant taxa of the invertebrate 

community. Although the sampling method used in 

the Cottonwood Lake area was not designed to capture 

ostracods, data on copepod diversity and abundance 

were similar to the findings ofBataille and Baldassare 

(1993). Kantrud, Krapu, and Swanson (1989) 

summarized a variety of studies, including research 

results from the Cottonwood Lake area, and found that 

the invertebrate communities in prairie wetlands are 

diverse and highly variable. Murkin and Ross (2000) 

noted that variability in the invertebrate community 

likely is related to both the variability in the amount of 

time water is present in the wetlands and the spatial 

variability imposed by the structure of vegetation in 

the wetlands. Understanding the controls on invertebrate communities in the prairie wetlands is a key to 

understanding their use by waterfowl because, as 

noted by Swanson and Duebbert (1989), the invertebrate communities are an essential food resource used 

by waterfowl. 

a: 7000 L.U 

1-

::::i 6000 

a: 

~ 5000 (/) 

--' 

<( 4000 :::> 

0 > 3000 

Ci 

~ 2000 

1000 

• 

• 

• 

WetlandTB 

• 

• 

z --' (!) "- 1- > u ::J ::J ::J w u 0 w ..., ..., <( (/) 0 z 0 

Figure 46. Seasonal changes in the abundance of microinvertebrates in Wetland T8 during the ice-free period of 1987. 

In general, limnological studies of prairie 

wetlands are providing valuable information on spatial 

and temporal patterns of invertebrate community 

structure. Coupling these patterns with environmental 

factors, such as variation in hydrological conditions 

and related chemical characteristics, provides the 

framework for relating physical and chemical 

processes to changes in invertebrate community 

composition, which is important for understanding the 

use of the wetlands by waterfowl. Spatial variation in 

the algal and microinvertebrate communities of the 

wetlands in the Cottonwood Lake area was greater 

than spatial variation in the major-ion chemical 

characteristics. In addition, spatial variability for 

nitrogen and phosphorus was greater than for major 

ions. Thus, replicate samples are useful in any study 

designed to relate biological communities to 

hydrological or chemical characteristics of prairie 

wetlands. 
52 CHAPTER3 

100 

80 

60 

40 

20 

0 

100 

w 

u 

z 

<l: 80 0 Copepoda 

z 

::J 

co 

<l: 60 

-' 

~ 

~ 40 LL 

0 

fz 

w 

u 

20 Cladocera 

a: 

w 

c... Rotifera 

0 

100 

80 Copepoda 

60 

40 

Cladocera 

20 

Rotifera 

0 z co a: a: <( w <( c... --, u.. ~ <( ~ z -' c:J c... 

:::> :::> :::> w 

~ --, --, <( (/J 

Wetland P1 

Transect C 

Wetland P1 

Transect E 

f- 6 u u w 0 z 0 

Figure 47. Seasonal changes in the relative abundance of the 

microinvertebrate community of Wetland P 1 during the icefree period of 1986. 

REFERENCES 

Arndt, J.L., and Richardson, J.L., 1993, Temporal variations 

in the salinity of shallow ground water from the 

periphery of some North Dakota wetlands (USA): 

Journal of Hydrology, v. 141 , p. 75-105. 

Barica, J., 1975, Geochemistry and nutrient regime of saline 

eutrophic lakes in the Erickson-Elphinstone district, 

southwestern Manitoba: Environment Canada 

Fisheries and Marine Service Technical Report 511 , 

82 p. 

Bataille, K.J., and Baldassare, GA. , 1993, Distribution and 

abundance of aquatic macroinvertebrates following 

drought in three prairie pothole wetlands: Wetlands, 

v. 13, p. 260-269. 

Britton, L.J., and Greeson, P.E., eds., 1988, Methods for 

collection and analysis of aquatic biological and 

microbiological samples: U.S. Geological Survey 

Techniques ofWater-Resources Investigations, book 5, 

chap. A4. 686 p. 

Crumpton, W.G, 1989, Algae in northern prairie wetlands, 

in van der Valk, A.G, ed., Northern Prairie Wetlands: 

Ames, Iowa, Iowa State University Press, p. 188-203. 

Kadlec, J.A., 1986, Input-output nutrient budgets for small 

diked marshes: Canadian Journal of Fisheries and 

Aquatic Sciences, v. 43, p. 2009-2016. 

Kadlec, J.A., van der Valk, A.G , and Murkin, H.R., 2000, 

The MERP nutrient budgets, in Murkin, H.R. , van der 

Valk, A.G, and Clark, W.R., eds., Prairie Wetland 

Ecology: The contribution of the Marsh Ecology 

Research Program: Ames, Iowa, Iowa State University 

Press, p. 37-54. 

Kantrud, H.A., Krapu, GL., and Swanson, GA., 1989, 

Prairie basin wetlands of the Dakotas: a community 

profile: U.S. Fish and Wildlife Service Biological 

Report 85(7.28), Ill p. 

Kantrud, H.A., Millar, J.B., and van der Valk, A.G, 1989, 

Vegetation of wetlands of the prairie pothole region, in 

van der Valk, A.G, ed., Northern Prairie Wetlands: 

Ames, Iowa, Iowa State University Press, p. 132-187. 

Kling, H., 1975, Phytoplankton successions and species 

distribution in prairie ponds of the Erickson-Elphinstone district, southwestern Manitoba: Environment 

Canada Fisheries and Marine Service Technical Report 

512, 31 p. 

LaBaugh, J.W., 1986, Wetland ecosystem studies from a 

hydrologic perspective: Water Resources Bulletin, 

v. 22, p. 1-10. 

1989, Chemical characteristics of water in northern 

prairie wetlands, in van der Valk, A.G , ed., Northern 

Prairie Wetlands: Ames, Iowa, Iowa State University 

Press, p. 56-90. 
CHAPTER3 53 

LaBaugh, J.W., and Swanson, G.A., 1988, Algae and 

invertebrates in the water column of selected prairie 

wetlands in the Cottonwood Lake area, Stutsman 

County, North Dakota, 1984: U.S. Geological Survey 

Open-File Report 88-451, 96 p. 

__ 1992, Changes in chemical characteristics of water in 

selected wetlands in the Cottonwood Lake area, North 

Dakota, U.S.A., 1967-1989, in Robarts, R.D., and 

Bothwell, M.L., eds., Aquatic Ecosystems in SemiArid Regions: Implications for Resource Management: 

Saskatoon, Saskatchewan, Environment Canada, 

National Hydrology Research Institute Symposium 

Series Number 7, p. 149-162. 

LaBaugh, J.W., Winter, T.C., Adomaitis, V.A., and 

Swanson, G.A., 1987, Hydrology and chemistry of 

selected prairie wetlands in the Cottonwood Lake area, 

Stutsman County, North Dakota, 1979-82: U.S. 

Geological Survey Professional Paper 1431,26 p. 

LaBaugh, J.W., Winter, T.C., and Rosenberry, D.O., 1998, 

Hydrologic functions of prairie wetlands: Great Plains 

Research, v. 8, p. 17-37. 

LaBaugh, J.W., Winter, T.C., Swanson, G.A., Rosenberry, 

D.O., Nelson, R.D., and Euliss, N.H. Jr., 1996, 

Changes in atmospheric circulation patterns affect 

midcontinent wetlands sensitive to climate: Limnology 

and Oceanography, v. 41, no. 5, p 864-870. 

Murkin, H.R., and Ross, L.C.M., 2000, Invertebrates in 

prairie wetlands, in Murkin, H.R., van der Valk, A. G., 

and Clark, W.R., eds., Prairie Wetland Ecology: The 

contribution of the Marsh Ecology Research Program: 

Ames, Iowa, Iowa State University Press, p. 201-247. 

Murkin, H.R., van der Valk, A. G., and Kadlec, J.A., 2000, 

Nutrient budgets and the wet-dry cycle of prairie 

wetlands, in Murkin, H.R., van der Valk, A. G., and 

Clark, W.R., eds., Prairie Wetland Ecology: The contribution of the Marsh Ecology Research Program: 

Ames, Iowa, Iowa State University Press, p. 99-121. 

Neely, R.K., and Baker, J.L., 1989, Nitrogen and 

phosphorus dynamics and the fate of agricultural 

runoff, in van der Valk, A. G., ed., Northern Prairie 

Wetlands: Ames, Iowa, Iowa State University Press, 

p.92-131. 

Robinson, G.G.C., Gurney, S.E., and Goldsborough, L.G., 

2000, Algae in prairie wetlands, in Murkin, H.R., van 

der Valk, A. G., and Clark, W.R., eds., Prairie Wetland 

Ecology: The contribution of the Marsh Ecology 

Research Program: Ames, Iowa, Iowa State University 

Press, p. 163-199. 

Sloan, C.E., 1972, Ground-water hydrology of prairie 

potholes in North Dakota: U.S. Geological Survey 

Professional Paper 585-C, 28 p. 

Stewart, R.E., and Kantrud, H.A., 1972, Vegetation of 

prairie potholes, North Dakota, in relation to quality of 

water and other environmental factors: 

U.S. Geological Survey Professional Paper 585-D, 

36 p. 

Swanson, G.A., 1978, A water column sampler for invertebrates in shallow wetlands: Journal of Wildlife 

Management, v. 42, p. 670-671. 

Swanson, G.A., and Duebbert, H.F., 1989, Wetland habitats 

of waterfowl in the prairie pothole region, in van der 

Valk, A. G., ed., Northern Prairie Wetlands: Ames, 

Iowa, Iowa State University Press, p. 228-267. 

Swanson, G.A., Winter, T.C., Adomaitis, V.A., and 

LaBaugh, J.W., 1988, Chemical characteristics of 

prairie lakes in south-central North Dakota- their 

potential for influencing use by fish and wildlife: U.S. 

Fish and Wildlife Service Technical Report 18, 44 p. 

van der Valk, A.G., 1981, Succession in wetlands: A 

Gleasonian approach: Ecology, v. 62, p. 688-696. 

__ ed., 1989, Northern Prairie Wetlands: Ames, Iowa, 

Iowa State University Press, 400 p. 

Vollenweider, R.A., 1974, A manual on methods for 

measuring primary production in aquatic environments, International Biological Programme Handbook 

Number 12 (2nd ed.): London, Blackwell Scientific 

Publications, 225 p. 
54 CHAPTER3 
CHAPTER4 

DYNAMICS OF A PRAIRIE POTHOLE WETLAND COMPLEX: 

IMPLICATIONS FOR WETLAND MANAGEMENT 

George A. Swanson1

, Ned H. Euliss, Jr.2, Bruce A. Hanson2, and David M. Mushet2 

1 U.S. Fish and Wildlife Service (Retired), Jamestown, North Dakota 

2U.S. Geological Survey, Northern Prairie Wildlife Research Center, 

Jamestown, North Dakota 

ABSTRACT 

Biotic communities of wetlands in the 

Cottonwood Lake area were influenced by the 

position each wetland occupied within the 

topographic and hydrologic landscape gradients. 

These gradients, together with climate, 

determined the hydrologic functions of the 

wetlands and, subsequently, the dissolved salt 

concentrations. The biotic communities were 

maintained and altered by natural fluctuations in 

the water levels and dissolved salt concentrations 

that established vegetative zones and their species 

structure. Variation in seasonal and annual 

precipitation in this semiarid area was associated 

with the abundance of seasonal wetlands and the 

dynamic characteristics of semipermanent 

wetlands. Long-term trends in climate conditions 

cycled the semipermanent wetlands between 

extremes of flooding and desiccation. The 

resulting variability in the water levels and the 

degree of water permanence, together with their 

interaction with ground water, was reflected in the 

variability of salinity in the wetlands. Because 

the biodiversity and productivity of prairie 

pothole wetland complexes is maintained by an 

abundance of temporary and seasonal wetlands 

and dynamic semipermanent wetlands, implications for creating, restoring, and preserving the 

wetland complexes in the prairie pothole region 

need to be based on an understanding of the 

response of wetland biotic communities to 

chemical and hydrologic gradients in the prairie 

wetlands. 

INTRODUCTION 

The semiarid climate of the prairie pothole 

region of North America, and the associated moisture 

deficits, affects the hydrological, chemical, and 

biological characteristics of prairie wetlands. The 

climate interacts with a glaciated landscape comprised 

of an abundance of heterogeneous, depressional 

wetlands, and causes the hydrogeochemical and 

biological characteristics in the dynamic wetland 

complexes to be repeatedly altered over time. 

Knowledge of the physical factors that determine the 

productivity and biological diversity of natural 

wetland complexes is a prerequisite to understanding 

biodiversity in the prairie pothole region (Swanson, 

1992). Management guidelines that do not consider 

the dynamics of the natural wetland complexes 

probably will have limited success in maintaining the 

productivity and biological diversity of the prairie 

pothole region (Euliss and others, 1999). 

A study to investigate the relation between 

waterfowl and prairie wetlands was initiated in the 

Cottonwood Lake area by U.S. Fish and Wildlife 

Service biologists in 1967 (fig. 48). An expansion of 

the study was initiated by U.S. Geological Survey 

hydrologists in 1979 to define the hydrologic characteristics of the wetlands (Winter and Carr; 1980, 

55 
56 CHAPTER4 

LaBaugh and others, 1987). Results of the now 

interdisciplinary study of the Cottonwood Lake area 

have led to an increased understanding of how the 

hydrologic setting affects wetland chemical characteristics and how interaction between hydrological and 

chemical characteristics affects wetland plant and 

animal communities (Winter and Carr, 1980; LaBaugh 

and others, 1987; Swanson and others, 1988). 

Although biological studies were initiated in 1967, 

information from the interdisciplinary study and 

information from a previous study of the hydrology 

and plant-community structure of wetlands in the 

vicinity of the Cottonwood Lake area (Eisenlohr and 

others, 1972; Stewart and Kantrud, 1971, 1972), could 

be used to describe long-term hydrologic trends and 

the response of wetland communities to climatic 

extremes that occurred during 1961-98. In addition, to 

relate the Cottonwood Lake area wetland complex to 

lakes and wetlands in the drift prairie, Missouri 

Coteau, and glacial outwash plain, a study of the 

chemical characteristics of lakes and wetlands in 

Stutsman and Kidder Counties, North Dakota, was 

completed by Swanson and others (I 988). 

Figure 48. Areal view of the Cottonwood Lake area looking west on July I, 1981. The tip of the cultivated right triangle in the lower 

left side of the photograph intersects the south end of Wetland P7. The east-west road bends around the north side of Wetland P II 

located in the background. 
CHAPTER4 57 

The Cottonwood Lake area was purchased by 

the U.S. Fish and Wildlife Service on August 6, 1963, 

to be managed as a Waterfowl Production Area. The 

91.8-ha area contains 75.1 ha ofuplands and 16.7 ha 

of wetlands. Eighteen percent of the land was 

cultivated at the time of purchase, and this land, which 

consisted of two fields, was seeded to non-native 

perennial grasses and legumes after Federal acquisition. The remaining upland is in native prairie and 

previously was exposed to grazing and prescribed 

burning. 

The Cottonwood Lake area is bordered on the 

east and west by pasture, on the north by planted 

hayland, and on the south by cultivated land that was 

converted to grassland in 1990 under the U.S. Department of Agriculture's Conservation Reserve Program. 

Uplands in the wetland complex were grazed until the 

area was purchased in 1963. The area initially was 

managed by the U.S. Fish and Wildlife Service by 

controlled burning that was terminated to accommodate this study. 

During periods of average water levels, the 

deep-marsh emergent zones of the semipermanent 

wetlands that occupy the Cottonwood Lake area, with 

the exception of Wetland P 11, were dominated by 

hardstem bulrush (Scirpus acutus) and broadleaf and 

narrow leaf cattail (T latifolia, Typha angustifolia) and 

their hybrid (T. x glauca). Wetland Pll typically was 

dominated by salt-tolerant plants such as prairie 

bulrush (Scirpus maritimus) and sago pondweed 

(Potamogeton pectinatus). Seasonal wetlands that 

functioned as ground-water recharge areas were 

dominated by swamp smartweed (Polygonum 

amphibium) and slough sedge ( Carex atherodes) 

(LaBaugh and others, 1987). Seasonal wetlands that 

functioned as ground-water flow-through systems, 

were dominated by slough sedge and sprangletop 

(Scolochloa festucacea ). 

METHODS 

Wetland water conditions and the response of 

emergent vegetation to hydrologic and chemical 

conditions were documented using ground and aerial 

photographs and 0.25-m2 plots of emergent vegetation 

located along randomly selected transects. Annual 

trends in wetland cover (open water/emergent vegetation ratio) and zonation were documented using 

vertical, low-altitude, 35-mm color photographs. 

Plant species within zones were identified in the field, 

and upland vegetation at the Cottonwood Lake area 

was described in July 1989. The species were identified and the canopy cover was estimated (Daubenmire, 

1959) from 130 quadrats (tables 5 through 1 0). Five 

20- x 50-cm quadrats, 8.75 m apart along four 

transects, were used. The transects were established at 

random on mesic and xeric sites and on sites with east, 

west, north, and south aspect slopes. Upland vegetation at the study site also was mapped (fig. 49) by 

vegetative community (Kruse and Bowen, 1996). Dry 

conditions in 1989 reduced mesic areas, and most 

mesic transects entered basin wet-meadow zones. The 

brush shown in figure 49 consists mostly of western 

snowberry (Symphoricarpos occidentalis) and the 

brome-alfalfa portions are areas that were cultivated 

prior to U.S. Fish and Wildlife Service ownership of 

the area. Because of the intensity of the work required 

to relate climate-induced variations in hydrology and 

chemistry to wetland biota, work was concentrated on 

five wetlands (T3, T8, PI, P8, and Pll). However, a 

broad overview of the response of wetlands in and 

near the Cottonwood Lake area to climate variability 

is presented first as a framework for discussion of the 

five wetlands. 

RESPONSE OF WETLANDS IN AND 

NEAR THE COTTONWOOD LAKE 

AREA TO CLIMATE VARIABILITY 

FROM 1961 TO 1998 

During 1961-98, the Cottonwood Lake area 

experienced the second worst drought (1988-92) and 

the greatest deluge (1993-98) in more than a century 

(Winter and Rosenberry, 1998; Rosenberry, Chapter 2 

of this report). The climate extremes were unique in 

that a historic deluge terminated a historic drought. 

The back-to-hack climate events provided a unique 

opportunity to document the response of a prairie 

pothole wetland complex to climate extremes that 

occurred over a short timeframe (1988-98). 

A wet period following a drought also occurred 

from 1962 to 1966 (Eisenlohr and others, 1972) but 

that wet period was shorter in duration and magnitude 

than the wet period from 1993 through 1998. 

However, 4 of the 5 years that followed the drought of 
58 CHAPTER4 

1961 described by Eisenlohr and others ( 1972) 

resulted in rises in water levels. The rises in water 

levels in 1962 and 1964 were caused by midsummer 

precipitation, and the rises in water levels in 1965 and 

1966 were caused by heavy runoff during spring 

snowmelt. During the wet conditions of 1962-66, 

dissolved salt concentrations were greatly reduced and 

emergent vegetation was drowned and eliminated in 

closed basins. In contrast, the dry conditions that 

occurred in 1973-74, 1976-77, and 1988-92 eventually 

caused even the most persistent semipermanent 

wetlands in the Cottonwood Lake area to become dry, 

and the wetland basins became dominated by dense, 

monotypic stands of emergent vegetation, primarily 

cattail, by the end of 1992. 

EXPlANATION 

Wetlands 

Forbs 

Trees 

Grass 

Figure 49. Upland vegetation in the Cottonwood Lake area, 1989. 

Brush with no understory 

Brush with grass/forb understory 

Grass with brush understory 

Brome/alfalfa 
CHAPTER4 59 

The water budget of each wetland, and the 

subsequent chemical environment and biotic 

community, was determined by the dominant flow 

pattern that contributed water to and removed water 

from the wetlands. Among the semipermanent 

wetlands, the dominant hydrologic feature that 

determined the dissolved salt concentration and the 

biotic community structure was whether or not the 

basin functioned as a closed system or an open 

integrated system with respect to surface flow. 

Among closed basins, the differences in salinity were 

related to the magnitude of seepage losses to ground 

water, which exports dissolved salts. Water loss to 

evapotranspiration concentrated dissolved salts and 

peaked during midsummer when plant respiration and 

temperatures were high, relative humidity was low, 

and warm winds blew persistently. The annual 

hydrologic response to gains and losses of water was 

usually a rapid rise in water levels during spring 

snowmelt and drawdown (declines in water levels) in 

late summer (Winter and Rosenberry, 1995; LaBaugh 

and others, 1996; Poiani and others, 1996). Hydroperiods (the periods when wetlands contain surface 

water) driven by annual moisture deficits facilitated 

the development of concentric bands of emergent 

vegetation dominated by wet-meadow, shallow-marsh, 

and deep-marsh species (Stewart and Kantrud 1972). 

Seasonal wetlands in the Cottonwood Lake area 

were dry by midsummer in most years and the centers 

of the basins were dominated by shallow-marsh plant 

species. In wet years, however, some seasonal 

wetlands held water until freezeup and the basins were 

dominated by deep-marsh plant species. In contrast, 

during dry years when the snowpack was low, the 

seasonal wetlands remained dry and the central zone 

was dominated by low-prairie and wet-meadow 

species. 

Closed (isolated) semipermanent wetlands in 

the area were the most dynamic in response to climate 

trends (fig. 50, A-B). The lack of an outlet caused 

water levels in the closed basins to rise rapidly in 

response to major precipitation events that flooded 

emergent vegetation and, on occasion, upland vegetation (fig. SOC). In contrast, complete drawdown (a 

loss of all wetland water) of the semipermanent 

wetlands by evapotranspiration during periods of 

drought resulted in exposed mudflats, seed germination, and the subsequent dominance of mudflats by 

emergent vegetation. The sequence in the response of 

individual wetlands to drought conditions and the 

magnitude of change that occurred in a given year was 

determined by the volume of water stored in each 

basin and by the persistence of ground-water discharge 

to the basin. Trends in dissolved salt concentrations 

followed water-level changes that responded to precipitation and evapotranspiration (LaBaugh and others, 

1996). 

Trends in climate conditions resulted in the 

semipermanent wetlands being cycled between 

extremes of complete drawdown (1961, 1973-74, 

1976-77, and 1988-92), which initiated seed germination, and flooding ( 1962-66 and 1993-98), which 

drowned emergent vegetation that had become 

established during previous drawdowns. High water 

levels occurred in the closed semipermanent wetlands 

southeast of the Cottonwood Lake area during 

1962-66 (fig. 51) (Eisenlohr and others, 1972). The 

high water levels, once established, persisted and 

caused the closed semipermanent wetlands in the 

Cottonwood Lake area to be dominated by open water 

during 1967-72 (Poi ani and others, 1996) and 1993 to 

the present (2002) (fig. SOB-C). During periods of 

high water levels, the closed semipermanent wetlands 

flooded into the surrounding prairie, eliminating 

emergent aquatic vegetation. 

All wetlands in the Cottonwood Lake area were 

dry during the drought of 1961 as reported by 

Eisenlohr and others ( 1972). The period that followed 

the drought of 1961 ( 1 962-1 966) was one of 

increasing water levels that peaked in 1966 following 

a major March blizzard. During the next 6 years 

( 1967-72), water stored in the closed semipermanent 

wetlands during 1962-66 persisted and hydrologic 

conditions in the wetlands remained relatively stable. 

The wetlands were dominated by open water and 

planktonic algae. The open-water zone, following the 

initial flooding, was characteristically surrounded by a 

narrow fringe of deep-marsh vegetation, primarily 

hardstem bulrush and cattail. Once the closed 

wetlands received a major contribution of water, the 

wetlands persisted because evapotranspiration was the 

dominant pathway for water loss (Eisenlohr and 

others, 1972). During this study, drought caused some 

of the semipermanent wetlands to dry up during the 

growing season, as in the dry periods of 1973-74 and 

1976-77. However, all of the semipermanent wetlands 

were completely dry and dominated by emergent 

vegetation during the drought of 1988-92. 
60 

A 

B 

c 

CHAPTER4 

Figure 50. Wetland P6 on October 1, 1983, 

dominated by deep-marsh vegetation 

established during the drought of 1976-77 

(A); Wetland P6 on October 28, 1997, 

dominated by open water following the 

deluge of 1993-98 (B); Wetland P7 on 

October 5, 1996, dominated by open water 

that flooded a 33-year old aspen stand located 

on the east side of the wetland (C). 
CHAPTER4 

Figure 51. Aerial view during May, I 970, of Wetland Pll (A), wetlands in the Cottonwood Lake area (B) and wetlands studied by 

Eisenlohr and others (1972) (C). 

61 
62 CHAPTER4 

As water levels declined from 1973 to 1992, 

basin substrates were exposed in the semipermanent 

wetlands in a sequence determined by the volume of 

water stored in each basin during 1962-66. Drawdown 

and an increase in dissolved salts caused seed 

germination of typical drawdown species (Stewart and 

Kantrud, 1971; Weller, 1978; van der Valk, 1981) 

when mudflats were exposed during the growing 

season. Annual plants dominated basin substrates 

during the first summer if drawdown occurred during 

the growing season. Annual plant dominance was 

succeeded by perennial plant dominance in subsequent 

years as water levels increased and stabilized. 

All of the closed semipermanent wetlands in the 

Cottonwood Lake area that were open-water basins 

prior to 1973 eventually were converted to dense, 

monotypic stands of emergent vegetation dominated 

by cattail. Wetlands P3 and P4 were the first semipermanent wetlands to dry up and be converted after 

1967. Wetland P4 contained three circular stands of 

hybrid cattail, one located in the center of the basin, 

that survived flooding during 1962-66. During the 

first year of drawdown, 1973, Wetland P4 was 

dominated by a dense stand of alkali blight 

(Chenopodium rubrum) (fig. 52A). During the second 

year, 1974, golden dock (Rumex maritimus) 

dominated this basin along with recently germinated 

Carex spp, sprangletop, hardstem bulrush and cattail 

(fig. 52B). In 1975, a year of rising water levels, wetmeadow and shallow-marsh plant species were 

eliminated from the central zone of the basin and 

cattail dominated the basin (fig. 52C). The cattail 

persisted as a dense monotypic stand for 18 years. The 

deluge of 1993-98 reversed the trend in climate 

conditions and converted Wetland P4 to an open-water 

basin, flooding out emergent vegetation that 

germinated during the drought of 1973-74 and the 

hybrid cattail that survived flooding during 1962-66 

(fig. 53A-C). Although cattail seedlings germinated 

during the first summer of desiccation in 1973, cattails 

did not dominate the basin until1975. The drought of 

1976-77 converted three additional semipermanent 

wetlands (P2, P6, and P7) from open-water basins to 

monotypic stands of cattail, and the drought of 1988-

92 converted the central open-water zone of all 

remaining semipermanent wetlands (P1 and P8) to 

dominant stands of cattail. 

Seed germination in most semipermanent 

wetlands in the Cottonwood Lake area during 

drawdown followed the pattern observed in Wetland 

P4. Wetlands P 1 and P 11, however, were exceptions 

because dissolved salt concentrations in those basin 

substrates following drawdown selectively influenced 

seed germination. 

Rising water levels during 1993-98 flooded out 

emergent vegetation established during the dry periods 

of 1973-74, 1976-77, and 1988-92 and converted 

Wetlands P1, P2, P4, PS, P6, and P7 from monotypic 

stands of emergent vegetation to open-water basins. 

By 1998, the closed semipermanent wetlands in the 

Cottonwood Lake area were dominated by open water 

as previously observed in the spring of 1967; 

maximum water levels in 1998, however, exceeded 

those in 1967. By 2002, Wetlands P2, P4, and PS 

integrated and formed a single wetland, and Wetland 

PI flooded into Wetland T3, causing Wetland T3 to 

temporarily be an extension ofWetland Pl. Water 

levels in closed basins in the study area are controlled 

primarily by precipitation and evapotranspiration, 

which produce dynamic hydroperiods compared to 

those in open basins where water levels do not rise 

much higher than the elevations of their outlets. 

Surface outflow was not a component of the 

water budget of most semipermanent wetlands (P1, 

P2, P4, PS, P6, P7, P9, and P11) in the Cottonwood 

Lake area or of seasonal Wetland T9 because the 

volumes of water contributed to the drainage basins 

were consistently less than the volumes required to fill 

the drainage basins to maximum storage capacity. The 

wetlands, with the exception of Wetland T9, routinely 

accumulated sufficient volumes of water from the 

drainage basins and ground-water discharge to persist 

for extended periods as semipermanent wetlands. 

Wetland T9 is located on a topographic high and due 

to the absence of ground-water input, functioned 

primarily as a seasonal wetland. The lack of surface 

outflow from semipermanent wetlands and the contribution of ground water to the water budget result in an 

accumulation of dissolved salts, which play a role in 

the chemical and biological characteristics of the 

wetlands. 

Wetlands P3, P8, and T2, which have surface 

outlets, were the only semipermanent wetlands that 

contained a significant stand of emergent vegetation 

following the deluge of 1993-98. Of the three open 

semipermanent wetlands in the study area, Wetland P8 

stored the largest volume of water. The elevations of 

the outlets of Wetlands T3 and P2 prevented the water 

level from exceeding the depth tolerance of hybrid 

cattail. 
CHAPTER4 63 

KEY WETLANDS 

The previous discussion provided a general 

description of the response of the Cottonwood Lake 

area wetland complex to climate variability that 

occurred from 1961 to 1998. This section describes 

the unique hydrological, chemical, and biological 

characteristics of five key wetlands (T8, T3, P1, P8, 

and P 11) that are located along a topographic and 

hydrologic gradient (Winter, Chapter 1 of this report). 

WETLANDT8 

Wetland T8, located near the highest part of the 

study area (fig. 54 A), functioned as a ground-water 

recharge basin; ground-water gradients were away 

from the basin for the entire study period. Precipitation and snowmelt runoff were the primary sources of 

water to this wetland; therefore, the water was low in 

dissolved salts. The volume of water stored in the 

basin usually did not exceed the volume lost annually 

to evapotranspiration and recharge; as a result, 

Wetland T8 contained water in the spring and usually 

was dry by midsummer. Dissolved salt concentrations 

in the water column were consistently low and the 

salts were dominated by potassium and calcium 

bicarbonate (LaBaugh and Swanson, 1992; LaBaugh 

and Swanson, Chapter 3 of this report). 

Plant and invertebrate species that occupied 

Wetland T8 reflected a seasonal water regime low in 

dissolved salts (Stewart and Kantrud, 1971; Kantrud, 

Krapu, and Swanson, 1989). Shallow-marsh vegetation dominated by slough sedge and swamp 

smartweed occupied the central zone of the wetland 

(fig. 54B) (LaBaugh and others,1987) along with a 

small stand of reed canary grass (Phalaris arundinacea). Deep-marsh emergents (cattail and bulrush) 

invaded when surface water persisted late into the 

growing season but were eliminated during periods of 

drought. Terrestrial, low-prairie and wet-meadow 

species invaded the central zone of the basin when the 

wetland was dry in early spring following the melting 

ofthe snowpack and the thawing of frozen soils (fig. 

54C). The invertebrate fauna was dominated by 

crustaceans (Anostraca, Conchostraca, Cladocera, and 

Copepoda), snails (Gyraulus circumstriatus, Stagnicola caperata, and Promenetus umbilicatellus ), and 

insects (Culicidae, Chaoboridae, Corixidae, 

Coleoptera, and Odonata). During wet periods when 

surface water persisted into the growing season, 

species that require an annual hydroperiod of extended 

flooding, such as the snails Stagnicola elodes and 

Helisoma trivolvis, colonized the basin. 

Amphibian communities of Wetland T8 also 

showed a marked temporal response to climate-driven 

hydrologic cycles. After the drought of 1988-92, 

amphibians did not reappear in the wetland until 1994 

although the wetland had reflooded the previous year. 

The initial amphibian community was composed 

mostly of boreal chorus frogs (Pseudacris triseriata 

maculata). Northern leopard frogs (Rana pipiens) 

appeared in 1995 and dominated the amphibian 

community through 1996. With prolonged inundation, 

tiger salamanders (Ambystoma tigrinum) greatly 

expanded their population and dominated the 

amphibian community in 1997. The tiger salamanders 

usually are not an important component of the 

amphibian community of Wetland T8 because the 

normal hydroperiod for that wetland involves a 

seasonal drawdown that interferes with the early 

development of salamanders. However, the current 

wet cycle is unusual in that conditions are much wetter 

than normal and Wetland T8 has not undergone a 

seasonal draw down since 1993. 

WETLANDT3 

Wetland T3, located at an intermediate elevation 

in the study area, functioned as a flow-through system 

that receives ground-water discharge and also 

recharges ground water. Ground water usually flowed 

into Wetland T3 along the north, east, and south sides 

and seeped out of Wetland T3 toward Wetland P 1 on 

the west side. Wetland T3 occasionally spilled into 

Wetland P 1 (Rosenberry and Winter, 1997), which, 

during extremely high water conditions (for example, 

during 1993-98), merged with Wetland T3. The 

volume of water stored in Wetland T3 is controlled by 

the elevation of the spill point to Wetland P 1 and 

typically is less than the volume lost annually to 

evapotranspiration and ground-water recharge; as a 

result, Wetland T3 has a seasonal water regime. The 

wetland is typical of all seasonal wetlands in the 

Cottonwood Lake area, with the exception of Wetland 

T9, that have storage capacities that are limited by 

overflow and that commonly become dry. 
64 

A 

CHAPTER4 

Figure 52. Wetland P4 on July 1, 1973, 

during a period of drawdown. A circular 

stand of hybrid cattail that survived 

flooding during 1962-66 is shown in the 

center (A); Wetland P4 on July 10, 1974, 

during a second year of draw down (B); 

aerial view of Wetland P4 on October 30, 

1975, showing three circular stands of 

hybrid cattail that survived flooding 

during 1962-66 and persisted as distinct 

units for 10 years (C). 
A 

c 

CHAPTER4 65 

Figure 53. Wetland P4 on October 30, 

1975, dominated by recently genninated 

cattail. (The persistent clump of hybrid 

cattail located in the center dominates the 

mixed-species stand of recently 

germinated cattail) (A); Wetland P4 on 

July 20, 1979, dominated by a monotypic 

stand of cattail that persisted for 18 years 

(B); Wetland P4 on October 28, 1997, 

during the deluge of 1993-98 that 

integrated Wetlands P2, P4, and P5 (C). 
66 CHAPTER 4 

Figure 54. Wetland T8 on May 5, 1975, 

following flooding (A); Wetland T8 on 

August 1, 1975, dominated by smartweed 

and sloughsedge following drawdown 

(B); Wetland T8 on July 23, 1991, 

dominated by Canada thistle (Cirsium 

arvense) during the drought of 1988-92 

(C). 
CHAPTER4 67 

Although Wetland T3 routinely overflowed, it 

consistently maintained dissolved salt concentrations 

that were higher than those in Wetland T8 (LaBaugh 

and Swanson, 1992). Dissolved salts in Wetland T3 

were dominated by calcium sulfate (LaBaugh and 

Swanson, 1992), but the water type changed from 

calcium sulfate to magnesium sulfate when persistent 

high water drowned emergent vegetation. At times of 

high water levels, the water column was highly turbid 

and dominated by planktonic algae, which caused a 

decline in calcium (LaBaugh and Swanson, 1992). 

The differences between the dissolved salt concentrations and the major-ion water types for Wetlands T3 

and T8 can be attributed to the fact that Wetland T3 

receives ground-water input and Wetland T8 does not 

receive ground-water input. The differences in 

salinity are reflected in the composition of the vegetation in the wetlands; the species in Wetland T3 can 

tolerate a wider range in salinity than the species in 

Wetland T8. 

The central zone of Wetland T3 was dominated 

by the shallow-marsh species sprangletop and slough 

sedge. During periods of drought, as described for 

Wetland T8, the plant species structure of the central 

zone was altered by invasions of species that normally 

occupy terrestrial sites, low prairie, and wet meadow 

(fig. 55A). Sprangletop routinely occupies seasonallyflooded zones of wetlands that receive ground-water 

input and that subsequently tend to have elevated salt 

levels (Stewart and Kantrud, 1971). For this reason, 

sprangletop is present in Wetland T3 but not in 

Wetland T8, which has no ground-water input and 

which has swamp smartweed as one of the dominant 

plants. When high water levels persisted late into the 

growing season in response to frequent and intense 

precipitation events, cattails became established in the 

central zone of Wetland T3 (fig. 55B). 

Invertebrates inhabiting Wetland T3 were 

similar to those in Wetland T8 and were dominated by 

species that respond to seasonal water regimes. 

However, during periods of extremely high water 

levels as in I993-98 (fig. 55C), Wetland PI can 

incorporate Wetland T3; when both wetlands coalesce, 

the invertebrate community is more representative of a 

semipermanent water regime. The amphibian 

community of Wetland T3 likewise is similar to that of 

seasonal Wetland T8 except when Wetland T3 is 

integrated with Wetland Pl. 

WETLANDPl 

Wetland PI is typical of the numerous, closed, 

oligosaline (Cowardin and others, 1979), semipermanent wetlands (Stewart and Kantrud, I97I) that are 

located at intermediate elevations in the prairie 

pothole region. Wetland PI functioned primarily as a 

ground-water discharge basin, although a limited 

amount of water was lost to ground water in late 

summer and in drier years (Winter and Rosenberry, 

I995; Winter, Chapter 1 of this report). Although 

some water loss to seepage occurred during seasonal 

flow reversals in late summer, this water loss was 

negligible compared to water loss by evapotranspiration. On the western side of the wetland, the groundwater gradient occasionally was away from the 

wetland toward Wetland P8, allowing Wetland PI to 

lose water and solutes through the ground-water 

system (Winter and Rosenberry, I995). During the 

drought of 1988-92, water levels declined to well 

below the bottom of the wetland, except on the side 

toward Wetland P6. 

The water type in Wetland P1 was predominately magnesium sulfate, except in 1967 and 1993-94 

when emergent vegetation was flooded and the water 

type changed to magnesium bicarbonate (LaBaugh 

and Swanson, 1992; LaBaugh and others, 1996). The 

differences in dissolved salt concentrations among 

prairie wetlands that do not overflow probably are 

determined by the relative inputs of and losses to 

ground water. The volume of salts brought in by 

ground water and the volume of salts lost by seepage 

to ground water appear to explain the differences in 

the salt concentrations among closed wetland basins 

and, subsequently, the difference in the biotic 

communities supported by the wetlands. 

During wet periods, Wetland PI stored large 

volumes of water (fig. 56A and C). Once the water 

was stored, high water levels persisted until drought 

conditions caused the water levels to decline (fig. 

56B). The high water levels observed in the spring of 

I967 persisted for 6 years until the drought of I973-

74. The water levels then declined until the wetland 

became dry in the fall of I97 4 after the growing 

season. Because the wetland became dry after the 

growing season, seeds germinated only on the narrow 

band of substrate exposed during the growing season 

and the substrate exposed after the growing season 

persisted as a barren mudflat until the wetland 

reflooded in the spring of 197 5. Thus, the plant 
68 

A 

c 

CHAPTER4 

Figure 55. Wetland T3 on July 23, 1991, 

dominated by Canada thistle during the 

drought of 1988-92 (A); Wetland T3 on 

July 27, 1994, showing a small stand of 

cattail (B); Wetland T3 integrated with 

Wetland P 1 on October 28, 1997, during 

the deluge of 1993-98 (C). 
CHAPTER4 69 

community of Wetland PI remained relatively stable 

for IO years until drawdown in the fall of I976. The 

partial drawdown that occurred during the spring of 

I976 established a narrow band of deep-marsh vegetation dominated by cattail and hardstem bulrush. The 

absence of a snowpack in the spring of I977 

minimized runoff to the wetland and the entire 

substrate was exposed during the spring and early 

summer of I977. However, the draw down during 

I977 was of short duration compared to the draw down 

of I988-92. Based on water levels of nearby wetlands 

described by Eisenlohr and others (1972), the entire 

substrate was exposed for the first time since the 

drought of I96I. 

The receding water levels during the spring of 

I977 caused an increase in dissolved salt concentrations in the water column, and only salt-tolerant plant 

species such as prairie bulrush, foxtail barley 

(Hordeumjubatum), hardstem bulrush, and 

sprangletop germinated on the exposed mudflat (fig. 

57 A). During the spring of I977, prior to the wetland 

becoming dry, specific conductance in the water 

column of the shallow isolated pools reached 7, 700 1-lS 

em -

1

. Thus, cattail did not germinate in the summer of 

I977 prior to complete drawdown. After complete 

drawdown, the open-water zone that dominated the 

center of the wetland prior to I977 was replaced by 

shallow-marsh vegetation dominated by sprangletop 

and a few stems ofhardstem bulrush (fig. 57B). The 

shallow-marsh emergents dominated the central 

vegetated zone of the wetland until I980 when the 

sprangletop drowned out in response to persistent high 

water levels(> I m) (fig. 57C). Sprangletop cannot 

tolerate persistent high water for extended periods but 

can dominate semipermanent wetlands for short 

periods following reflooding. Thus, although 

sprangletop in the central zone of Wetland PI was 

eliminated after 3 years of high water, the shallowmarsh species continued to dominate the shallowmarsh zone along the shoreline. The central zone of 

Wetland PI remained open (fig. 58A), surrounded by 

the wide band of deep-marsh vegetation that 

germinated in I976, until the drawdown of I988-92. 

The drawdown of I988-92, in contrast to the 

draw down of I97 6-77, persisted for 4 years, causing 

Wetland PI to become dry and water levels to fall 

below the bottom of the wetland. During I988-92, 

precipitation falling on the dry mudflat diluted salts. 

When the water level fell below the bottom of the 

wetland, for a short period the basin functioned as a 

ground-water recharge basin. This change in function 

reduced dissolved salt concentrations on the exposed 

mudflat such that specific conductance was 2, I80 J.lS 

em -

1 in spring of I992 and allowed cattail to 

germinate, unlike in I977 when only salt-tolerant 

species germinated. As the drought continued, the 

specific conductance of the water declined in response 

to the dilution of salts caused by subsequent precipitation (LaBaugh and others, I996; fig. 34A in Chapter 3 

of this report). Salts were not lost to deflation as was 

observed for some of the saline wetlands in the area 

that were exposed to wind action because seed 

germination on the exposed mudflat and the protection 

by emergent vegetation prevented deflation. The shift 

in hydrologic function thus reduced dissolved salt 

concentrations and altered the species structure of the 

biotic community of Wetland PI. During I988-92, 

cattails dominated the central zone of the wetland. 

Thus, the response of the plant community ofWetland 

PI differed between the draw downs of I97 6-77 and 

I988-92 because the dynamics of the dissolved salt 

concentrations differed. 

Wetlands PI and P8 were the last semipermanent wetlands in the Cottonwood Lake area to be 

dominated by monotypic stands of cattail in the central 

zone during I973-92. The cumulative effect of the 

abundant precipitation that fell from I993 through 

I998 reversed the trend caused by the drawdowns of 

I973-74, I976-77, and I988-92. Cattail flooded out, 

and Wetland PI currently (2002) is dominated by open 

water (fig. 56C). The water levels observed in I998 

exceeded the water levels observed in the spring of 

I967, and Wetland PI flooded into Wetland T3 for the 

first time since I967 (figs. 58B and 55C). 

The deep-marsh zone of Wetland PI usually was 

dominated by hardstem bulrush and broadleaf and 

narrowleaf cattail and their hybrid. The shallowmarsh zone was dominated by sprangletop. A variety 

of annual, biannual, and perennial species germinated 

on the exposed mudflats during drawdown (Poiani and 

others, I996). Prairie bulrush was observed only 

during drawdown and persisted as viable seed during 

extended periods of flooding. Plant communities that 

occupied Wetland PI and their response to the 

dynamic water regimes are described by Poiani and 

others, ( I996). 
70 

B 

c 

CHAPTER4 

Figure 56. Wetland PI on June 1, 1967, 

following a rise in water levels during 

1962-1966 (A); Wetland PI on October 

29, 1992, dominated by deep-marsh 

vegetation following the drought of 1988-

92 (B); Wetland P 1 on September 25, 

1999, dominated by open water following 

a rise in water level initiated by the 

deluge of 1993-98 (C). 
CHAPTER4 

A 

71 

Figure 57. Wetland PI on September 

12, 1977, following drawdown (A); 

Wetland PI on September 9, 1978, 

dominated by shallow-marsh vegetation 

that germinated on the mudflat inside the 

deep-marsh zone (B); Wetland PI on 

July I, 1984, fo llowing flooding that 

eliminated shallow-marsh vegetation in 

the center of the wetland (C). Shallowmarsh vegetation persisted in the seasonally flooded zone shown in the 

background. 
72 CHAPTER4 

The dominance of sprangletop in the central 

zone ofWetland PI following the drought of 1976-77 

facilitated the rapid return to open water in the central 

zone of the wetland because less water is required to 

drown out sprangletop than to drown out cattail. This 

is especially true of hybrid cattail, which can tolerate 

greater water depths than either broadleaf or 

narrow leaf cattail. Once established, hybrid cattails 

can develop monotypic stands that persist for extended 

periods as in Wetland P4 where cattails persisted for 

I8 years (fig. 53 B). Hybrid cattail is relatively new to 

the prairie pothole region (Kantrud, I992). The ability 

of this hybrid to dominate oligosaline wetlands, 

probably to the detriment of bulrush, appears to have 

altered aquatic communities throughout much of the 

prairie pothole region. 

During the summer of I978-79, following the 

drawdown of I976-77 and the establishment of 

shallow-marsh vegetation in the central zone of 

Wetland PI, a large number of semipermanent 

wetlands in the vicinity of the Cottonwood Lake area 

contained a central zone of sprangletop surrounded by 

bands of cattail and hardstem bulrush. The presence 

of shallow-marsh plants in the center of semipermanent wetlands surrounded by deep marsh is not 

common in the prairie pothole region (Stewart and 

Kantrud, I972). 

Because Wetland PI lacked a surface outlet, the 

wetland usually accumulated a large enough volume 

of water to contain water most years. The lack of a 

surface outlet also produced a dynamic water regime 

that influenced dissolved salt concentrations and 

initiated major changes in the plant and animal 

communities of the wetland. In response to climatic 

extremes, the wetland functioned as a ground-water 

discharge basin, when water persisted in the basin, and 

a ground-water recharge basin for short periods when 

the water table dropped below the bottom of the 

wetland during extended periods of drought (Winter 

and Rosenberry, I998). Winter and Rosenberry 

(1998) attributed some of the water loss from Wetland 

PI to transpiration of ground water along the wetland 

perimeter. As a result of the shift in function, although 

only for a relatively short period of time, dissolved salt 

concentrations were altered and a major change in the 

response of the seed bank to draw down was initiated. 

The basin water budget maintained a semipermanent 

water regime and a biotic community that reflected the 

hydroperiod and the dissolved salt concentrations of 

the wetland. 

Invertebrates that occupied Wetland PI are 

typical of species that occupy oligosaline semipermanent wetlands in the prairie pothole region (Kantrud, 

Krapu, and Swanson, I989). Midge larvae (Diptera: 

Chironomidae) are an important element in the 

invertebrate fauna of semipermanent prairie wetlands 

and to diets of waterfowl (Swanson, I977). The 

maximum density of chironomid larvae, principally 

Chironomus ten tans and Glyptotendipes pari pes, in 

Wetland PI exceeded I40,000 individuals m-2 in 

October I983 (Nelson, I989). The invertebrate 

community of Wetland PI differed from the invertebrate communities of Wetlands T3 and T8 primarily 

by the presence of large pond snails (Lymnaea 

stagnalis) and amphipods (Hyalella azteca and 

Gammarus lacustris). As noted for Wetlands T8 and 

T3, crustaceans ( Cladocera and Copepoda) were well 

represented in the invertebrate community of Wetland 

PI (LaBaugh and Swanson, Chapter 3 of this report). 

Amp hi pods (scuds) routinely invade wetlands during 

the summer by clinging to waterfowl and aquatic 

mammals (Swanson, I984) but are eliminated 

annually when seasonal wetlands become dry and 

during drought in wetlands that have a more 

permanent water regime. The reflooding of Wetland 

P 1 in I993 restored the semipermanent water regime, 

but large numbers of H. azteca were not observed until 

3 years later. G lacustris did not appear in Wetland PI 

until I998. The large shallow-marsh zone that 

occupied the northwest bay of Wetland PI was 

dominated by sprangletop and supported invertebrate 

species similar to those in Wetlands T3 and T8. 

Tiger salamanders were an important 

component of the amphibian community of Wetland 

Pl. Boreal chorus frogs, wood frogs (Rana sylvatica), 

and northern leopard frogs also were observed in 

Wetland PI, but tiger salamanders comprised the 

largest proportion of the population, especially during 

wet cycles when the wetland did not undergo significant seasonal drawdowns. Salamanders likely 

influenced the presence of other amphibians in 

Wetland PI, especially frogs, since tadpoles are an 

important salamander food (Lannoo and others, I994; 

Lannoo, I996). 
CHAPTER4 

B 

73 

Figure 58. Aerial view looking south 

toward the Cottonwood Lake area on July 

19, 1989, showing Wetlands P4 (lower 

left), P2 (lower right), and open water in 

the central zones of Wetlands PI (center) 

and P8 (right edge) during the early stage 

of the drought of 1988-92 (A); Wetland 

T I on October 28, 1997, looking east 

toward Wetland Pl into Wetland T3 

located in the background (B); aerial view 

of the Cottonwood Lake area on October 

I, 1976, showing the persistence of open 

water in Wetland P8 located in the lower 

left corner (C). 
74 CHAPTER4 

WETLANDP8 

Wetland P8 (fig. 58C) differed hydrologically 

from Wetland P 1 in that the storage capacity of the 

basin was fixed by the elevation of the surface outlet 

to Wetland P9 (fig. 59 A). Runoff from the contributing drainage basin of Wetland P8 and ground-water 

discharge from the sand deposit in the southern part of 

the Cottonwood Lake area (Winter, Chapter 1 of this 

report) exceeded the storage capacity of the basin in 

most years, causing a high annual turnover of water 

and solutes in the wetland. This high turnover 

stabilized water levels and dissolved salt concentrations that, with the exception of extreme drought, were 

consistently lower than those in Wetland P 1 although 

Wetland P8 is lower in elevation (LaBaugh and others, 

1996; fig. 34B in Chapter 3 ofthis report). 

The water levels, dissolved salt concentrations, 

and plant community of Wetland P8 remained 

relatively stable for 23 years until the drought of 1988-

92 (fig. 59B). The droughts of 1973-74 and 1976-77 

had very little effect on the water levels on Wetland 

P8. By using water levels of the wetlands studied by 

Eisenlohr and others (1972) as proxies for the water 

levels in Wetland P8 prior to 1967, the water levels in 

Wetland P8 probably did not decline appreciably for 

27 years until the drought of 1988-92. The stable 

water levels in Wetland P8 and the associated low 

dissolved salt concentrations provided ideal conditions 

for establishing floating mats of cattail. A floating mat 

dominated the shoreline of the wetland and portions of 

the mat periodically broke loose and drifted across the 

basin due to wind action. The central zone of Wetland 

P8 at maximum water levels exceeded the depth 

tolerance of hybrid cattail, which was maintained only 

by the persistent floating mat connected to the 

shoreline. 

The stable water levels and low dissolved salt 

concentrations of Wetland P8, along with the floating 

mat of cattail, resemble the hydrologic and chemical 

characteristics of wetland basins located east of the 

prairie pothole region in glaciated regions dominated 

by a moisture surplus. Under the influence of a 

moisture surplus, contributed volumes of water 

annually exceed basin storage capacity and water 

levels rise until stabilized by fixed outlets or substantial seepage to ground water. Floating cattail mats are 

not common in much of the prairie pothole region 

because water levels are generally too dynamic to 

permit their establishment and maintenance. The 

dynamic water regimes of closed basins in the prairie 

pothole region periodically drown out cattail that 

germinates on mudflats during complete drawdown. 

During the drought of 1988-92 when Wetland 

P8 experienced complete drawdown, the cattail mat 

was stranded on the substrate and the plants responded 

by rooting in the substrate. High water that followed 

the deluge of 1993-98 approached the depth tolerance 

ofhybrid cattail rooted in the center of the wetland and 

the plants began to flood out (fig. 59C.) 

The chemical characteristics of Wetland P8 also 

differed from those of Wetland Pl. Wetland P8 was 

dominated by magnesium bicarbonate water 

maintained by anaerobic conditions that persisted 

beneath the floating cattail mat allowing sulfate 

reduction to remove sulfur from the ground-water 

inputs (LaBaugh and Swanson, 1992). A major-ion 

change from magnesium bicarbonate to magnesium 

sulfate occurred when ground water entered the 

wetland beyond the stranded cattail mat during the 

drought of 1988-92. 

Wetlands P8 and T3, as stated previously, are 

open basins having fixed outlets. However, unlike 

Wetland T3, which contained water only seasonally, 

Wetland P8 maintained a semipermanent water 

regime, containing water throughout the year in all but 

the driest years. The wetlands also differed in depth 

and in the volume of water stored in each basin. The 

volume of water stored in Wetland P8 exceeded the 

annual loss to evapotranspiration whereas the annual 

loss to evapotranspiration in Wetland T3 exceeded the 

volume of water stored. As a result, Wetland P8 

functioned as a semipermanent wetland dominated by 

a central zone of deep-marsh vegetation and open 

water (Stewart and Kantrud, 1971). 

The macroinvertebrates found in Wetland P8 

were similar to those found in Wetland P 1. Snails, 

Lymnaea stagnalis and Helisoma trivolvis, were 

common as were amphipods, H. azteca and G 

lacustris. The G lacustris, however, were more 

persistent in Wetland P8 than in the other semipermanent wetlands in the Cottonwood Lake area. This 

persistence reflected the stable water conditions and 

strong ground-water influence on Wetland P8. G 

lacustris populations in the prairie pothole region are 

more common in wetlands having stable water levels 

caused by ground-water discharge. The drought of 

1988-92 eliminated amphipods in Wetland P8, but the 
CHAPTER4 

B 

c 

75 

Figure 59. Wetland P8 on July I, 1981, 

dominated by an open-water zone 

surrounded by a mat of floating cattail 

(A); Wetland P8 on October 29, 1992, 

dominated by deep-marsh vegetation 

following drawdown (B); Wetland P8 on 

October 28, 1997, dominated by deepmarsh vegetation persisting in the central 

zone following the deluge of 1993-98 (C). 
76 CHAPTER4 

wet period beginning in 1993 fostered the reestablishment of H. aztec a populations by 1995. G lacustris 

were not observed through 1997. Future long-term 

monitoring should provide insight into the recovery 

period for the G lacustris. 

The amphibian community of Wetland P8 also 

was similar to that of Wetland P 1. Tiger salamanders 

dominated the amphibians captured after 1992 

although chorus, wood, and leopard frogs also were 

present. As mentioned previously, tiger salamanders 

are effective aquatic predators and likely exert considerable influence on frogs through predation on 

tadpoles. 

WETLANDPll 

Wetland P11 (fig. 60A-C) functioned as a 

ground-water discharge basin and supported plants 

and invertebrates that tolerate elevated salt concentrations. Based on hydrologic modeling (Winter and 

Carr, 1980), Wetland P 11 probably does not lose water 

to ground water because the wetland is a discharge 

area for local and regional ground-water flow systems. 

The large ratio of surface area to water volume for 

Wetland P 11 results in evapotranspiration being large 

relative to the volume of water stored in the wetland. 

Similar hydrologic conditions cause wetlands in the 

prairie pothole region to have high concentrations of 

dissolved salts dominated by sodium sulfate (Swanson 

and others, 1988). Water levels in Wetland P11 

remained relatively stable until the drought of 1988-92 

when the wetland became dry. With the return of 

water in 1993-98, the basin was dominated by an 

open-water pool that flooded into the adjacent upland. 

Dissolved salt concentrations were greatly reduced by 

dilution and by the removal of salts by deflation 

during the drawdown of 1988-92 (LaBaugh and 

others, 1996; fig. 34C in Chapter 3 of this report). 

The central zone of Wetland P 11 was never 

dominated by emergent vegetation. Emergent vegetation that did occupy Wetland P 11 was dominated by 

prairie bulrush that established along the shoreline 

where salt concentrations were favorable and the water 

depth did not exceed the depth tolerance for bulrush. 

Spring seeps in topographically low, highly saline 

hydrologic sumps such as Wetland P 11 are consistently lower in dissolved salts than in the lake water 

(Swanson and others, 1988). Seeps along the 

shoreline of Wetland P 11 supported cattail, hardstem 

bulrush, and common reed (Phragmites australis), 

plants that could not tolerate the salt concentrations in 

the lake proper. Clumps of hardstem bulrush and 

cattail have been observed in the center of saline lakes 

where ground-water discharge lower in dissolved salts 

enters the lake proper (Swanson and others, 1988). 

Broadleaf cattail is less tolerant of elevated salt 

concentrations than narrow leaf cattail or their hybrid 

(Stewart and Kantrud 1972). 

When Wetland P11 was dry during 1988-92, 

exposed salt crystals (fig. 60B) that concentrated in the 

dry basin were removed by wind action and deposited 

in the upland (fig. 60C). A portion of the windblown 

salt, however, appeared to have blown beyond the 

drainage basin of Wetland P 11. The flooded openwater zone was dominated by sago pondweed, a 

submerged vascular plant that can tolerate elevated 

salt levels. When the wetland was dry, it remained 

free of plants with the exception of saltwort 

(Salicornia rubra), a salt-tolerant species that invades 

exposed salt flats. 

Invertebrates that inhabited Wetland P 11 were 

dominated by microcrustacea and fairy shrimp 

common in wetlands of intermediate salt content ( 5 to 

15,000 J.!S cm-1). During 1967-87, the salinity of 

Wetland P11 was consistently greater than the salinity 

of other semipermanent wetlands located higher in the 

landscape, and Wetland P 11 was the only wetland in 

which sodium was the most abundant cation. Sago 

pond weed that dominated the open water of Wetland 

P 11 was attractive to fall migrating canvasbacks 

(Aythya valisineria) and tundra swans (Cygnus 

columbianus) (fig. 60A), which were never observed 

on other semipermanent wetlands in the Cottonwood 

Lake area. 

An interesting pattern in the composition of the 

invertebrate community of Wetland P11 was observed 

when the basin refilled following the 1988-92 drought. 

Prior to the flood of 1993, nearly 80 percent of the 

invertebrate fauna was represented by fairy shrimp. 

When the wetland flooded, the community shifted to 

one dominated by other crustaceans ( Cladocera and 

Copepoda) and then gradually shifted to one 

dominated by the amp hi pod H. aztec a. In 1997, the 

amp hi pod G lacustris was observed in Wetland P 11 

for the first time following the 1993 flood, and future 

increases in this species are anticipated as the wetland 

increases in permanence. 
CHAPTER4 

A 

B 

77 

Figure 60. Wetland P 11 on October II , 

1985, visited by tundra swan that feed on 

sago pondweed tubers (A); Wetland PII 

on October 1, 1990, during drawdown 

that exposed salt crystals (B); Wetland 

P11 on June 10, 1991 , duringdrawdown 

(C). Windrows of salt crystals that were 

blown from the basin and deposited in the 

upland southeast of the basin are shown. 
78 CHAPTER4 

The amphibian community of Wetland P 11 also 

reflected the influence of wetland function on 

dissolved salt concentrations. In 1992, an amphibian 

community was not detected in Wetland P 11 · in fact ' ' 

no amphibians were found in the wetland despite an 

intensive effort to document their presence prior to the 

1993 flood. When Wetland P 11 flooded, the reduced 

salt concentrations resulted in the solute concentration 

changing to one fiftieth of what it had been just prior 

to the wetland becoming completely dry in 1992, and 

amphibians recolonized the wetland. In contrast to the 

other wetlands in the Cottonwood Lake area however ' ' Wetland P 11 was immediately recolonized by tiger 

salamanders and animals were found in 1993 immediately after the wetland flooded. The tiger salamander 

population exploded in subsequent years and reached 

a major peak in 1996. Interestingly, though, most of 

the animals captured from 1994 through 1996 were 

large larvae (neonates) that did not metamorphose into 

their terrestrial form. The large invertebrate 

community of Wetland P 11 may have affected the 

physiological response of the tiger salamander and 

caused the animal to remain in the neonate form. In 

1997, the salamander population in Wetland P 11 

declined substantially, the total number of trapped 

animals decreased more than 83 percent; at the same 

time, invertebrate abundance decreased by about 72 

percent. The decline in invertebrates and in tiger 

salamanders in Wetland P 11 may be related to the 

reduced availability of vegetative litter, which 

provides important substrates for production of algae 

that provides a forage base for invertebrates (Euliss 

and others, 1999). With the exception of chorus frogs, 

all other amphibians found in wetlands in the Cottonwood Lake area were also found in Wetland P11. 

Chorus frogs were abundant in Wetland P 11 only in 

1996 following several years of very high water levels 

that coincided with relatively low specific conductance. 

HYDROLOGICAL, CHEMICAL, 

AND BIOLOGICAL INTERACTIONS 

IN A WETLAND COMPLEX AND 

THEBROADERPRAIIDEPOTHOLE 

REGION 

SPATIAL RELATIONS 

Biodiversity within the Cottonwood Lake area 

and within the prairie pothole region in general, can be 

attributed to differences in basin topographic settings 

and to wetland basin interaction with ground water, 

which determines the general salinity of the wetlands. 

Chemical gradients across the wetland complex of the 

Cottonwood Lake area tracked hydraulic gradients 

forming the framework for biological diversity. 

Wetlands located in topographically high areas were 

seasonally flooded. Some of these wetlands received 

discharge from local ground-water flow systems, 

while others functioned as ground-water recharge 

basins, and contained surface water low in dissolved 

salts dominated by potassium and calcium 

bicarbonate. Wetlands located at intermediate 

altitudes in the landscape were seasonally or semipermanently flooded. Some of these wetlands received 

discharge from local ground-water flow systems and 

others functioned as ground-water flow-through 

systems. Wetlands of this type contained moderate 

levels of dissolved salts dominated by magnesium 

bicarbonate and magnesium sulfate. Wetlands located 

in topographically low areas were semipermanently 

flooded and functioned as ground-water discharge 

basins, losing water primarily to the atmosphere. 

Ground-water discharge basins contained elevated 

levels of dissolved salts dominated by sodium sulfate. 

Although salt gradients that occur among closed 

basins within a wetland complex reflect landscape 

features, open basins that discharge surface water are 

exceptions. 

As dissolved salt concentrations increase or 

decrease in response to hydrologic setting, the 

composition of the plant and animal communities that 

occupy each basin shifts. As salt concentrations 

increase, they may exceed the tolerance of saltresistant emergent species such as prairie bulrush; 

such basins often are dominated by barren shorelines 

and open water with submerged vascular plants such 
CHAPTER4 79 

as widgeon grass (Ruppia maritima) (Stewart and 

Kantrud, 1971 ). Brine shrimp (Artemia salina ) and 

widgeon grass occupy basins in which specific 

conductance approaches 35,000 f.lS cm-1 in southcentral North Dakota (Swanson and others, 1988). 

The differences in major-ion structure and dissolved 

salt concentrations for the wetlands accounted for the 

trends in plant communities observed in Minnesota by 

Moyle (1945) and in North Dakota by Stewart and 

Kantrud (1972). Stewart and Kantrud (1971) used 

specific conductance to develop subclasses in their 

classification of prairie wetlands and demonstrated 

that plant-species structure in prairie wetlands is a 

function of specific conductance, which is an index of 

dissolved salt concentration. 

Biodiversity within a wetland complex also is 

influenced by variations in the hydroperiod (water 

permanence), which was used by Stewart and Kantrud 

( 1971) to classify prairie wetlands. The hydroperiod 

structure (the ratio of temporary, seasonal, and 

semipermanent wetlands within a glacial landform) of 

basins that comprise a wetland complex differs 

between the drift prairie, which has a preponderance 

of temporary and seasonal wetlands; the Missouri 

Coteau, which has a greater number of semipermanent 

wetlands; and glacial outwash plains, which have a 

large number of shallow saline wetlands. The 

composition of wetland classes has a major influence 

on the distribution of waterfowl during the breeding 

season, which changes seasonally and annually in 

response to changing climatic conditions that fill or 

desiccate temporary and seasonal wetlands. 

TEMPORAL RELATIONS-- RESPONSE TO 

CLIMATE EXTREMES 

The dynamics of the aquatic communities that 

occupied wetland basins in the Cottonwood Lake area 

during the study period were driven by climate events 

that initiated partial or complete drawdown and 

subsequent flooding of the wetland basins. As the 

climate cycled between drought and deluge, several 

climate events contributed to rapid rises in water 

levels and subsequent changes in plant-community 

structures in closed basins in the study area. The 

record snowpacks during March 1966 and during the 

winter of 1996-97 and the deluge of July 1993 caused 

rapid rises in water levels in response to surface 

runoff. Periods of rising water ( 1962-66 and 1993-98) 

in the Cottonwood Lake area caused closed-basin 

semipermanent wetlands to drown emergent vegetation and to shift the plant community to one dominated 

by submerged vascular plants and planktonic algae. 

Eventually, a narrow, concentric band of deep-marsh 

and shallow-marsh species dominated the newly 

established shorelines. 

Partial or complete drawdown during periods of 

drought exposed basin substrates during the growing 

season. The plant-community composition on the 

newly exposed substrates reflected the species 

structure of the seed bank of each substrate (Stewart 

and Kantrud, 1971; Weller, 1978; van der Valk, 1981; 

Weller, 1981; Kantrud, Krapu, and Swanson, 1989; 

Kantrud, Millar, and van der Valk, 1989; Poiani and 

Johnson, 1989; Poiani and others, 1996). Annuals 

dominated during the first year of drawdown, with 

species structure determined by the salt content of the 

substrate (Stewart and Kantrud, 1971 ). Perennials 

eventually dominated the substrates, usually in 2 to 3 

years following the initial exposure of the mudflats. 

The emergent species structure was determined by the 

time frame of the draw down, which influenced salinity, 

and the water depth that prevailed following 

reflooding of the basin. 

The plant-community response to the impact of 

drought on basin interaction with ground water varied 

with the duration of the drought. Droughts of short 

duration concentrated salts and allowed salt-tolerant 

plants to germinate and eventually dominate the plant 

community; seeds of salt-tolerant species germinated 

in oligosaline wetlands during drawdown, when 

specific conductance in the water column approached 

7,700 J.!S cm-1. In contrast, extended periods of 

drought caused the water levels to fall below the 

bottoms of the wetlands (Winter and Rosenberry, 

1998), and permitted seasonal precipitation to dilute 

salts (LaBaugh and others, 1996) that accumulated in 

the basins during drawdown. When the water levels 

dropped below the bottoms of semipermanent 

wetlands, the functions of the basins shifted from a 

ground-water flow-through or ground-water discharge 

to ground-water recharge. The reduction in salt 

concentration altered the dominant emergent plant 

community and, subsequently, the water depth and 

time required to drown out emergent vegetation and 

reopen the wetlands. Once the central zone of the 

closed semipermanent wetlands was dominated by 

cattail, the cattail persisted until rising water during 

the deluge of 1993-98 exceeded the depth tolerance of 
80 CHAPTER4 

hybrid cattail, the most persistent of the cattail species. 

When the central zone was dominated by sprangletop, 

a shallow-marsh species, wetlands reopened in 4 years 

at a depth that would not have drowned out hybrid 

cattail. The frequency of change in wetland function, 

in response to trends in water levels initiated by 

climate extremes, appears to account for most of the 

variation in the major-ion structure and dissolved salt 

concentrations among the closed wetland basins. 

Sloan (1972) reported that the salt concentrations in 

the wetlands are a function of the net loss of water to 

surface and ground water. The dissolved salt concentrations were altered by (1) precipitation that fell on 

dry exposed substrates, moving the salts into the 

substrate; (2) deflation when dry basins were exposed 

to wind action that blew salts out of the basin; and (3) 

deluge, which diluted the salts. 

The extreme change in climate that occurred 

between the dry period of 1988-92 and the wet period 

of 1993-98 also caused a major shift in the limnological characteristics of eusaline, polysaline and mesosaline wetlands. This major shift, in tum, caused major 

changes in the flora and fauna in the wetlands. Closed 

basins previously dominated by sago pondweed, 

widgeon grass, and brine shrimp (Swanson and others, 

1988) decreased in salinity in response to increases in 

the volume of water and became dominated by 

fresher-water flora and fauna. Some of the closed 

saline lakes that were less than 1 m deep or dry in 

1992 currently (2002) contain 5-7 m of water and 

support fish. The loss of salt-tolerant species caused 

migrating waterfowl that normally concentrate on 

saline wetlands during fall migration to avoid the 

wetlands because the abundant, highly preferred foods 

these wetlands produce during drier periods were no 

longer present or abundantly available. The impact of 

the loss of preferred foods on the physiological 

condition of waterfowl during migration is unknown. 

Some of the closed wetlands shifted to open systems 

in response to the deluge of 1993-98 and subsequently 

exported previously stored dissolved salts. 

Ground water that enters wetland basins 

contributes dissolved salts to closed basins and ground 

water that leaves wetland basins by surface flow or 

ground-water recharge removes dissolved salts from 

the basins. Precipitation that falls directly on a 

wetland basin contributes fewer salts per unit volume 

than runoff from surface flow or ground-water 

discharge. Water that travels through glacial till 

contributes more dissolved salts per unit volume than 

water from other sources. Surface outflow is more 

efficient than ground-water recharge in transporting 

dissolved salts because the clay content of glacial till 

reduces permeability and retards flow (Swanson, 

1990). Sand deposits and fractures in the till, however, 

can expedite the flow of water and the movement of 

the dissolved salts (Winter, Chapter 1 of this report). 

The large volume of water lost to evapotranspiration also affects major-ion concentrations within the 

water column of wetlands. Salts that are least soluble 

precipitate as dissolved salt concentrations increase, 

leaving the most soluble salts to dominate the water 

column. Wetlands in the prairie pothole region of 

south-central North Dakota that retain the highest 

dissolved salt concentrations usually are dominated by 

sodium sulfate (Swanson and others, 1988). Large 

shallow basins that are high in dissolved salt concentrations(> 30,000 J.!S cm-1

) generally do not support 

emergent vegetation and subsequently lose salts by 

wind deflation when substrates are exposed during 

drought (Swanson and others, 1988). 

The response of open semipermanent wetlands 

in the Cottonwood Lake area to climate extremes 

differed from that of closed basins in the area. The 

storage capacity of the open basins was limited 

because the basins spilled when the water level rose 

above the elevation of the surface outlet. The surface 

outflows reduced the variability in water level, 

lowered the dissolved salt concentrations, and 

promoted the growth of floating cattail mats in the 

semipermanent wetlands. The floating cattail mats 

were affected only when extreme drought stranded the 

mats on the bottoms of the wetlands and allowed the 

mats to root into the dry substrate; subsequent 

reflooding drowned the stranded cattail mats rooted in 

the substrate and reopened the vegetation canopy as 

described for the closed basins. The dynamic water 

regimes of the prairie pothole region prevent the 

dominance of wetlands by floating mats of vegetation 

common to areas of moisture surplus where wetland 

water levels are relatively stable and dissolved salts 

are diluted. Hence, flooding that drowned emergent 

vegetation in the closed basins had less impact in the 

open basins. 

Open wetland basins that periodically overflow 

(for example, Wetland P8) export salts. The salt 

content varies as a function of the time required to 

replace the volume of water stored in the basin with an 

equal volume of precipitation, runoff, and groundwater discharge (turnover rate). As turnover rates 
CHAPTER4 81 

increase, the dissolved salt concentration in the water 

column can be expected to approach the concentration 

of the dominant contributing water source. The 

dominant water source will vary, however, with 

climatic conditions that alter the ratio of runoff to 

ground-water discharge. 

Wetlands in the Cottonwood Lake area that 

stored less water than lost by seepage and evapotranspiration dried up each summer and functioned as 

seasonal wetlands. Seasonal wetlands that functioned 

as ground-water flow-through basins had higher 

dissolved salt concentrations and were dominated by 

different salt and plant species than wetlands that 

functioned primarily as ground-water recharge basins. 

The biological communities of the seasonal wetlands 

reflected salt concentrations that varied in relation to 

each wetland's position in the ground-water flow 

system and its exchange with atmospheric water. 

Some seasonal wetlands in the prairie pothole 

region were not affected by a fixed outlet. These 

wetlands (for example, Wetland T9) occupied closed 

basins and routinely stored a volume of water that was 

greater than what was lost to evapotranspiration and 

ground-water recharge. Wetlands of this type are 

highly dynamic compared to seasonal wetlands 

maintained by a fixed outlet and, during periods of 

above-average precipitation, may shift to a water 

regime often resembling that of semipermanent 

wetlands. When this occurs, the central zone of the 

wetland shifts from having a dominance of shallowmarsh species to having a dominance of deep-marsh 

species, and the wetland classification changes from 

seasonal to semipermanent. Other seasonal wetlands 

that were maintained by fixed outlets, but that held 

water longer into the summer and fall during wet 

periods, were dominated by deep-marsh species such 

as bulrush and cattail. During periods of drought 

when the seasonal wetlands were dry during the 

growing season, the trend reversed with plants 

normally occupying low-prairie and wet-meadow 

zones (Stewart and Kantrud, 1971) colonizing the 

wetlands. 

The dynamic nature of wetlands in the Cottonwood Lake area excludes species of vertebrates and 

invertebrates that cannot tolerate periodic desiccation. Wetland aquatic invertebrates that are periodically eliminated from the wetlands during extended 

periods of drought use a number of methods to recolonize following seasonal or annual drawdowns induced 

by droughts. However, the recolonization of the 

wetlands by flying insects capable of rapidly recolonizing temporary and seasonal wetlands following 

normal seasonal drawdowns may be much slower 

following extended periods of drought because fewer 

flooded wetlands are available to provide refugia for 

the recolonizing stocks (Swanson, 1984). A few 

permanent lakes in and near the Cottonwood Lake 

area retain water during periods of severe drought but 

most have high dissolved salt concentrations and 

support a fauna that is salt tolerant and may not be 

physiologically capable of colonizing freshwater 

habitat (Swanson and others, 1984). George Lake 

(Salt Lake), located northwest of Streeter, North 

Dakota, in Kidder County, exceeds 33m in defth and 

its water has a conductivity of20,000 J..LS cm-

(Swanson and others, 1988). This lake supports 

Copepoda, Odonates, and Coleoptera but is devoid of 

fish and most freshwater invertebrates. 

Flightless invertebrates face even greater 

challenges when recolonizing freshly flooded 

wetlands. Common recolonization mechanisms 

include eggs and cysts resistant to drying and freezing, 

diapause, aestivation, waterproof secretions, 

epiphragms (snails), burrowing, and passive dispersal. 

Passive dispersal mechanisms include wind (Pennak, 

1989), the digestive tracts ofbirds (Proctor, 1964, 

Proctor and others, 1967), and clinging to more mobile 

fauna (Swanson, 1984). Ostracods and clams have 

been observed clinging to migrating Hemiptera and 

Coleoptera (Fryer, 1974), and amphipods can be 

carried in the feathers of waterfowl (Segerstrale, 1954; 

Rosine, 1956; Swanson, 1984). Peck (1975) observed 

H azteca and G. lacustris on the fur of muskrat 

( Ondatra zibethicus) and beaver (Castor canadensis). 

However, the drainage of wetlands increases the 

distance between wetlands that contain water and may 

disrupt transporting mechanisms or delay introductions. 

Fish and decapods were never found in wetlands 

in the Cottonwood Lake area, but leopard frogs, 

chorus frogs, wood frogs, and tiger salamanders were 

abundant, especially during periods of high water. 

Although several species of toads were found in the 

area (Wheeler and Wheeler, 1966), toads were not 

seen from 1967 through 1991 or from 1992 through 

1997 despite an intensive trapping program for 

amphibians on each wetland in the area. Only one 

western painted turtle ( Chrysemys picta belli), approximately 15 em in length, was found during 32 years of 

observations in the area. The turtle was observed 
82 CHAPTER4 

north ofWetland P1 during July 1985 and was moving 

toward Wetland P2. Although absent from the Cottonwood Lake area, fish, decapods, and turtles were 

commonly found in the Pipestem Creek area, located 

12.9 km to the east of the study area. 

Muskrats were common in the Cottonwood 

Lake area during periods of high water but were 

absent during extreme drought when all basins in the 

area were dry. During the drought of 1988-92, all 

wetlands in the vicinity of the Cottonwood Lake area 

were dry most of the time, but muskrats likely 

survived the drought in a few permanent lakes located 

in the Missouri Coteau and in the James River and 

Pipestem Creek located east of the Missouri Coteau. 

When water returned to the wetlands during 1993-98, 

muskrats repopulated the Cottonwood Lake area, but 

the high water eventually removed most of the deepmarsh vegetation that is attractive to muskrats and 

their populations are currently declining. Due to the 

lack of emergent vegetation in semipermanent 

wetlands during periods of high water (1962-67 and 

1993-98), winter denning by muskrats was restricted 

primarily to bank burrows. During 1974-87, the 

Cottonwood Lake area was attractive to muskrats 

because cattail dominated during drawdown and some 

of the basins contained sufficient water depth to 

~upport a wintering population. Mink (Mustela vis on), 

m general, followed the pattern ofuse described for 

muskrats but were less dependent on water depth for 

winter survival. 

With the exception of the ubiquitous tiger 

salamander, aquatic predators (for example, fish) were 

absent from wetlands in the Cottonwood Lake area. 

The area is a habitat for breeding waterfowl, and 

abundant invertebrate populations in the shallow 

waters are readily available for waterfowl food. 

Breeding pairs and hens with broods have been 

observed to concentrate on wetlands that support an 

abundant invertebrate population and will feed at night 

to take advantage of this food source (Swanson and 

Sargeant, 1972). 

IMPLICATIONS FOR WETLAND 

MANAGEMENT 

BIODIVERSITY AND PRODUCTIVITY 

Wetlands can be managed through water-level 

manipulations, preservation policies that maintain 

wetland-complex integrity, and restoration technology. 

Wetland vegetation also has been manipulated by 

chemical and mechanical methods and through 

grazing and burning designed to open wetlands 

dominated by dense stands of emergent vegetation. 

Dense monotypic stands of cattail, for example, 

discourage use of a wetland by waterfowl and 

encourage use by roosting blackbirds that depredate 

sunflowers (Weller, 1981; Kantrud, 1986). 

Knowledge of the physical factors that determine 

biodiversity and productivity in wetland complexes is 

needed to develop wetland-management strategies. 

Landscape features, together with prevailing climate 

conditions, determine the biotic communities that 

occupy a prairie pothole wetland complex. Landscape 

position and climate determine the hydrologic 

functions of the individual wetlands within a complex, 

including their hydroperiods, their water chemistry, 

and the composition of wetland classes and subclasses 

within the complex. A wide variety of wetland types, 

in tum, determines the spectrum of aquatic plant and 

invertebrate communities that are available to attract 

and support a variety of wildlife species. 

The moisture deficit characteristic of the climate 

in the prairie pothole region in North Dakota 

maintains an abundance of temporary and seasonal 

wetlands and nonintegrated (closed) semipermanent 

wetlands and lakes. The wet-meadow and shallowmarsh species that dominate the temporary and 

seasonal wetlands and the dynamic species structure 

of the semipermanent wetlands, which is related to salt 

gradients, provide the basic framework for the 

biodiversity and productivity of wetland complexes in 

the prairie pothole region. Preserving and restoring 

wetland complexes that are capable of maximizing 

biodiversity in the prairie pothole region requires 

knowledge of the landscape ecology and the response 

of biotic communities to the hydrologic and chemical 

gradients that are related to the landscape features. 

Glaciated landscapes, dominated by abundant, 

heterogeneous depressions, set the stage for dynamic 
CHAPTER4 83 

wetland complexes that are biologically diverse and 

highly productive under the influence of a semiarid 

climate. Wetlands that have seasonal flooding of 

emergent vegetation (wet-meadow, shallow-marsh, 

and occasionally terrestrial vegetation) each spring 

produce abundant invertebrate populations that 

respond to the shallow, warm water rich in 

decomposing organic matter. Semipermanent 

wetlands cycle between periods of flooding and 

desiccation in response to climate extremes and 

periodically produce abundant invertebrate populations that respond to flooded, decomposing, emergent 

vegetation. The decomposing vegetation and 

subsequent release of nutrients in the wetlands attracts 

and supports invertebrates for years following inundation. Wetland complexes composed of a balance of 

temporary, seasonal, and semipermanent wetlands 

(Stewart and Kantrud, 1971) provide ideal habitats for 

breeding waterfowl because the temporary and 

seasonal wetlands provide pair habitat and the 

semipermanent wetlands provide renesting and brood 

rearing habitat. Temporarily and seasonally flooded 

zones in the semipermanent wetlands provide plant 

and animal communities similar to those provided by 

the temporary and seasonal wetlands, but the zones in 

the semipermanent wetlands are very transient and 

they respond to the dynamic water levels caused by 

climate variations. 

The dynamic water regimes characteristic of 

midcontinent climate cycles routinely recycle wetland 

communities from drawdown to prolonged flooding; 

such regimes produce biological communities that are 

highly productive and similar to those of newly 

flooded reservoirs (Euliss and others, 1999). 

Prolonged inundation decreases the availability of 

plant nutrients, whereas oxidative processes that 

accompany drawdowns facilitate nutrient release and 

ultimately foster the development of plant communities that make significant contributions to the nutrient 

and detritus pool upon reflooding. The sharp increase 

in wetland productivity when wetlands reflood 

following a dry phase is the basis for artificially 

flooding and draining wetlands to enhance waterfowl 

populations (Cook and Powers, 1958; Kadlec and 

Smith, 1992) and for the modern-day practice of moist 

soil management (Fredrickson and Taylor 1982). The 

basic underpinnings of the water regimes of prairie 

wetlands relate to nutrient releases from the aerobic 

decomposition of accumulated macrophyte litter by 

terrestrial hyphomycetes (Barlocher and others, 1978). 

The water regimes of prairie wetlands differ from 

those of reservoirs, however, in that the climate-driven 

cycles for the wetlands are routinely repeated, rather 

than being a single event such as the initial filling of 

reservoirs (Jenkins, 1961 ). 

WETLAND PRESERVATION AND 

RESTORATION 

The most effective management approach for 

maintaining biodiversity in the prairie pothole region 

is to preserve and restore wetland complexes that have 

been degraded or drained. Wetland-preservation 

strategies should consider wetland complexes as a 

whole and need to focus on the important wetlands 

that cannot be created because of their complex 

hydrology. Wetland-preservation strategies need to 

also consider processes that degrade wetlands and 

have the potential to eliminate wetland basins. 

Wetland-complex degradation can take many forms, 

some of which are subtle and disguised by extended 

timeframes. Deposition of wind borne and water 

borne silt in wetlands, for example, is a subtle 

accumulative process that is disguised each spring as 

vegetative growth resumes. Selective removal of 

wetland types that comprise a wetland complex, such 

as the removal (draining) of seasonal wetlands, will 

alter the hydrology of a complex by removing groundwater recharge basins from the complex. Diverted 

water then will concentrate in the remaining basins in 

the watershed and potentially influence biodiversity 

by converting the closed basins to integrated open 

systems as the basins respond to increases in the 

volume of water stored. This process has the potential 

for converting closed basins that cycle between open 

water and drawdown to open basins that stabilize 

water levels, trapping waterborne silt and 

subsequently promoting a dominance of hybrid cattail 

that responds to silted basins of decreased depth. As 

depth decreases in response to silt deposition, the 

hybrid cattail can no longer be eliminated by flooding 

if the maximum pool elevation is fixed by a surface 

outlet. Cattail-dominated wetlands are common in the 

drift prairie where open basins have been exposed to 

silt deposition for extended periods (Swanson, 1992). 

In wetlands that have lost storage capacity from silt 

deposition, cattail can only be eliminated naturally for 

short periods during severe drought. The relatively 

recent appearance of hybrid cattail in the prairie 
84 CHAPTER4 

pothole region (Kantrud, 1992) has tempered the 

impact that a rise in water levels can have on dense 

monotypic stands of emergent vegetation dominated 

by cattail; the hybrid cattail tolerates greater depths 

and salt concentrations than the broadleaf cattail 

(Stewart and Kantrud, 1971 ). 

To maintain biodiversity in the prairie pothole 

region, wetland-restoration programs are essential for 

wetland complexes that have been degraded or 

drained. Ground-water flow-through basins that are 

drained, for example, continue to receive groundwater discharge that subsequently evaporates and 

deposits salts. These drained flow-through basins are 

prime candidates for restoration because they are not 

productive of agricultural crops due to saturated soils 

and high dissolved salt concentrations. Drained 

ground-water recharge basins, however, are productive 

of agricultural crops, and special incentives will be 

required to restore those wetlands. When restored, 

both types of wetlands provide prime wildlife habitat; 

thus, their restoration is essential to the goal of 

targeting wetland complexes with restoration 

programs. 

Management techniques that are capable of 

manipulating water levels and dissolved salt concentrations have great potential for restoring prairie 

wetlands. Through water-level manipulations, natural 

drawdown and flooding cycles that make prairie 

wetlands so productive can be duplicated. By 

installing control structures in restored wetlands, water 

can be cycled between extremes in a short timeframe, 

with less dependence on prevailing climate conditions, 

providing adequate water supplies are available. 

Drawdown and reflooding (for example, moist soil 

management; Fredrickson and Taylor, 1982) can be 

used to expose mudflats to germinate prolific seedproducing plants in the wetlands. 

Although controlled drawdowns have been used 

extensively to produce plants that are attractive to 

waterfowl, water-level manipulations designed to 

benefit invertebrate communities and increase production of aquatic invertebrates, such as midges (Chironomidae) and Cladocera, that are attractive to waterfowl 

(Swanson, 1977; Euliss and Grodhaus, 1987) have not 

been fully utilized. As water levels increase in closed 

seasonal wetlands during wet periods, emergent 

vegetation drown and populations of midges and other 

invertebrates increase. When precipitation declines, 

shorter seasonal hydroperiods return and receding 

water levels result in invertebrates concentrating in the 

shrinking pools of water. Waterfowl and shorebirds 

respond to the concentrated, high-protein, food 

resources provided by the invertebrates by foraging 

heavily in the shallow wetlands. Swanson ( 1977) 

observed breeding pairs and hens with broods 

responding to dense populations of midges produced 

in recently flooded wetlands, barnyard ponds, wastestabilization ponds, and recently flooded impoundments. 

Drawdown also can be used to increase the 

availability of benthic midge larvae to breeding and 

migrating waterfowl by concentrating the larvae in 

shallow waters (Swanson and Meyer, 1977). During 

emergence, midges concentrate on the water surface, 

and hens with broods have been observed feeding at 

night on the abundant, highly available, food 

(Swanson and Sargeant, 1972). Breeding pairs also 

have been observed foraging heavily at night on the 

emerging midges in the prairie wetlands. 

A variety of wetland classes can be duplicated 

with restoration programs if adequate water supplies 

are available. However, the ability to manage 

dissolved salt concentrations through water manipulation, is limited because salts are routinely exported in 

the water discharged to achieve drawdown. Thus, to 

maintain the high dissolved salt concentrations 

required by plant and animal species, such as widgeon 

grass and brine shrimp, that are attractive to 

waterfowl, (Swanson and others, 1984), restored 

wetlands can be allowed to draw down and concentrate salts through evapotranspiration. 

If adequate water control is possible, restored 

wetlands have the potential for duplicating many of 

the aquatic communities observed to benefit 

waterfowl in the prairie wetlands. However, watercontrol structures very often have design limitations 

that prevent complete drawdown, and many structures 

are designed to deeply flood wetlands to eliminate 

persistent emergent deep-marsh plants. Research on 

the relation between waterfowl, water control, and 

prairie wetland ecology will provide insight into the 

hydrological, chemical, and biological conditions that 

produce optimal waterfowl habitat. 

WATERFOWL 

The advantage of a wetland complex to 

waterfowl is that each basin is unique hydrologically 

and chemically and the basins respond to climatic 
CHAPTER4 85 

trends somewhat differently over time. Waterfowl 

exploit the abundant high-protein invertebrate foods 

that are produced in the prairie wetlands (Swanson and 

Duebbert, 1989), and temporary and seasonal wetlands 

contribute to the biological diversity of the wetland 

complexes and play an important role in the ecology 

of migrating and breeding waterfowl. Because 

breeding waterfowl require isolation to maximize 

body mass and reduce intraspecific strife, paired birds 

vigorously compete for optimal habitat and can 

expend significant body energy spacing themselves 

from other pairs of the same species. The lost body 

energy reduces the amount of energy available for egg 

production. During the breeding season, paired drakes 

routinely chase paired hens to remove competing pairs 

from the feeding sites and to provide ample space 

among breeding pairs for courtship. Breeding hens 

require access to readily available high-protein food 

resources to support egg development and have been 

observed to successfully complete five clutches of 

eggs, after previous nests have been destroyed, 

provided the food resources are available. 

Temporary and seasonal wetlands that produce 

abundant invertebrate populations early in the spring 

provide an ideal habitat for breeding waterfowl, 

especially for dabbling ducks that are restricted to 

feeding on or near the water surface or in shallow 

water. Diving ducks can effectively feed in the deeper 

waters of semipermanent wetlands but also use 

seasonal wetlands during spring migration. The 

semipermanent wetlands provide an ideal habitat for 

breeding birds as the breeding season progresses and 

are especially valuable once the seasonal wetlands 

become dry and emerging insects become more 

available in the semipermanent wetlands. Semipermanent wetlands also provide habitat for waterfowl 

broods and often provide the only midseason habitat 

available for brood rearing. The wetlands provide the 

high-protein foods needed by broods during early 

development and by molting ducks after the breeding 

season to support feather development. Some 

waterfowl species also require high-protein foods to 

develop stored reserves required for fall migration. 

Occasionally, fall precipitation will refill the 

temporary and seasonal wetlands that can provide 

these foods and the high-energy foods required for 

staging and migrating waterfowl and shore birds. 

During drought conditions, when temporary and 

seasonal wetlands are dry early in the breeding season, 

breeding waterfowl fly north of the prairie pothole 

region to reach stable water or they concentrate on 

large permanent prairie lakes and do not breed. When 

seasonal wetlands are dry during the breeding season, 

waterfowl production is severely impaired (Swanson 

and Duebbert, 1989). Thus, wetland complexes 

provide stability to breeding waterfowl populations 

except during periods of extreme drought when the 

wetlands are completely dry. Droughts, however, 

periodically reset the biological communities of the 

semipermanent wetlands and, thus, maintain the high 

productivity of prairie wetlands. 

Nonintegrated (closed) prairie pothole wetland 

complexes provide ideal habitats for nonpiscivorous 

birds during the breeding season because the wetlands 

maintain an aquatic habitat free of most vertebrate 

aquatic predators that compete for invertebrate foods 

with laying females and their broods (Swanson and 

Nelson, 1970; Bouffard and Hanson, 1997; Cox and 

others, 1998). Open wetland complexes that eventually flow into permanent lakes and rivers that support 

fish are frequently colonized by fish during wet 

periods. Fathead minnows (Pimephales promelas) 

have been known to invade open wetlands during 

spring runoff and persist for several years, dominating 

the wetland fauna until being eliminated by winterkill 

or summer dessication. Tile drainage systems used in 

some areas of the prairie pothole region to drain 

wetlands also provide a corridor for fish invasions 

from associated drainage systems. Fish were not 

present in the Cottonwood Lake area during this study 

because the wetlands were not integrated, periodically 

contained elevated dissolved salt concentrations, were 

occasionally dry, and were anaerobic or often frozen to 

the bottom of the wetland during the winter months. 

Saline lakes, with their unique flora and fauna, 

support a variety of waterfowl and shore birds that are 

not found in fresh-water wetland complexes located 

higher in the landscape. A moisture deficit, therefore, 

contributes to the biological diversity and productivity 

of the prairie pothole region by supporting flora and 

abundant invertebrate fauna not found in regions of 

moisture surplus. Stable water levels in regions 

having a moisture surplus tend to support fish and 

wetlands dominated by emergent vegetation that are 

less attractive to most species of breeding waterfowl 

and shore birds found in the prairie pothole region. 
86 CHAPTER4 

CONCLUSIONS 

Biotic communities that were present in the 

wetlands in the Cottonwood Lake area, at any point in 

time, were determined by a dynamic physical environment that set boundaries on species structure. Each 

wetland was unique in its hydrogeochemical response 

to the physiographic setting and climate trends and in 

the biological community it produced. The synergistic 

effects ofhydroperiods and dissolved salt concentrations established boundaries on wetland species 

structure and, subsequently, on biological diversity 

within the wetland complex. The position that each 

wetland occupied within the landscape with respect to 

topographic and hydrologic gradients determined the 

hydrologic function of the wetland. The hydrologic 

regime and the subsequent chemical environment were 

determined by wetland basin, climate, and ground 

water interactions. The semiarid conditions characteristic of the Cottonwood Lake area maintained seasonal 

wetlands and resulted in semipermanent wetlands 

cycling between extremes of flooding and desiccation. 

Hydroperiods determined the number and area of 

vegetative zones that occupied each wetland, and 

dissolved salt concentrations determined the species 

structure of the biotic communities that dominated 

each zone. Major-ion concentrations were determined 

by the hydrologic functions of the wetlands and were 

modified by the hydroperiods. Biological diversity 

within the wetland complex was maintained by a 

wetland class structure that reflected variations in the 

hydroperiods and the dissolved salt concentrations. 

The spectrum of wetland classes and subclasses that 

comprised the wetland complex determined the 

diversity of the plant and invertebrate communities 

and the avian and other vertebrate communities of the 

prairie wetlands. 

To be reproductively successful, waterfowl 

require wetlands that provide isolation and abundant, 

highly available, high-protein food resources. 

Restoring and preserving wetland complexes in the 

prairie pothole region to maximize productivity and 

biodiversity requires an understanding of the 

landscape ecology and the response of wetland biotic 

cmnmunities to hydrologic and chemical gradients 

that are a result of landscape position. The deluge of 

1993-98 that followed the severe drought of 1988-92 

caused a major decline in dissolved salt concentrations 

in the prairie wetlands in south-central North Dakota. 

The shift in aquatic community structure from one that 

tolerated elevated salt concentrations to one that 

reflected a relatively fresh-water environment altered 

the biological community of the wetlands in a specific 

and predictable fashion. It would be of value to 

document the timeframe required to achieve the 

volumes of water and the dissolved salt concentrations 

that persisted in closed prairie wetlands prior to 1988. 

The timeframe required to produce the aquatic 

community structure that existed prior to the deluge of 

1993-98 would provide additional insight into prairie 

wetland ecology. The time required to evaporate 

water during the 1993-98 wet period and to import 

salts lost to deflation during 1988-92 could be considerable, perhaps a decade or more. 

East of the prairie pothole region in areas of 

moisture surplus, surface water is abundant and 

relatively stable but waterfowl pair densities are low. 

Although waterfowl species structure varies to some 

degree between areas of moisture extremes, the value 

of the dynamic prairie wetlands in maintaining North 

American waterfowl populations is not disputed. The 

future of waterfowl production in North America lies 

in maintaining the integrity of the sensitive prairie 

pothole wetland complexes to insure the availability of 

productive aquatic habitats. Overcoming the effects of 

drainage and siltation are major challenges to 

preserving the productivity and biodiversity of the 

prairie wetlands as well as to the waterfowl populations of North America. 

ACKNOWLEDGMENTS 

Appreciation is extended to James W. LaBaugh, 

Donald 0. Rosenberry, and Thomas C. Winter for 

extensive consultation and critical review of this 

manuscript. 

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CHAPTER4 89 

Table 5. Species composition, occurrence (percent), and average canopy cover for vegetation in mesic upland sites in the Cottonwood 

Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent- 4 percent, 3 = 5 percent- 24 percent, 4 = 25 

percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent- 94 percent, and 7 = 95 percent- 100 percent. 

Qccyrren£~ (uerc~nt) Canou1: cov~r 

Achillea millefolium L. 7 2 

Agropyron caninum L. 10 2 

Agropyron repens (L.) Beauv. 47 2 

Ambrosia psilostachya DC. 17 2 

Andropogon gerardii Vitman 10 3 

Anemone canadensis L. 43 2 

Apocynum cannabinum L. 23 3 

Artemisia ludoviciana Nutt. 3 2 

Asclepias ovalifolia Dene. 3 2 

Aster simplex Willd. 68 2 

Bromus inermis Leyss. 33 2 

Calamagrostis stricta (Timm.) Koel. 13 2 

Carex L. 83 3 

Cirsium arvense (L.) Scop. 13 3 

Elaeagnus commutata Bemh. 10 3 

Fragaria virginiana Duchn. 3 2 

Galium boreale L. 17 3 

Gentiana andrewsii Griseb. 3 3 

Glycyrrhiza lepidota Pursh 30 3 

Helianthus maximilianii Schrad. 20 3 

Heuchera richardsonii R. Br. 10 3 

Hordeum jubatum L. 3 3 

Liatris ligulistylis (A. Nels.) K. Schum 20 1 

Liatris punctata Hook. 3 2 

Lilium philadelphicum L. 3 3 

Mentha arvensis L. 7 2 

Muhlenbergia richardsonis (Trin.) Rydberg 3 1 

Onosmodium molle Michx. 3 3 

Panicum virgatum L. 10 2 

Poa pratensis L. 70 3 

Populus tremuloides Michx. 3 4 

Potentilla anserina L. 50 3 

Rumex crispus L. 3 3 

Solidago canadensis L. 27 3 

Solidago mollis Bartl. 10 3 

Solidago rigida L. 3 3 

Sonchus arvensis L. 63 2 

Spartina pectinata Link 23 3 

Stachys palustris L. 17 2 

Symphoricarpos occidentalis Hook. 20 3 
90 CHAPTER4 

Table 6. Species composition, occurrence (percent), and average canopy cover for vegetation in xeric upland sites in the Cottonwood 

Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = < 1 percent, 2 = 1 percent - 4 percent, 3 = 5 percent - 24 percent, 4 = 25 

percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent- 94 percent, and 7 = 95 percent- 100 percent. 

Occurrence (Dercent) Canog~ cover 

Achillea millefolium L. 35 2 

Agropyron caninum L. 40 2 

Agropyron cristatum (L.) Gaertn. 5 3 

Agropyron smithii Rydb. 15 2 

Ambrosia psilostachya DC. 20 2 

Amorpha canescens Pursh 5 2 

Anemone patens L. 20 3 

Artemesia frigida Will d. 40 2 

Artemesia ludoviciana Nutt. 25 3 

Aster falcatus Lindl. 50 2 

Astragalus crassicarpus Nutt. 5 2 

Astragalus jlexuosus (Hook.) G. Don. 15 2 

Bouteloua gracilis (H.B.K.) Lag. Ex Griffiths 25 3 

Bromus inermis Leyss. 35 3 

Carex L. 95 2 

Cerastium arvense L. 15 2 

Chenopodium album L. 10 

Chrysopsis villosa (Pursh) Nutt. 25 3 

Cirsium jloodmanii (Rydb.) Arthur 10 3 

Comandra umbellata (L.) Nutt. 5 2 

Echinacea angustifolia DC. 5 3 

Erysimum asperum (Nutt.) DC. 5 2 

Gallium boreale L. 10 2 

Grindelia squarrosa (Pursh) Dun. 10 3 

Hedeoma hispidum Pursh 5 2 

Koeeleria pyramidata (Lam.) Beauv. 25 3 

Liatris punctata Hook. 25 2 

Phlox hoodii Rich. 20 3 

Poa pratensis L. 55 3 

Potentilla norvegica L. 5 2 

Psoralea argophylla Pursh 5 2 

Psoralea esculenta Pursh 10 3 

Ratibida columifera (Nutt.) Woot & Standi 15 2 

Rudbeckia hirta L. 5 3 

Senecio integerrimus Nutt. 5 2 

Solidago missouriensis Nutt. 10 3 

Stipa comata Trin. & Rupr. 50 3 

Stipa spartea Trin. 15 3 

Stipa viridula Trin. 35 3 

Vicia americana Muhl. Ex Willd. 5 1 

Viola pedatifida G. Don 5 2 
CHAPTER4 

Table 7. Species composition, occurrence (percent), and average canopy cover for vegetation in north aspect upland sites in the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent- 4 percent, 3 = 5 percent- 24 percent, 

4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent- 94 percent, and 7 = 95 percent- 100 percent. 

Occurrence (Dercent) CanO,Ul;: cover 

Achilla millefolium L. 20 2 

Agropyron caninum L. 5 1 

Agropyron repens (L.) Beauv. 25 2 

Amalanchier alnifolia Nutt. 20 3 

Ambrosia psilostachya DC. 5 1 

Anemone canadensis L. 10 3 

Andropogon gerardii Vitman 10 3 

Apocynum cannabinum L. 10 3 

Artemesia ludoviciana Nutt. 25 2 

Aster falcatus Lindl. 10 2 

Bromus inermis Leyss. 75 2 

Carex L. 45 2 

Cirsium arvense (L.) Scop. 30 2 

Crataegus rotundifolia Moench. 5 3 

Elaeagnus commutata Bemh. 5 3 

Gallium boreale L. 35 3 

Geum triflorum Pursh 5 2 

Lithospermum canescens (Michx) Lehm. 5 2 

Parthenocissus vitacea (Knerr.) Hitchc. 5 2 

Poa pratensis L. 50 3 

Populus tremuloides Michx. 30 4 

Prunus virginiana L. 5 6 

Ratibida columnifera (Nutt.) Woot. & Standi. 10 2 

Salix exigua Nutt. 10 3 

Solidago canadensis L. 20 3 

Solidago mollis Bartl. 10 3 

Solidago rigida L. 15 3 

Stipa spartea Trin. 5 2 

Symphoricarpos occidentalis Hook. 65 4 

Toxicodendron rydbergii (small) Greene 5 2 

Urtica dioica L. 15 2 

91 
92 CHAPTER4 

Table 8. Species composition, occurrence (percent), and average canopy cover for vegetation in south aspect upland sites in the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent- 4 percent, 3 = 5 percent- 24 percent, 

4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent- 94 percent, and 7 = 95 percent- 100 percent. 

Qccyrrence (gercent} Canog~ cover 

Achillea millefolium L. 45 2 

Agropyron caninum L. 35 2 

Allium textile A. Nels. & Macbr. 5 1 

Ambrosia psilostachya DC. 25 1 

Amorpha canescens Pursh 10 3 

Andropogon gerardii Vitman 5 1 

Andropogon scoparius Michx. 10 4 

Anemone cylindrica A. Gray 20 2 

Anomone patens L. 20 3 

Artemesia ludoviciana Nutt. 55 3 

Aster falcatus Lindl. 25 2 

CarexL. 40 2 

Cirsium jloodmanii (Rydb.) Arthur 20 2 

Echinacea angustifolia DC. 10 2 

Elaeagnus commutata Bemh. 5 3 

Galium boreale L. 15 2 

Helictotrichon hookeri (Scribn.) Henr. 15 2 

Lactuca oblongifolia Nutt. 20 2 

Poa pratensis L. 100 4 

Rosa arkansana Porter 20 2 

Senecio integerrimus Nutt. 10 3 

Solidago rigida L. 20 2 

Sphaeralcea coccinea (Pursh) Rydb. 5 2 

Stipa comata Trin. & Rupr. 15 3 

Stipa viridula Trin. 15 3 

Trogopogon dubius Scop. 5 

Vicia americana Muhl & Willd. 15 2 

Viola pedatifida G. Don 10 2 
CHAPTER4 93 

Table 9. Species composition, occurrence (percent), and average canopy cover for vegetation in east aspect upland sites in the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent- 4 percent, 3 = 5 percent- 24 percent, 

4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent- 94 percent, and 7 = 95 percent- 100 percent. 

Q~curr~n~~ (D~rc~ntl Ci!D!!DI co1::~r 

Agropyron caninum L. 15 1 

Agropyron repens (L.) Beauv. 55 3 

Amorpha canescens Pursh 10 3 

Apocynum cannabinum L. 5 1 

Artemesia ludoviciana Nutt. 60 2 

Aster falcatus Lindl. 20 

Bromus inermis Leyss. 25 2 

CarexL. 45 1 

Crataegus rotundifolia Moench. 15 3 

Dichanthelium leibergii (Vasey) Freckmann 10 2 

Echinacea angustifolia DC. 10 2 

Elaeagnus commutata Bernh. 5 3 

Galium boreale L. 75 3 

Helianthus rigidus (Cass.) Desf. 10 4 

Lactuca oblongifolia Nutt. 40 1 

Linum perenne L. 10 2 

Linum rigidum Pursh 5 1 

Onosmodium molle Michx. 5 3 

Poa pratensis L. 100 4 

Prunus virginiana L. 10 3 

Rosa arkansana Porter 20 2 

Solidago missouriensis Nutt. 25 2 

Sphaeralcea coccinea (Pursh) Rydb. 5 3 

Symphoricarpos occidentalis Hook. 25 3 
94 CHAPTER4 

Table 10. Species composition, occurrence (percent), and average canopy cover for vegetation in west aspect upland sites in the Cottonwood Lake area in July 1989. Canopy cover values range from 1 to 7; 1 = <1 percent, 2 = 1 percent- 4 percent, 3 = 5 percent- 24 percent, 

4 = 25 percent- 49 percent, 5 =50 percent- 74 percent, 6 = 75 percent- 94 percent, and 7 = 95 percent- 100 percent. 

CanOl!):: cover Occurrence £uercent) 

Agropyron caninum L. 50 2 

Agropyron cristatum (L.) Gaertn. 5 3 

Agropyron repens (L.) Beauv. 10 2 

Agropyron smithii Rydb. 15 2 

Andropogon scoparius Michx. 15 3 

Anemone cylindrica A. Gray 10 2 

Amorpha canescens Pursh 5 3 

Artemesia frigida Will d. 25 2 

Artemesia ludoviciana Nutt. 10 2 

Asclepias viridiflora Raf. 5 2 

Aster falcatus Lindl. 60 2 

Bouteloua gracilis (H.B.K.) Lag ex Griffiths 25 2 

Bromus inermis Leyss. 15 3 

Calamovilfa longifolia (Hook.) Scribn. 45 2 

Calylophus serrulatus (Nutt.) Raven 5 2 

Campanula rotundifolia L. 10 2 

Carex L. 95 3 

Echinacea angustifolia DC. 20 2 

Elaeagnus commutata Bemh. 10 2 

Helianthus rigidus (Cass.) Desf. 50 2 

Koelaria pyrimidata (Lam.) Beauv. 10 3 

Lactuca oblongifolia Nutt. 15 2 

Liatrus punctata Hook. 15 2 

Linum rigidum Pursh 5 2 

Melilotus officina/is (L.) Pall. 5 

Mirabilis hirsuta (Pursh) MacM. 5 2 

Oxytropis lambertii Pursh 5 3 

Physalis virginiana P. Mill. 5 3 

Poa pratensis L. 65 3 

Potentilla arguta Pursh 5 3 

Potentilla pennsylvanica L. 5 2 

Ratibida columifera (Nutt.) Woot & Standi. 10 3 

Rosa arkansana Porter 35 2 

Solidago mol/is Bartl. 10 3 

Stipa spartea Trin. 35 3 

Symphoricarpos occidentalis Hook. 10 3 

Solidago rigida L. 25 3 

Sphaeralcea coccinea (Pursh) Rydb. 10 2 

Stipa comata Trin. & Rupr. 55 2 
CHAPTERS 

SIMULATION OF HYDROLOGY AND VEGETATION 

DYNAMICS OF PRAIRIE WETLANDS IN THE COTTONWOOD 

LAKE AREA 

Karen A. Poiani, The Nature Conservancy of Hawaii, Honolulu, Hawaii 

W. Carter Johnson, South Dakota State University, Brookings, South Dakota 

ABSTRACT 

Several generations of spatial simulation 

models of water depth and of wetland vegetation 

have been developed on the basis of long-term 

data sets for the Cottonwood Lake area in North 

Dakota. WETSIM 1.0 and 2.0 were developed 

for semipermanent wetlands in the area to 

simulate temporal and spatial dynamics of water 

depth and vegetation cover. Despite the use of 

simple model structure and algorithms, the 

models accurately simulated the general characteristics of the wetland vegetation cycle. Initially, 

the models were used to assess the potential 

effects of climate change on wetland and wildlife 

resources, but the models have wider applicability 

for the management of natural and restored 

wetlands. The most recent model version is 

WETSCAPE. WETSCAPE is based on the 

surface-water submodel in WETSIM 2.0 but 

includes multiple wetland basins and a groundwater submodel necessary to simulate the 

hydrology of temporary and seasonal wetlands. 

INTRODUCTION 

The development and use of simulation models 

has increased the understanding of complex ecosystems (Jorgensen, 1983). Although the models 

necessarily simplify highly complex ecological 

processes and interacting components, they also help 

elucidate relations between the structure and function 

of ecosystems and are useful in exploring past and 

future dynamics of ecosystems. Developing simulation models usually is more practical and less 

expensive than conducting field experiments, particularly for large systems; however, the models must be 

thoroughly evaluated and tested before results are 

deemed reliable for use in management decisions and 

planning. 

Various simulation models have been developed 

to manage freshwater and coastal wetlands (Costanza 

and Sklar, 1985; Mitsch and others, 1982, 1988). 

These models include energy- and nutrient-budget 

models, tree-growth or gap-dynamic models, habitatsuccession or spatial ecosystem models, hydrology 

models, and economic models (Mitsch and others, 

1982). For example, several gap-dynamic, treegrowth models have been developed for southern 

wetland forests (Phipps, 1979; Pearlstine and others, 

1985). These models "grow" individual trees on 

various sized plots and incorporate the effects of 

flooding, depth to water table, shade tolerance, 

overtopping and crowding, and probability of death 

and reproduction (Phipps, 1979). One such gap model 

(FORFLO) was used to explore the potential effects of 

a river diversion project on the bottomland forest 

communities in the Santee River floodplain in South 

Carolina (Pearlstine and others, 1985). The model 

predicted a loss of as much as 97 percent of the 

existing bottomland forest under the new hydrologic 

95 
96 CHAPTERS 

regime. The model also showed that an alternative 

water release schedule could retain much of the native 

cypress-tupelo forest. 

Several spatially-defined wetland succession 

models have been developed for use in wetland 

management and impact assessments (Browder and 

others, 1985; Boumans and Sklar, 1990; Costanza and 

others, 1990; Lee and others, 1992; Ellison and 

Bedford, 1995). These models simulate vegetation 

changes in grid cells on the basis of biotic and abiotic 

ecosystem processes. The models use different 

mathematical approaches, including probabilistic (for 

example, Browder and others, 1985), process-based 

(for example, Costanza and others, 1990), and rulebased (for example, Lee and others, 1992) calculations. The Coastal Ecological Landscape Spatial 

Simulation (CELSS) model is an example of a habitatsuccession model used to examine both natural 

changes and human-induced impacts to a large 

marsh/estuarine complex in south Louisiana (Sklar 

and others, 1985; Costanza and others, 1990). 

Management of northern prairie wetlands for 

wildlife (Swanson and Duebbert, 1989) depends on a 

thorough understanding of the climate-driven vegetation and hydrologic cycles that these wetlands exhibit 

(Kantrud and others, 1989; Swanson and others, 

Chapter 4 of this report). The processes underlying 

the cyclic patterns have been extensively described in 

van der Valk and Davis (1978), van der Valk (1981), 

and Weller (1994). The first mathematical simulation 

model of prairie wetland vegetation and hydrology 

was based on the conceptual model of van der Valk 

( 1981) and field data from the Cottonwood Lake area. 

Initially, the Cottonwood Lake area model was used to 

assess the potential effects of human-induced climate 

change on wetland and wildlife resources (Poiani and 

Johnson 1991, 1993b, Poiani and others, 1995) but the 

model also has potential for wetland management. 

This chapter discusses the general structure and testing 

of the model and the work performed to date on the 

assessment of potential climate change and gives an 

example of how the model can guide the management 

of natural and restored wetlands. 

SPATIAL SIMULATION MODEL 

STRUCTURE AND DEVELOPMENT 

A spatially-defined, rule-based simulation 

model was developed for semipermanent wetlands in 

the Cottonwood Lake area (Poiani and Johnson, 

1993a). The Cottonwood Lake area was chosen 

because of its extensive, long-term data sets for water 

levels, air temperature, precipitation, and vegetation 

(Winter and Carr, 1980; LaBaugh and others, 1987). 

The model was developed primarily using data for 

semipermanent Wetland Pl. A seed bank study of 

semipermanent Wetlands PI and P4 also was 

conducted (Poiani and Johnson 1988, 1989). 

The first model (WETSIM 1.0) consisted of 

interacting hydrologic and vegetation submodels. The 

hydrologic submodel calculated daily water levels 

during the growing season (May-October) on the basis 

of air temperature and precipitation. The model 

estimated the initial spring water level from total 

spring precipitation. To illustrate, the water-level 

change from late fall to May 1 was calculated from the 

following empirically-derived equation (P < 0.001, R2 

= 87.7): water-level change= 1.68 + 4.59 * sprpre, 

where sprpre = March and April direct precipitation + 

estimated snowmelt runoff (all values are in centimeters). 

Data for Wetland P 1 indicated that contributions 

from snowmelt runoff were significant only when the 

March maximum snow depth was greater than 30 em. 

Therefore, snowmelt runoff was calculated for years 

when the snow depth was greater than 30 em by 

determining the water equivalent for the March 

maximum snow depth (a 1 : 1 0 ratio of equivalent water 

to snow depth was assumed) and multiplying the water 

equivalent by 1.5 to account for drifting into the basin 

(Shjeflo, 1968). 

Water-level changes during the growing season 

were calculated daily. Hydrologic inputs were from 

direct precipitation and estimated runoff from heavy 

summer or fall rains. An analysis of the daily waterlevel changes for Wetland P1 showed that for rainfall 

events greater than 2.4 em, the water level often rose 

in excess of direct precipitation input. This excess rise 

in water level, attributed to runoff, was calculated 

from an empirical, nonlinear equation for precipitation 

events greater than 2.4 em: runoff= 0.164 * precipl.25 

(all values are in centimeters). Outputs due to 
CHAPTERS 97 

evapotranspiration were calculated using a modified 

version of the climatic water-budget method of 

Thomthwaite and Mather (1957). Calculations of 

evapotranspiration using this method relied on daily 

air temperature, latitude, day length, a heat index that 

was calculated from monthly mean temperature, and a 

vegetation scalar (Poiani and Johnson, 1993a). The 

vegetation scalar was an empirically-calibrated coefficient derived from comparisons of calculated 

evapotranspiration versus actual water loss in Wetland 

P 1. Daily values of evapotranspiration calculated 

using the Thomthwaite method were increased by 25 

percent to better represent actual losses. 

In the hydrologic submodel, the elevation of the 

bottom of the wetland was set as the lower limit for the 

water level. Negative water levels (for example, the 

elevation of the water table) were restricted to upland 

and wetland fringe cells, and water levels in the 

deepest part of the basin were assumed to remain near 

ground level. As shown in Chapter 1 of this report, the 

water table under the central part of Wetland Pl fluctuated several decimeters during 5 consecutive dry years 

( 1988-92). While ground-water processes were not 

considered directly in the submodel, calculations of 

runoff and evapotranspiration included inputs to and 

outputs from the local ground-water system. Improvements to ground-water components were made in 

subsequent versions of WETSIM 1.0. 

The vegetation submodel calculated the amount 

and distribution of emergent vegetation and open 

water in a wetland. Topographic characteristics of a 

basin were determined from a Geographic Information 

System (GIS) by interpolating surveyed point 

elevations. Within the submodel, a wetland basin and 

some of the surrounding upland were represented by a 

grid having uniform cells. The cell size was 

determined according to the size of the wetland and 

the appropriate resolution for vegetation processes. 

Simulations of semipermanent Wetlands Pl and P4 in 

the Cottonwood Lake area had a cell size of 9.3 m2. 

The topographic characteristics determined from the 

GIS and the water levels generated by the hydrologic 

submodel were provided to the vegetation component, 

and the water depth in each cell was calculated 

monthly (water depth= water level- ground-surface 

elevation). 

An initial vegetation type was assigned to each 

cell and was thereafter updated monthly during the 

growing season. In general, the vegetation composition for each grid cell at any time, t, was a function of 

COV1 = f(WD1,WD1_b LENWD1, LOC, COV1_b t), 

where COV =the vegetation type, WD =the water 

depth, LENWD = the time period at that water depth, 

and LOC =the cell location (for example, simulating 

the 'vegetative spread' of an emergent zone). 

The vegetation submodel used eight vegetation 

types (table 11). Four of the types-upland vegetation, combined wet-meadow/shallow-marsh 

emergents, deep-marsh emergents, and open 

water-represented relatively permanent zones and 

corresponded to a specific range of water depths. The 

remaining four types-seedlings, mixed species of 

drawdown plants (for example, mixed plants), mixed 

species of shallow- and deep-marsh emergents (for 

example, mixed emergents ), and exposed 

soil-represented the more transient vegetation types 

present during the dry phase of the vegetation cycle. 

Both submodels were developed and calibrated 

using 1979-89 data for Wetland Pl. Initial parameter 

estimates were based on methods and values in the 

literature and personal observations (Poiani and 

Johnson, 1993a). Simulations ofthe hydrology and 

vegetation for Wetland Pl were performed with firstcut versions of the submodels. The output was 

evaluated by comparison with actual data, and, based 

on the comparisons, several calculations and coefficients in both submodels were adjusted to produce 

more accurate results (Poiani and Johnson, 1993a). 

The simulations reported below were based on the 

latter version ofWETSIM 1.0, unless otherwise 

indicated. 

WETSIM 1.0 also was modified (WETSIM 2.0) 

to produce a 30-year simulation. Modifications 

(Poiani and others, 1996) included (1) seed germination restricted late in the growing season (for example, 

seeds germinate in 95 percent of the dry cells in May 

and June, 30 percent in July and August, 10 percent in 

September, and 0 percent in October); (2) the addition 

of stochastic components to kill emergents by flooding 

and to establish upland, wet-meadow/shallow-marsh, 

and deep-marsh types; (3) the use of the BlaneyCriddle method (McGuinness and Bordne, 1972) to 

compute evapotranspiration; and (4) the use of EPIC, a 

stand-alone soil-water simulation model (Sharpley and 

Williams, 1990), to generate surface runoff and 

subsurface inflow to the wetland. 
98 CHAPTERS 

Table 11. Rules for changes in vegetation submodel. COV =vegetation type, WDC =water-depth category, MON =month, LENWDC = 

time period in water-depth category (months), LOC =cell location. Water-depth categories: (1) <-55 em, (2) -55 to -10 em, (3) -9 to 3 em, 

(4) 4 to 19 em, (5) 20 to 55 em, and (6) >55 em. OW= open water, DM =deep-marsh emergents, MSM =wet-meadow/shallow-marsh 

emergents, UPL =upland vegetation, SEEDL =seedlings, MIXPLT =mixed young plants, MIXEM =mixed species of shallow- and deepmarsh emergents, EXPSOIL =exposed soil. 

( 1) Wetland draw down: 

lfMON =MAY or JUN and COV = OW or EXPSOIL and WDC = 1 or 2 or 3 then COV = SEEDL (95 percent) or COV = 

EXPSOIL (5 percent) 

lfMON = WL or AUG and COV =OW or EXPSOIL and WDC = 1 or 2 or 3 then COV = SEEDL (30 percent) or COV = 

EXPSOIL (70 percent) 

lfMON = SEP and COV =OW or EXPSOIL and WDC = 1 or 2 or 3 then COV = SEEDL (10 percent) or COV = 

EXPSOIL (90 percent) 

lfMON = OCT and COV = OW or EXPSOIL and WDC = 1 or 2 or 3 then COV = EXPSOIL (1 00 percent) 

(2) Fate of drawdown: 

If COV = SEEDL and WDC = 1 or 2 or 3 then COV = MIXPLT 

If COV = SEEDL or EXPSOIL and WDC = 4 or 5 or 6 then COV = OW 

If COV = MIXPLT and WDC = 5 or 6 then COV = OW 

(3) Spring following drawdown: 

If COV = MIXPLT and MON =MAY and WDC = 1 then COV = MSM 

If COV = MIXPLT and MON =MAY and WDC = 2 or 3 or 4 or 5 then COV = MIXEM 

If COV = MIXPLT and MON =MAY and WDC = 6 then COV = OW 

( 4) Flooding of mixed emergents: 

lfCOV = MIXEM and WDC = 4 or 5 or 6 and LENWDC> 15 then COV =OW (35 percent) 

( 5) Spread of deep marshl: 

IfWDC = 3 or 4 or 5 and LENWDC > 4 and LOC =next to cell with COV = DM then 

COV = DM (95 percent) 

(6) Spread of wet meadow/shallow marshl: 

lfWDC = 2 and LENWDC> 12 and LOC =next to cell with COV = MSM then COV = MSM (95 percent) 

(7) Establishment of open water: 

IfWDC = 6 and LENWDC> 12 then COV =OW (85 percent) 

(8) Establishment of upland: 

lfWDC = 1 and LENWDC> 12 then COV = UPL (95 percent) 

1

This change was permitted to occur only once per growing season (for example one cell neighborhood spread). 

VALIDATION AND TESTING 

WETSIM 1.0 parameters were tested using a 

data set for semipermanent Wetland P4, which is 

similar to Wetland P 1. Initial conditions of the 

emergent vegetation, open water, and water levels 

were from actual data (1979). Three 11-year simulations were run for (1) the water-level changes in 

Wetland P4 from 1979-89, (2) the vegetation changes 

in Wetland P4 from 1979-89 using actual water levels, 

and (3) the vegetation changes in Wetland P4 from 

1979-89 using water levels calculated from the 

hydrologic submodel. The first simulation evaluated 

the hydrologic submodel, the second evaluated the 

vegetation submodel without confounding error from 

hydrologic calculations, and the third evaluated the 

combined performance of both submodels. 

To assess model performance, output from the 

three simulations was compared to actual historic data. 

Although the vegetation submodel included eight 

vegetation types, only six (open water, deep marsh, 

mixed emergents, mixed plants, seedlings, and 

exposed soil) could be evaluated from the historic 

aerial photos. The wet-meadow/shallow-marsh and 

upland types were not clearly discernible on the 

photos and, therefore, could not be quantified. Details 

of those model evaluations can be found in Poiani and 

Johnson (1993a) and are summarized below. 
CHAPTERS 99 

HYDROLOGIC SUBMODEL 

The hydrologic submodel satisfactorily 

calculated the water-level changes in Wetland P4 for 

most years. Years with 'average' conditions were 

represented most accurately. Calculated water levels 

were within 5 em of observed values 46 percent of the 

time and within 10 em of observed values 75 percent 

of the time. However, calculations of potential 

evapotranspiration were underestimated late in the 

growing season during years when the wetland was 

nearly dry. The method of estimating potential 

evapotranspiration did not consider directly the effects 

of water and sediment temperatures, which increase 

evaporation as a wetland dries up. Calculations of 

spring refill also fell short of actual peaks in years 

after Wetland P4 was nearly dry. Therefore, adjustments to evapotranspiration calculations were made in 

WETSIM2.0. 

VEGETATION SUBMODEL 

In general, the vegetation submodel closely 

simulated changes in the amount and distribution of 

emergent vegetation and open water in Wetland P4. 

Calculated values of open water ranged from 0 to 8 

percent and actual values ranged from 0 to 7 percent, 

generally with the correct spatial distribution. 

However, seeds germinated on exposed mudflats too 

late in the growing season in the vegetation submodel. 

These areas in the actual wetland remained bare 

because a dry bed occurred late in the growing season 

and detritus impeded germination. Therefore, adjustments to germination seasonality were made in 

WETSIM 2.0. The timing of emergent growth and 

death (for example, conversions between deep-marsh 

emergents and open water) also was not always 

accurate although the percent and distribution were 

often correct. However, model stochasticity in 

WETSIM 2.0 improved the performance of the model. 

OVERALL MODEL 

In general, the model accurately represented the 

observed water-level and vegetation changes in 

Wetland P4 during the test period. The typical 

seasonal cycle of spring refill and summer draw down 

in prairie wetlands was reproduced. The 1979-89 test 

period, however, did not include extremely wet 

climatic conditions but, rather, included a number of 

slightly below average rainfall years followed by 

several years of drought. 

As previously discussed, model performance 

also was evaluated for a wider range of weather 

conditions (the complete weather cycle) by modifying 

the model to produce a 30-year simulation for Wetland 

Pl. WETSIM 2.0 (Poiani and others, 1996) depicted 

known wet-dry periods during 1961-92 (fig. 61) and 

simulated the wettest period prior to 1993 ( 1966-70) 

and the moderately wet periods (1975, 1978-80, and 

1983-86) accurately. However, water volumes in 

1981-82 and 1987 were underestimated. The four 

known low-water periods (1961, 1973-74, 1976-77, 

and 1988-1992) were reproduced (fig. 61), and waterlevel declines were remarkably accurate for some of 

those periods. Simulated vegetation dynamics 

generally were realistic for low water periods except 

in those years when simulated water levels differed 

from actual water levels by more than 10 to 20 em. 

Figure 61. Observed water volume in Wetland P 1 compared 

to calculated water volume obtained using WETSIM 2.0 

model, 1961-92. 

CLIMATE CHANGE STUDIES 

WETSIM 1.0 also has been used in several 

climate impact studies (Poiani and Johnson, 1991, 

1993b; Poiani and others, 1995). All simulations for 

this study were performed for semipermanent Wetland 

PI using climate data for 1979-89 adjusted for a 

double-C02 climate. Climate adjustments included 

(1) projections from the Goddard Institute for Space 

Studies (GISS) climate model ('GISS simulation'), 

(2) a range of possible temperature (+2°C and +4°C) 
100 CHAPTERS 

and annual precipitation (-20, -10, 0, +10, and +20 

percent) changes ('sensitivity simulations'), and 

(3) a +2°C increase in temperature combined with a 

1 0-percent increase or decrease in total growing 

season precipitation that varied by season (all months, 

spring, summer, and fall) ('seasonal simulations'). The 

simulated water-level and vegetation changes obtained 

using the climate adjustments were compared to the 

simulated water-level and vegetation changes obtained 

using data for 1979-89 with no adjustments ('current 

climate'). The same starting water level (558.54 m 

above sea level) and initial vegetation conditions 

(8,000 m2 emergent cover and 8,300 m2 open water) 

were used in all simulations. 

GODDARD INSTITUTE FOR SPACE 

STUDIES (GISS) SIMULATION 

Simulations were performed for Wetland P 1 

using temperature and precipitation projections from 

the GISS general circulation model (GCM). GCMs 

provide an initial, albeit limited, assessment of the 

sensitivity of climate to atmospheric additions of 

greenhouse gases (Kittel and others, 1995). Output 

from the large grid cells of the GISS model was 

interpolated by Takle and Zhong (1990) to the 

Jamestown station located about 40 km from the study 

area. Projected changes in temperature and precipitation varied by month (table 12), and daily values for 

each month were adjusted accordingly. Initially, 

simulations were run with the original version of the 

vegetation submodel in which seeds could germinate 

on exposed mudflats at any time during the growing 

season (Poiani and Johnson, 1993a). 

Marked differences in wetland hydrology 

occurred between the GISS and current climate 

simulations (Poiani and Johnson, 1993b ). The water 

levels in Wetland PI were significantly lower in the 

greenhouse gas climate simulations, and, although the 

water levels rose each spring, the wetland dried up 

almost every year (fig. 62). Peak water levels were 

almost 0.5 m lower than in the current climate simulations. Differences in the ratio of vegetation to open 

water also occurred between the two simulations 

(Poiani and Johnson, 1993b ). The open-water area 

declined slightly in the current climate simulations due 

to drought late in the decade (fig. 63A). In contrast, in 

the GISS simulations, the open-water area decreased 

quickly from the initial 51 percent to 0 percent by year 

4 (fig. 63B). 

The area of wet-meadow/shallow-marsh vegetation also declined in the GISS simulations and went 

from 4,600 to 2,100 m2 (fig. 63B). The area for this 

vegetation type expanded almost 20 percent (from 

4,600 to 5,400 m2

) in the current climate simulations 

(Poiani and Johnson, 1993b). The implications of the 

differences in the ratios of vegetation to open water to 

the quality of waterfowl breeding habitat in the prairie 

pothole region were discussed in Poiani and Johnson 

( 1991 ). Other studies using statistical models suggest 

more droughts occurring in a future greenhouse gas 

climate, resulting in fewer breeding ponds and much 

smaller duck populations (Larson, 1994, 1995; 

Sorenson and others, 1998). 

To illustrate the effect of seedling germination 

on vegetation dynamics and the use of the model in 

exploring related questions, the GISS simulation was 

rerun in WETSIM 1.0 using a modified vegetation 

submodel that restricted germination late in the 

growing season (see earlier section "Spatial Simulation Model - Structure and Development"). Wetland 

vegetation dynamics were significantly different in the 

Table 12. Adjustments to daily temperature and precipitation data as projected by the GISS general 

circulation model for Jamestown, North Dakota. Data from Takle and Zhong (1990). Details in Poiani 

and Johnson (1993b). 

Month 

March 

April 

May 

June 

July 

August 

September 

October 

Temperature increase 

(OC) 

not used 

not used 

3.1 

3.2 

3.2 

3.3 

4.9 

4.3 

Precipitation change 

(percent) 

+29 

+ 8 

+ 7 

+ 5 

+ 3 

+ 19 

-17 

-4 
CHAPTERS 101 . 

GISS simulations when late-season seedling germination was restricted. For example, the wetland did not 

become vegetated with mixed emergents in the center 

of the basin by year 4 (fig. 64) as in the initial simulation (fig. 63B), and more than 1,300 m2 of open water 

was still present in the wetland in year 8 (fig. 64). 

Many cells in the center of the basin did not become 

colonized by vegetation because either water levels in 

the wetland dropped too late in the season to allow 

germination or drawdown plants were killed by high 

water the following spring (fig. 62). A small area ( 450 

m2

) of mixed emergents did become established in 

year 7. By the end of the simulation period, the openwater area had declined significantly and the vegetative spread of the deep-marsh zone covered most of 

the wetland basin (fig. 64). These results indicate that 

a simple parameter such as the timing of seedling 

germination may strongly affect the overall vegetation 

cycle in semipermanent prairie wetlands. The results 

also illustrate the use of a simulation model for 

exploring potential impacts to wetlands. 

558.9 

{f) 

Cl:...J 558.7 

Ww 

~> Ww 558.5 ~...J 

z<t: -W 558.3 w· Cfl ow 

::J> 558.1 ~0 -co 

~<( 557.9 <( 

557.7 

1 

I 

I 

Enhanced 

" ''I I 

\ I \ 

\'\1 \ : \ ~ I I I II 

,,, '/ 

2 3 4 5 6 7 8 9 10 11 

YEAR OF SIMULATION 

Figure 62. Model output of water-level changes in Wetland 

PI for an 11-year simulation under current climate and 

enhanced greenhouse gas climate conditions. Temperature 

and precipitation adjustments for the enhanced greenhouse gas 

climate were from the GISS general circulation model. The 

entire wetland was dry at approximately 557.7 meters above 

sea level. (From Poiani and Johnson, 1993b). 

SENSITIVITY SIMULATIONS 

To supplement the results obtained on the basis 

of the GCM projections, 10 hypothetical climate 

change simulations (+2°C or +4°C temperature 

combined with - 20, - 10, 0, +10, or +20 percent 

precipitation) were performed for Wetland P 1 using 

WETSIM 1.0. The ranges of temperature and precipitation changes are the approximate ranges used in 

other climate models for the United States central 

grasslands (Houghton and others, 1990). The 

sensitivity simulations elucidated several additional 

important aspects of wetland behavior. The 

hydrologic processes were, in general, more sensitive 

to temperature and precipitation changes than the 

vegetation processes (Poiani and Johnson, 1993 b). 

Water levels ranged from relatively high to very low in 

the various simulations. In contrast, nearly all (9 of 

the 1 0) GISS simulations eventually produced more 

frequent low-water periods and emergent plant colonization in the wetland basin than what was produced by 

the current climate simulations. The wetland went 

from a nearly balanced ratio of vegetation to open 

water to a completely vegetated basin. 

The GISS simulations also showed that there 

may be a threshold temperature beyond which precipitation changes become less important to wetland 

hydrology. Precipitation change had much less effect 

on wetland water levels with a temperature increase of 

+4°C than with a temperature increase of +2°C (Poiani 

and Johnson, 1993b). Wetland hydrology was more 

sensitive, however, to increases versus decreases in 

precipitation at the more moderate temperature 

increase (+2°C). These results stress the need for 

accurate projections of climate change, particularly at 

the regional scale (Kittel and others, 1995). 

SEASONAL SIMULATIONS 

A second series of hypothetical climate change 

simulations perfonned for Wetland PI (Poiani and 

others, 1995) identified several important aspects of 

wetland behavior. The climate change simulations 

were run with a temperature increase of 2°C and a 

total growing season precipitation increase or decrease 

of 10 percent. Precipitation changes were applied 

either evenly across all growing season months, that 

is, May through October (all months), or to individual 

seasons (spring, summer, or fall). 
102 CHAPTERS 

Figure 63A. Model output of emergent vegetation cover and open-water distributions for years 

1, 4, 7, and 11 of an 11-year simulation for semipermanent Wetland P 1 under current climate. 

Simulations were on August 1 of each year using unrestricted seed germination. (From Poi ani 

and Johnson, 1991) 

The response of Wetland Pl to precipitation 

changes was similar for all months, summer, and fall. 

Even a 1 0-percent increase in precipitation in those 

seasons produced lower water levels with a +2°C 

temperature increase than the current climate due to 

increased evapotranspiration (Poiani and others, 

1995). Wetland hydrology was most impacted by 

spring precipitation changes. Thus, potential climate 

changes that affect spring refill, such as spring precipitation changes and snowmelt, may have the greatest 

impact on prairie wetland hydrology and vegetation. 

The vegetation response was relatively consistent 

across all simulations. Seven of the eight seasonal 

simulations produced drier conditions with no openwater areas and greater vegetation cover than did the 

current climate simulations (Poiani and others, 1995). 

Simulation results also showed that relatively small 

water-level changes during dry years can significantly 

impact the proportion of emergent vegetation in a 

basin. Emergent vegetation was persistent in shallow, 

fluctuating water. 
CHAPTERS 103 

11 

EXPLANATION 

Open water 

Mixed emergents 

Deep marsh 

D Wet meadow/ shallow marsh 

D Upland 

Figure 63B. Model output of emergent vegetation cover and open-water distributions for 

years I, 4, 7, and II of an !!-year simulation for semipermanent Wetland PI under an 

enhanced greenhouse gas climate as projected by the GISS general circulation model. 

Simulations were on August I of each year using unrestricted seed germination. (From 

Poiani and Johnson, 1991) 

MANAGEMENT AND RESTORATION APPLICATIONS 

A prolonged drought in central North Dakota 

during 1988-92 caused the quality of wetland habitat 

for nesting waterfowl in the Cottonwood Lake area to 

significantly decline (Swanson and others, Chapter 4 

of this report). Many of the semipermanent wetlands 

in the area became covered by emergent vegetation 

(primarily cattail) and contained few or no open-water 

areas, which lead to maximum production (Weller and 

Spatcher, 1965, Weller and Frederickson, 197 4 ). 

These were opportune conditions to illustrate the use 

of the wetland model in estimating the water depth and 

duration needed by either natural or artificial sources 

to restore productive habitat. Although water levels 

currently cannot be manipulated in the Cottonwood 

Lake area, the experiment shows the potential of the 

model as a management and restoration tool. 
104 CHAPTER 5 

11 

EXPLANATION 

Open water Deep marsh 

Exposed soil Wet meadow/shallow marsh 

Mixed emergents Upland 

Drawdown plants 

Figure 64. Model output of emergent vegetation cover and open-water distributions for years l, 4, 

7, 8, 10, and ll of an 11-year simulation for semipem1anent Wetland P l under an enhanced 

greenhouse gas climate as projected by the GISS general circulation model. Simulations were on 

August 1 of each year. Seed gem1ination was limited according to the timing of the draw down. 
Management simulations were performed for 

Wetland P4. The effects of four water depths (75, 100, 

125, and 150 em) were simulated for a 5-year period. 

Simulations were performed with both fluctuating and 

constant water-level conditions. 'Fluctuating-depth' 

simulations had an average water depth of75, 100, 

125, or 150 em in the deepest part of the basin, with a 

range of 15 em above or below this level over the 

course of the growing season. This range of fluctuations approximated the observed range of water-level 

changes in semipermanent wetlands. Constant waterlevel conditions were assigned to each of the four 

depths for the duration of the simulations. 

As expected, the water-depth simulations 

produced markedly different ratios of vegetation to 

open water. Among the fluctuating-depth simulations, 

open water ranged from about 1 to 68 percent, and 

among the constant-depth simulations, from about 11 

to 73 percent (table 13 ). The percentage of open water 

in the fluctuating-depth simulations increased most 

significantly between 100 em (17 percent) and 125 em 

(51 percent). In contrast, the largest increase in open 

water in the constant-depth simulations was between 

75 em (11 percent) and 100 em (42 percent) (fig. 65; 

table 13). 

There also were differences between the 

constant and fluctuating conditions for the same water 

depth (fig. 65). For example, a constant depth of 100 

em in Wetland P4 produced about 42 percent open 

water, but an average water depth of 100 em produced 

only about 17 percent open water (table 13). The 

percentage of open water was always greater for a 

given water depth with constant conditions. The 

desired 50:50 ratio of vegetation to open water was 

best approximated by the 125-cm water depth with 

fluctuating conditions. A nearly balanced ratio of 

CHAPTERS 

vegetation to open water (58:42) also occurred in the 

100-cm constant-depth simulation (table 13). 

The death of deep-marsh emergents and the 

conversion to open water occurred in all eight simulations during year 3 due to deep water conditions for 

greater than 12 growing season months. Previous 

model evaluations indicated that the timing of 

emergent growth and death was not represented 

adequately by a simple threshold function (see section 

"Validation and Testing"). The death of emergent 

vegetation from flooding is highly variable (Shay and 

Shay, 1986), even among species in a single genus 

(Grace, 1989). Thus, model-generating information 

on water depths needed for establishing open-water 

areas in Wetland P4 is more reliable than that for 

flooding duration given the current model limitations. 

The water-depth simulations also produced 

differences in the wet-meadow/shallow-marsh vegetation area (table 13). In six of the simulations, the area 

of this vegetation type either expanded from the 

original3,800 m2 (the 75-cm fluctuating and constant 

depth and the 1 00-cm constant depth), declined 

slightly (the 125-cm fluctuating and constant depths), 

or remained the same (the 1 00-cm fluctuating depth) 

(table 13). In contrast, the area significantly declined 

over 5 years in the two deepest-depth simulations (the 

150-cm fluctuating-depth and the 150-cm constantdepth). A large rise in water levels caused an 

expansion of the deep-marsh zone upslope (fig. 65), 

but the steep-sided basin did not allow the wetmeadow/shallow-marsh zone to migrate upslope. 

Results for the wet-meadow/shallow-marsh zone 

should be interpreted with caution, however, because 

few data were available to test this aspect of the model 

(Poiani and Johnson, 1993a). 

Table 13. Percentage of deep-marsh emergents and open water and area of wet-meadow/shallow-marsh emergents. 

Values are for end of 5-year management scenarios for Wetland P4. 

Water-depth Deep-marsh Open water Wet-meadow/shallow-marsh 

simulation emergents emergents 

(em) (percent) (m2) 

7 5 fluctuating 99 1 5,500 

75constant 89 11 6,800 

1 00 fluctuating 83 17 3,800 

1 00 constant 58 42 5,200 

125 fluctuating 49 51 3,700 

125 constant 37 63 3,500 

150 fluctuating 32 68 100 

150 constant 27 73 300 

105 
106 

Fluctuating water levels 

5 Open water 

Deep marsh 

CHAPTER S 

Constant water levels 

75 centimeters 

100 centimeters 

125 centimeters 

150 centimeters 

EXPlANATION 

D Wet meadow/shallow marsh 

D Upland 

Figure 65. Model output of emergent vegetation cover and open-water distributions for eight 

5-year water-depth simulations for Wetland P4. Resu lts are shown for October of year 5. 
CHAPTERS 107 

These results indicate that a spatially-defined 

model that integrates hydrology, topography, and 

vegetation dynamics can provide target water levels 

for managers to reach desired habitat characteristics 

either for natural or restored wetlands. The simulations particularly are useful for wetlands that have 

controlled water levels or in designing basins for 

restoring and constructing wetlands. Model simulations should be less expensive to conduct than field 

experiments, particularly once a model is tested 

adequately and deemed reliable. 

FUTURE NEEDS AND DIRECTIONS 

The WETSIM models accurately represented 

wetland processes despite being based on simple 

algorithms and relations. The timing and magnitude 

of wetland drawdown and refill, and vegetation 

dynamics, closely followed observed values. Yet, 

while the models satisfactorily simulated major 

wetland dynamics during the test period, considerable 

work still is needed before the models can be used 

reliably to direct management or to assess more 

comprehensively the ramifications of human-induced 

impacts on prairie wetlands and waterfowl. 

Several needs currently are being addressed 

through the development of a wetland landscape 

simulator (WETSCAPE). WETSCAPE expands 

WETSIM 2.0 to include ground-water dynamics and 

the hydrology of temporary and seasonal wetlands that 

typically occur near semipermanent wetlands in 

glaciated prairie landscapes. The response of these 

more ephemeral wetlands to climate change or 

management practices may differ from that of 

semipermanent wetlands. Shallow temporary and 

seasonal wetlands warm early in the spring and 

provide an abundant invertebrate community for 

migrating waterfowl and laying dabbling ducks 

(Swanson, 1988; Murkin and others, 1992). When 

these wetlands are dry in the spring or have a 

shortened annual hydroperiod, waterfowl production 

declines, even if semipermanent wetlands are in good 

condition (Swanson and Duebbert, 1989). 

Models for all three wetland types (temporary, 

seasonal, and semipermanent) need to be tested 

beyond the Cottonwood Lake area for regional 

transferability. While model testing using the Cottonwood Lake data set will continue, data also are being 

collected from a wetland landscape in Deuel County, 

South Dakota (Hubbard and others, 1988). This site 

will enable the calibration and testing ofWETSIM and 

WETSCAPE for a warmer and wetter portion of the 

prairie pothole region. The site has ground-water, 

surface-water, and vegetation data for 13 years, 

making it the site with the second longest data set for 

prairie wetlands in the Dakotas. 

ACKNOWLEDGMENTS 

We gratefully acknowledge the long-term 

consultation and data provided by Tom Winter and 

George Swanson. Both have been instrumental in 

model development and testing. Past funding for this 

work was provided by the N orthem Prairie Wildlife 

Research Center, National Ecology Research Center, 

Patuxent Wildlife Research Center, and U.S. Environmental Protection Agency Global Change Program. 

Eugene Takle of Iowa State University provided GISS 

climate projections. The South Dakota Agricultural 

Experiment Station provided funds to interpolate GISS 

projections for the Dakotas. Tim Kittel provided 

valuable advice on various aspects of climate-impact 

studies. 

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