U.S. Department of the Interior

U.S. Geological Survey

Environmental Setting of the Yellowstone 

River Basin, Montana, North Dakota, and 

Wyoming

By Ronald B. Zelt, Greg Boughton, Kirk A. Miller, Jon P. Mason, and Laura M. Gianakos

Water-Resources Investigations Report 98-4269

Prepared as part of the

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Cheyenne, Wyoming

1999
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by the 

U.S. Government

For additional information write to:

District Chief

U.S. Geological Survey, WRD

2617 E. Lincolnway, Suite B

Cheyenne, Wyoming 82001-5662

Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286, Denver Federal Center

Denver, Colorado 80225

Information regarding the National Water-Quality Assessment (NAWQA) Program is available on the Internet via the World 

Wide Web. You may connect to the NAWQA Home Page using the Universal Resource Locator (URL):

<http://wwwrvares.er.usgs.gov/nawqa/nawqa_home.html>
CONTENTS iii

CONTENTS

Page

Abstract ............................................................................................................................................................................... 1

Introduction......................................................................................................................................................................... 1

Purpose and scope..................................................................................................................................................... 3

Previous studies......................................................................................................................................................... 3

Environmental setting ......................................................................................................................................................... 4

Physiography, by R.B. Zelt ....................................................................................................................................... 5

Climate, by R.B. Zelt ................................................................................................................................................ 8

Geology, by R.B. Zelt ............................................................................................................................................... 8

Structure .......................................................................................................................................................... 8

Volcanic fields and uplifts..................................................................................................................... 8

Structural basins.................................................................................................................................... 13

Stratigraphy..................................................................................................................................................... 14

Paleozoic rocks ..................................................................................................................................... 14

Triassic and Jurassic systems ................................................................................................................ 17

Cretaceous system................................................................................................................................. 17

Tertiary system ...................................................................................................................................... 20

Quaternary system................................................................................................................................. 20

Glaciation ........................................................................................................................................................ 20

Seismicity........................................................................................................................................................ 21

Coal and hydrocarbon deposits ....................................................................................................................... 21

Metallic mineral deposits ................................................................................................................................ 23

Absaroka volcanic field ........................................................................................................................ 23

Beartooth uplift and Cretaceous volcanic centers................................................................................. 25

Bighorn, Owl Creek, and Pryor Mountains .......................................................................................... 26

Bighorn and Powder River Basins ........................................................................................................ 26

Wind River Basin and Wind River Range ............................................................................................ 27

Vegetation, by R.B. Zelt............................................................................................................................................ 27

Surface water, by K.A. Miller ................................................................................................................................... 31

Basin characteristics........................................................................................................................................ 31

Streamflow characteristics .............................................................................................................................. 32

Annual streamflow................................................................................................................................ 32

Flow duration ........................................................................................................................................ 36

Floods and droughts ........................................................................................................................................ 40

Floods.................................................................................................................................................... 40

Droughts................................................................................................................................................ 40

Water-quality characteristics........................................................................................................................... 41

Suspended sediment.............................................................................................................................. 42

Dissolved solids .................................................................................................................................... 44

Stream ecology, by R.B. Zelt and L.M. Gianakos .................................................................................................... 46

Physical stream habitat ................................................................................................................................... 46

Aquatic biological communities .................................................................................................................... 48

Longitudinal zonation .......................................................................................................................... 48

Algae .................................................................................................................................................... 48

Invertebrates ......................................................................................................................................... 50

Fish ....................................................................................................................................................... 50

Ground water, by J.P. Mason..................................................................................................................................... 51

Wind River Basin ............................................................................................................................................ 51

Quaternary aquifers............................................................................................................................... 51
iv ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

CONTENTS--Continued

Page

Tertiary and Cretaceous aquifers........................................................................................................... 53

Pre-Cretaceous aquifers ........................................................................................................................ 53

Bighorn Basin.................................................................................................................................................. 53

Quaternary and Tertiary aquifers .......................................................................................................... 54

Mesozoic and Paleozoic aquifers.......................................................................................................... 54

Powder River Basin......................................................................................................................................... 54

Quaternary and Tertiary hydrogeologic units ....................................................................................... 54

Mesozoic and Paleozoic hydrogeologic units....................................................................................... 55

Williston Basin................................................................................................................................................ 55

Anthropogenic factors............................................................................................................................................... 55

Population, by G.K. Boughton and R.B. Zelt ................................................................................................. 55

Land cover and land use, by G.K. Boughton and R.B. Zelt............................................................................ 57

Agriculture ............................................................................................................................................ 61

Urban areas............................................................................................................................................ 63

Minerals extraction................................................................................................................................ 63

Metals mining ............................................................................................................................... 63

Hydrocarbon production............................................................................................................... 64

Coal mining .................................................................................................................................. 66

Water use, by G.K. Boughton ......................................................................................................................... 70

Waste disposal, by G.K. Boughton ................................................................................................................. 71

Disposal to streams ............................................................................................................................... 71

Disposal on land.................................................................................................................................... 73

Disposal in air ....................................................................................................................................... 74

Disposal underground ........................................................................................................................... 74

Integrated environmental settings, by R.B. Zelt ....................................................................................................... 74

Ecoregions....................................................................................................................................................... 74

Environmental stratification............................................................................................................................ 75

Summary ............................................................................................................................................................................. 80

References .......................................................................................................................................................................... 80

Glossary .............................................................................................................................................................................. 93

Figures

1. Map showing location of Yellowstone River Basin study unit and selected cities ............................................ 2

2. Shaded relief map showing physiographic provinces, Yellowstone River Basin .............................................. 6

3. Graph of hypsometric curve for Yellowstone River Basin ................................................................................ 7

4-6. Maps showing:

4. Average annual precipitation, Yellowstone River Basin, and graphs showing mean daily

precipitation and temperature at selected stations .............................................................................. 10

5. Generalized depth to Precambrian basement structure, Yellowstone River Basin................................... 12

6. Generalized geology, Yellowstone River Basin........................................................................................ 15

7. Diagrammatic sections showing principal geologic structures and groupings of geologic units, 

Yellowstone River Basin............................................................................................................................... 16

8. Generalized correlation chart of Paleozoic, Mesozoic, and Cenozoic rocks, Yellowstone River Basin............ 18

9-10. Maps showing:

9. Principal areas of metallic mineral deposits, Yellowstone River Basin ................................................... 24

10. Potential natural vegetation, Yellowstone River Basin............................................................................. 28

11. Graph of conifer-forested and total area, by elevation zone, Yellowstone River Basin..................................... 29
CONTENTS v

Figures--Continued

Page

12. Map showing hydrography and selected streamflow-gaging stations, Yellowstone River Basin...................... 33

13-15. Graphs showing:

13. Statistical summary of daily mean streamflow for period of record at selected streamflow-gaging 

stations, Yellowstone River Basin ...................................................................................................... 37

14. Streamflow duration curves for selected periods at selected streamflow-gaging stations, 

Yellowstone River Basin..................................................................................................................... 39

15. Statistical summary diagrams of suspended-sediment and dissolved-solids concentrations at 

selected streamflow-gaging stations, Yellowstone River Basin ......................................................... 43

16-21. Maps showing:

16. Generalized aquifers in semiconsolidated and consolidated rocks located at or near the land 

surface, Yellowstone River Basin ....................................................................................................... 52

17. Population density, by census block group, 1990, Yellowstone River Basin ........................................... 58

18. Land cover and land use, Yellowstone River Basin ................................................................................. 59

19. Indian reservations, national forests and grasslands, and national parks and recreation areas, 

Yellowstone River Basin .................................................................................................................... 60

20. Oil and gas fields, Yellowstone River Basin ............................................................................................ 65

21. Coal types and major coal mine lease areas, Yellowstone River Basin ................................................... 68

22. Graphs showing estimated water use, 1990, Yellowstone River Basin ............................................................. 72

23. Map showing ecoregions, Yellowstone River Basin.......................................................................................... 76

24. Schematic diagram and map of environmental settings of Yellowstone River Basin........................................ 78

Tables

1. Basin characteristics and annual streamflow statistics at selected sites, Yellowstone River Basin ................... 34

2. Mean annual runoff for major tributaries and the mainstem as measured at selected sites, 

Yellowstone River Basin............................................................................................................................... 36

3. Maximum daily suspended-sediment discharge, flood of May 1978, Powder River, 

Wyoming and Montana................................................................................................................................. 41

4. Annual departure from mean annual discharge at selected streamflow-gaging stations, 

Yellowstone River Basin, water years 1987-96 ............................................................................................ 41

5. Statistics for total dissolved-solids concentration at selected sites, September-February, during 

non-drought and drought periods, upper Wind and Yellowstone Rivers, 1969-94....................................... 42

6. Correlation of suspended-sediment and dissolved-solids concentration with instantaneous 

discharge at selected sites, Yellowstone River Basin.................................................................................... 44

7. Seasonal variability in total dissolved-solids concentration in stream samples collected at 

site 26, Little Bighorn River near Hardin, Montana, 1970-96...................................................................... 46

8. Human activities contributing to contamination of surface and ground water .................................................. 56

9. Estimated annual applications of fertilizers in the Yellowstone River Basin .................................................... 61

10. Livestock numbers in the Yellowstone River Basin, 1987................................................................................. 61

11. Estimated annual applications of the 12 most extensively used herbicides in the Yellowstone 

River Basin.................................................................................................................................................... 63

12. Principal oil fields in the Yellowstone River Basin............................................................................................ 67

13. Reported surface- and ground-water use by category, 1990, Yellowstone River Basin .................................... 71

14. Names and descriptions of major reservoirs in the Yellowstone River Basin .................................................. 73

15. Toxic Release Inventory emissions in the Yellowstone River Basin, 1993 ....................................................... 73

16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the 

Yellowstone River Basin that lie within the Wind River, Bighorn, and Powder River Basins..................... 97

17. Shallow hydrogeologic units in the Powder River Basin, Montana and Wyoming........................................... 109

18. Hydrogeology of southeastern Montana ............................................................................................................ 111
vi ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

CONVERSION FACTORS AND VERTICAL DATUM

Multiply By To obtain

millimeter (mm) 0.03937 inch

centimeter (cm) 0.3937 inch

meter (m) 3.281 foot

kilometer (km) 0.6214 mile

square kilometer (km2

) 0.3861 square mile

cubic meter (m3) 35.31 cubic foot (ft3

)

barrel (bbl), petroleum 42 gallon

barrel (bbl), petroleum 0.159 cubic meter (m3

)

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

kilogram (kg) 2.205 pound (lb)

megagram (Mg) 1.102 short ton (2,000 lb)

Temperature can be converted to degrees Celsius (°C) or degrees Fahrenheit (°F) by using the following equations:

°C = 5/9 (°F- 32)

 °F = 9/5 (°C) + 32

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived 

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.
ABSTRACT 1

ABSTRACT

Natural and anthropogenic factors influence 

water-quality conditions in the Yellowstone River 

Basin. Physiography parallels the structural geologic setting that is generally composed of several 

uplifts and structural basins. Contrasts in climate 

and vegetation reflect topographic controls and the 

midcontinental location of the study unit. Surfacewater hydrology reflects water surpluses in mountainous areas that are dominated by snowmelt runoff, and arid to semiarid conditions in the plains 

that are dissected by typically irrigated valleys in 

the remainder of the study unit. Principal shallow 

aquifers are Tertiary sandstones and unconsolidated Quaternary deposits.

Human population, though sparsely distributed in general, is growing most rapidly in a few 

urban centers and resort areas, mostly in the northwestern part of the basin. Land use is areally dominated by grazing in the basins and plains and 

economically dominated by mineral-extraction 

activities. Forests are the dominant land cover in 

mountainous areas. Cropland is a major land use 

in principal stream valleys. Water use is dominated by irrigated agriculture overall, but mining 

and public-supply facilities are major users of 

ground water. Coal and hydrocarbon production 

and reserves distinguish the Yellowstone River 

Basin as a principal energy-minerals resources 

region. Current metallic ore production or 

reserves are nationally significant for platinumgroup elements and chromium.

The study unit was subdivided as an initial 

environmental stratification for use in designing 

the National Water-Quality Assessment Program 

investigation that began in 1997. Ecoregions, geologic groups, mineral-resource areas, and general 

land-cover and land-use categories were used in 

combination to define 18 environmental settings 

in the Yellowstone River Basin. It is expected that 

these different settings will be reflected in differing water-quality or aquatic-ecological characteristics.

INTRODUCTION

Beginning in 1991, the U.S. Congress appropriated funds for the U.S. Geological Survey (USGS) to 

commence full-scale implementation of the National 

Water-Quality Assessment (NAWQA) Program. Goals 

of the program include the identification, description, 

and explanation of major natural and anthropogenic 

factors that influence water-quality conditions (Gilliom 

and others, 1995). Important components of the 

NAWQA Program are investigations of more than 

50 major river basins and aquifers, called study units. 

Water-quality conditions measured at sites in these 

study units may be misinterpreted unless spatial patterns and environmental characteristics of associated 

drainages or flow-path areas are understood.

The Yellowstone River Basin (YRB) was among 

the set of 13 NAWQA study-unit investigations begun 

in 1997. The YRB study unit (fig. 1) consists of the 

entire 182,000 km2 area drained by the Yellowstone 

River and its tributaries, including the Wind/Bighorn, 

Powder, Tongue, and Clarks Fork Yellowstone Rivers. 

The Yellowstone River is the largest tributary of the 

Missouri River (Missouri Basin Inter-Agency Committee, 1969), and the mean annual discharge (361 m3/s) of 

the Yellowstone at its confluence with the Missouri in 

western North Dakota represents about 55 percent of 

their combined discharge (Shields and others, 1997). 

Environmental Setting of the Yellowstone River Basin, 

Montana, North Dakota, and Wyoming

By Ronald B. Zelt, Greg K. Boughton, Kirk A. Miller, Jon P. Mason, and Laura M. Gianakos
2 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
INTRODUCTION 3

The purpose of the study-unit investigation is to 

increase the scientific understanding of surface- and 

ground-water quality and the factors that influence 

water quality in the YRB. Important water-quality 

issues in the YRB generally are related to effects of 

land and water management practices, but some are 

related to natural factors. For example, sedimentation 

problems in streams and reservoirs in parts of the YRB 

are likely a result of naturally erodible soils and natural 

processes such as post-wildfire landslides and debris 

flows, as well as human land-use activities that remove 

vegetation, disturb soils, and steepen or overload 

slopes. Other major water-quality issues include trace 

elements, toxic compounds, and salinity (Miller and 

Quinn, 1997).

Purpose and Scope

This report describes the environmental settings 

of the YRB, including natural and anthropogenic factors believed to influence water-quality conditions. 

Specifically, the report provides historical and baseline 

information for the topical studies to be undertaken as 

part of the NAWQA study-unit investigation of the 

YRB. An initial stratification (subdivision into areas 

having greater homogeneity than the whole YRB) of 

the study unit based on its environmental settings also 

is presented.

Previous Studies

Early explorers of the YRB include Larocque in 

1805, Clark in 1806, Colter in 1807-08, Drouillard in 

1808, Henry in 1809, Hunt in 1811, Wyeth in 1833, and 

Raynolds and Maynadier in 1859-60 (U.S. Department 

of the Interior, 1965; Missouri Basin Inter-Agency 

Committee, 1969). The U.S. Army sent exploratory 

expeditions into the YRB, beginning with the 1856 

expedition led by Lt. G.K. Warren, accompanied by 

geologist F.V. Hayden. Hayden mapped the general 

geology of the Wind River Basin in 1859-60 and the 

Yellowstone-Teton area in 1868. The WashburnDoane expedition of 1870 ascended the Yellowstone 

River and received official credit for the “discovery” of 

what became Yellowstone National Park. Also in 1870, 

the Yale Scientific Expedition, led by O.C. Marsh, 

explored the Bighorn Basin. The U.S. Geological and 

Geographical Survey of the Territories began in 1871, 

under the direction of Hayden, and produced impressive documentation of the geologic and scenic wonders 

of the Yellowstone Plateau, due in large part to the photography by W.H. Jackson and paintings by Thomas 

Moran.

Settlement and development of the YRB followed the period of exploration. Numerous studies 

have been conducted in local areas and of individual 

resources or environmental factors within this vast 

area. Selected previous studies that were broad in geographic extent and that examined multiple geographic 

or environmental factors are the topic of the remainder 

of this section.

A general reconnaissance of the northern Great 

Plains was made by the USGS in 1924 to classify the 

public domain into three principal land-suitability categories: land that would be best suited to production 

under irrigation, by dry-farming methods, or as grazing 

land (Aldous and Deeds, 1929). That study included 

part of the YRB east of the Rocky Mountains and north 

of 43oN latitude, and described the physiography, soils, 

and vegetation.

The U.S. Department of the Interior’s Missouri 

River Project was a basin development program active 

from 1945-65 that produced separate studies of seven 

subbasins that together compose the YRB (for examples, the reader is referred to U.S. Department of the 

Interior, 1949; 1965). The purpose of these studies was 

to promote the use and management of the public lands 

in the best interests of the public, consistent with proper 

conservation practices (U.S. Department of the Interior, 1965). A recommendation resulting from that program was the sale of a large part of the vacant, 

unreserved public domain that would contribute more 

to the local economy in private ownership (U.S. 

Department of the Interior, 1965). 

During 1964-69, a comprehensive study of the 

Missouri River Basin was conducted to guide longrange planning for multipurpose development of water 

and related land resources during 1970-2020 (Missouri 

Basin Inter-Agency Committee, 1969). The recommendations made in the final report included the following: (1) construction of 78 reservoirs to provide 

2.6 million m3

 of additional storage; (2) development 

of 2,500 km2 of land for irrigation; (3) protecting 

179 km of stream banks against erosion; (4) designating 507 km of scenic rivers; and (5) acquisition of 

186 km2 of land for wildlife management (Missouri 

Basin Inter-Agency Committee, 1969). The planners 

foresaw conflicts between environmental values 
4 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

related to scenic beauty and preservation of wild 

streams and economic-development values related to 

an extensive system of reservoirs and irrigation 

projects. Given the surface-water emphasis of the 

interagency study, a subsequent study (Taylor, 1978) of 

the Missouri River Basin was done to describe its 

ground-water resources and indicate areas where information was lacking. 

During the late 1960s and through the 1970s, a 

series of water-resources assessments were conducted 

in several moderately large study areas included in the 

YRB (for examples, the reader is referred to Miller, 

1979; 1981; Taylor, 1968). The Yellowstone Impact 

Study, begun in 1974 by the Montana Department of 

Natural Resources and Conservation, evaluated potential physical, biological, and water use impacts of water 

development on the middle and lower sections of the 

Yellowstone River Basin in Montana (Newell, 1977). 

Due to the importance of ground water for domestic 

and stock consumption, studies of several areas of the 

Powder River Basin emphasized the major aquifers in 

those areas. Several ground-water studies from this 

period examined the potential hydrologic effects of the 

anticipated development of coal resources in the Powder River Basin (Slagle and others, 1985; Van Voast 

and others, 1978; Van Voast and Thompson, 1982). A 

notable large-area study examining a broader scope of 

effects of coal development was the Northern Great 

Plains Resource Program (1974)

Following enactment of the Surface Mining 

Control and Reclamation Act of 1977 (PL 95-87), 

requiring the assessment of hydrologic impacts prior to 

issuance of mining permits, a series of reports summarizing the hydrology of coal provinces were prepared 

by the USGS. The drainages of the Clarks Fork Yellowstone River, Pryor Creek, and smaller tributaries to 

the Yellowstone River in its westernmost Great Plains 

reach were characterized by Slagle and others (1986). 

Peterson and others (1987) described the resources of 

much of the area drained by the Bighorn-Wind and 

Shoshone Rivers upstream of Bighorn Lake. Slagle 

and others (1983) characterized the drainages of the 

Tongue River, Rosebud Creek, and smaller tributaries 

to the middle reach of the Yellowstone River. Lowry 

and others (1986) described the hydrology and setting 

of the Powder River Basin. Drainages of the lower 

reach of the Yellowstone River were characterized by 

Slagle and others (1984). 

About the northeastern half of the YRB was 

included in the study area of the USGS’ Northern Great 

Plains Regional Aquifer-System Analysis (RASA) that 

began in 1981. The objectives of that study were to 

describe the regional trends of ground-water chemistry, 

to determine the mechanisms controlling the water 

chemistry, and to identify flow directions and areas of 

recharge (Busby and others, 1995). The study focused 

on Lower Cretaceous through Paleozoic aquifers.

During 1986-90, the USGS conducted studies of 

the water-quality of the Powder River. The objectives 

were to (1) determine temporal trends in selected water 

properties and chemical constituents (Cary, 1989; 

1991), and (2) develop a water-quality model of the 

river to evaluate water-management alternatives (Lindner-Lunsford and others, 1992). Also during the late 

1980s, an atlas of the water resources and water hazards of Wyoming was compiled (Ostresh and others, 

1990).

ENVIRONMENTAL SETTING

The physical, chemical, hydrological, and ecological characteristics of an area are considered to compose its environmental setting (Stark and others, 1996). 

In a study unit as large and diverse as the YRB, strong 

contrasts among these characteristics exist. The waterquality characteristics of the study unit are expected to 

similarly be diverse and to reflect the environmental 

contrasts among the areas of water sources and those 

areas through which the water moves before exiting the 

YRB. This description of the environmental setting is 

not exhaustive, but concentrates on the factors believed 

to influence water quality and aquatic ecology in this 

study unit. 

Both natural and anthropogenic factors affecting 

water quality are described. Natural factors discussed 

in this report are those which generally control waterquality characteristics in the absence of human activity. 

Although water and land use practices sometimes overshadow natural factors, understanding the natural 

diversity of the YRB is believed to be important in 

understanding its water-quality conditions. Natural 

factors include physiography, climate, geology, vegetation, and hydrology. Anthropogenic factors, including 

population, land cover and land use, water use, and 

waste disposal, are described following the natural factors.
ENVIRONMENTAL SETTING 5

Physiography

Physiography refers to the physical morphology 

of the earth’s surface. The study unit lies in parts of the 

Great Plains, Middle Rocky Mountains, Wyoming 

Basin, and Northern Rocky Mountains physiographic 

provinces (fig. 2) (Fenneman and Johnson, 1946). A 

digital map (G.P. Thelin, U.S. Geological Survey, 

unpub. data, 1992) of Fenneman’s physical divisions at 

a nominal scale of 1:7,500,000 was used to summarize 

the distribution of physiographic provinces in the YRB. 

Digital elevation models (U.S. Geological Survey, 

1987) for all 1- by 1-degree quadrangles of latitude and 

longitude in the study unit were assembled and used to 

summarize elevation. Elevation ranges from 564 m 

above sea level at the mouth of the Yellowstone River 

to about 4,200 m on the crest of the Wind River Range.

Fifty-five percent of the study unit lies in the 

Great Plains Province (fig. 2). The Great Plains feature 

gently rolling topography in general, but some areas of 

sharply dissected badlands have developed in easily 

erodible shales. Most of this area is part of the unglaciated Missouri Plateau section of the Great Plains, but 

the extreme northeastern end of the YRB lies within the 

area subjected to continental glaciation. The unglaciated Missouri Plateau section displays the greatest variety of landforms in the Great Plains (Trimble, 1980). 

The topography of the unglaciated plains largely 

reflects fluvial dissection of the ancient outwash plain 

that extended eastward from the Rocky Mountains 

(Trimble, 1980). The sediments of this surface slope to 

the east at an average gradient of about 1.9 m/km 

(Howard and Williams, 1972). In places, the plateau is 

surmounted by higher erosional remnants of an older 

depositional surface (Trimble, 1980). Most rivers of 

this section flow in broad alluvial valleys established 

prior to the advance of continental ice sheets (Trimble, 

1980). Elevations in the Great Plains Province range 

from 570 to 2,200 m within the study unit, averaging 

1,070 m. Local relief typically is less than 150 m, but 

in parts of the Powder River Basin increases to about 

300 m.

The Middle Rocky Mountains Province (fig. 2) 

contains 35 percent of the study unit and features landforms varying from mountain ranges and high plateaus 

to intermontane basins, such as the Bighorn Basin. 

Elevations in this province range from 1,040 to about 

4,200 m within the study unit, averaging 2,090 m. 

Centered in this part of the study unit is the Bighorn 

Basin, bordered on the west by the Absaroka Range 

and Beartooth Mountains, on the south by the Owl 

Creek Mountains, and on the east and northeast by the 

Bighorn and Pryor Mountains. To the north, the basin 

is topographically open where it merges into the Crazy 

Mountains Basin (or Syncline) of south-central 

Montana (Lageson and Spearing, 1988), not shown in 

figure 5. 

Most of the mountain ranges have a high axial 

crest surmounting a broad, high-elevation erosional 

surface that slopes gently outward until dropping off 

abruptly to the lowlands. Remnants of this surface are 

found on gravel-capped mesas in the adjacent basins. 

Lower erosional levels are evidenced by lower mesas 

and by broad, paired alluvial terraces (Howard and 

Williams, 1972, p. 29).

Badlands topography has developed in parts of 

the Bighorn Basin where the Willwood Formation of 

Tertiary age crops out (Mears and Marston, 1990). 

Local relief is 150 to 300 m in this basin, which is topographically classified as plains with high hills (Mears 

and Marston, 1990).

The Bighorn and Wind River mountain ranges 

are broad anticlines flanked by hogbacks. The 

Beartooth Mountains are high plateaus atop several 

uplifted blocks that are dissected by numerous glaciated river valleys (Page and Zientek, 1985). The Absaroka Range consists of thick volcanic deposits 

presenting a rugged topography dominated by deep, 

steep-sided valleys with erodible slopes and large areas 

of cliffs and talus (Despain, 1990). All of these mountain ranges feature local relief typically in excess of 

1,000 m.

The volcanic Yellowstone Plateau lies west of 

the Absaroka Range and features an undulating, almost 

level surface averaging about 2,500 m in elevation. 

Both the Yellowstone Plateau and the Absarokas are 

topographically classified as tablelands with high 

relief, being 50 to 80 percent gently sloping terrain, 

with 50 to 75 percent of that occurring in uplands 

(Mears and Marston, 1990).

A few broad valleys with large meandering rivers are located within the Middle Rockies, but stream 

gradients within the mountainous areas generally are 

steep. Dense vegetation and armored streambeds resist 

erosion, but where soft rocks are exposed or vegetation 

is disturbed, rapid erosion occurs (Missouri Basin 

Inter-Agency Committee, 1969). The drainage network of the Middle Rocky Mountains Province features several deep canyons, such as the Bighorn and
6 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 7

Wind River Canyons, where rivers cut through mountain ranges rather than following lowland courses 

around the barriers. These canyons are attributed to 

superimposition of stream courses from the surface of 

sediments that formerly filled the basins to higher levels than at present (Howard and Williams, 1972).

The part of the Wyoming Basin physiographic 

province contained within the study unit, composing 

about 7 percent of the study unit, corresponds to the 

Wind River Basin. The boundary between this province 

and the Great Plains is the subtle surface expression of 

the Casper Arch (fig. 5); this gap south of the Bighorn 

Range is the only area where the mountainous western 

border of the Great Plains is interrupted. The Wind 

River Basin is bounded on the north by the Owl Creek 

Mountains, on the west by the Wind River Range 

(fig. 2), and on the south by minor ranges associated 

with the Sweetwater uplift (fig. 5). Terrace and pediment surfaces form extensive tablelands, and locally 

prominent badlands are eroded into soft Tertiary sediments (Keefer, 1965). Elevation in this province ranges 

from 1,440 to 3,600 m within the study unit, averaging 

about 1,840 m. Local relief is typically 150 to 300 m.

The Northern Rocky Mountains Province (fig. 2) 

contains about 3 percent of the study unit. This province is separated from the Middle Rocky Mountains 

Province only by the Yellowstone River valley and 

Yellowstone Plateau. Elevation in this province ranges 

from 1,310 to 3,350 m within the study unit, averaging 

about 1,930 m.

A hypsometric curve for the YRB illustrates 

(fig. 3) that only 2.3 percent of the study unit lies more 

than 3,100 m above sea level, which roughly corresponds to the alpine zone above timberline (compare, 

Despain, 1990; Thilenius and Smith, 1985). Also, 

about 82 percent of the study unit is less than 2,100 m in 

elevation and, thus lies below the densely forested 

mountain zone, discussed elsewhere in this report.
8 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Climate

Climate in the study unit ranges from cold and 

moist in the mountainous areas to temperate and semiarid in the plains areas. Primarily because of its midcontinental location, the basin’s weather is characterized by 

fluctuations and extremes (Missouri Basin Inter-Agency 

Committee, 1969). The interaction between air masses 

originating in the Gulf of Mexico, the northern Pacific 

Ocean, and the Arctic regions is largely responsible for 

the seasonal climate regimens found within the study 

area. The Gulf air tends to dominate in spring and early 

summer, but Arctic air dominates in winter (Missouri 

Basin Inter-Agency Committee, 1969).

Mean annual temperatures range from less than 

0oC at Yellowstone Lake to about 10oC along the Bighorn River valley in Montana (National Climatic Data 

Center, digital data, 1994). Annual temperature 

extremes range from about -40oC during the winter to 

hotter than 38oC during the summer (Western Regional 

Climate Center, digital data, 1997). Temperatures generally are coldest in January, when average daily lows 

range from less than -18oC in higher elevations to about 

-8oC near Livingston, Mont. (fig. 4) (Western Regional 

Climate Center, digital data, 1997), and average monthly 

temperature ranges from less than -13oC in Yellowstone 

National Park to about -3oC near Livingston, Mont. 

(National Climatic Data Center, digital data, 1994). July 

normally is the warmest month, with average daily highs 

ranging from about 22oC in higher elevations to about 

32oC in some valleys of the Great Plains and Wyoming 

Basin Provinces (Western Regional Climate Center, digital data, 1997). Average July temperature ranges from 

about 12oC in higher elevations to about 24oC in some 

valley locations (National Climatic Data Center, digital 

data, 1994). The average frost-free period ranges from 

less than 10 days at high elevations (Marston and Anderson, 1991) to more than 140 days on the plains and in 

lower basins (U.S. Department of the Interior, 1965). 

The climate is cold enough at some sites to form permanent ice (permafrost) in the ground: local permafrost 

occurs at elevations as low as 2,400 to 2,600 m on north 

slopes near Yellowstone National Park (Pierce, 1979).

In the YRB, 40 to 45 percent of the annual precipitation falls during April through June at most locations 

(fig. 4), but this seasonality diminishes in mountainous 

areas. Mean annual precipitation ranges from about 

150 mm (5.9 in.) in the central parts of the Bighorn and 

Wind River Basins to more than 1,500 mm (59 in.) at 

high elevations in the mountains near Yellowstone 

National Park (Oregon Climate Service, 1995a, 1995b). 

Snowfall composes a substantial part of annual precipitation in most years, with average annual snowfall ranging from less than 300 mm (12 in.) in parts of the 

Bighorn Basin to more than 5,200 mm (200 in.) near 

Yellowstone National Park (Western Regional Climate 

Center, digital data, 1997). The mountain ranges in the 

study unit cause precipitation to vary strongly with elevation, because in mountainous terrain, most of the spatial variation in precipitation is explained by orographic 

effects of the large-scale terrain features (Daly and others, 1994). Annual precipitation in the plains areas generally is more variable from year to year, and less than in 

the mountains (Slagle and others, 1983).

Evaporation varies with temperature, which, in 

turn, is strongly affected by elevation (Reider, 1990). 

Evaporation in the YRB is affected more by prevailing 

wind and sky conditions than by latitude, as shown by 

published maps of mean annual evaporation for 1956-70 

(Farnsworth and others, 1982). Evaporation is greatest 

in the windswept basins and prairies where the mean 

annual total generally exceeds 900 mm (35 in.), and surpasses 1,100 mm (43 in.) in parts of the Bighorn and 

Powder River Basins and Yellowstone River valley. In 

the cool, often cloud-shrouded highlands of the Absaroka and Beartooth Mountains, mean annual evaporation is less than 500 mm (Martner, 1986). Evaporation 

and precipitation together distinguish the moist, mountain forest ecosystem from the lower-elevation regions 

where evaporation exceeds precipitation (Ostresh and 

others, 1990; Marston and Anderson, 1991).

Geology

Structure

The YRB contains parts of three geologic provinces: the uplifts and basins of the Rocky Mountain foreland, the Yellowstone Plateau, and the Absaroka 

volcanic field (Snoke, 1993). The structural framework 

of uplifts and sedimentary basins is shown in figure 5. 

This section of the report presents a brief overview of the 

principal structural features of the study unit.

Volcanic Fields and Uplifts

The Absaroka volcanic field and the Yellowstone 

Plateau are the two major post-Laramide (Late Cretaceous through Paleocene) volcanic fields in the YRB. 
ENVIRONMENTAL SETTING 9

The Beartooth, Bighorn, Owl Creek, and Wind River 

Mountain ranges are the major uplifted areas. The basement-cored uplifts commonly feature complex structures, including foliation, small-scale folds, shear 

zones, and igneous dikes (Brown, 1993). 

The Absaroka volcanic field of Tertiary age is the 

largest, least-studied volcanic province in the conterminous western United States (Snoke, 1993). This massive volcanic field covers about 23,000 km2 to a 

maximum thickness of about 1,500 m (Smedes and 

Prostka, 1972), and extends well beyond the Absaroka 

Range itself, so that some workers name it the 

Absaroka-Gallatin volcanic province (Chadwick, 

1970). The volcanic rocks are mostly composed of 

andesite and dacite; units are chiefly flows, breccias, 

and stocks. Eruptive centers are aligned along two 

subparallel northwest-trending structural zones (Chadwick, 1970), one of which includes the Emigrant-Mill 

Creek, Independence, New World, and Sunlight mineral 

districts (fig. 9) (Elliott and others, 1983; 1993). An 

overall pattern of progressively younger eruptive centers transgressing from northwest to southeast has been 

suggested (Snoke, 1993). The vent areas for the volcanics were predominantly calc-alkaline stratovolcanoes 

and shield volcanoes composed of lava flows, flow 

breccias, mudflows, avalanche debris, and tuff in a chaotic assemblage called vent facies (Smedes and Prostka, 

1972). Outward from the volcanic centers, the vent 

facies rocks interfinger with, and grade into, reworked 

volcanic sedimentary rocks--mainly volcanic conglomerate, breccia, volcanic sandstone and siltstone, and airfall tuff that together are called alluvial facies (Smedes 

and Prostka, 1972). The volcanic field may have originally covered much of the Bighorn Basin (Love, 1939; 

Smedes and Prostka, 1972), and extends into the southcentral part of the Beartooth uplift (Montagne, 1982, 

p. 12). However, there is no evidence to indicate what 

was the northernmost extent of this volcanic field 

(Smedes and Prostka, 1972). 

The Yellowstone Plateau is the product of volcanic eruptions of Quaternary age. Each of three cycles of 

volcanic activity climaxed with an explosive eruption 

that produced a voluminous rhyolitic ash flow and a 

large collapse caldera (Christiansen and Blank, 1972). 

Rhyolites predominate among the volcanic rocks of the 

plateau, but basalts also occur. The enormous caldera, 

70 by 45 km across and formed in the third volcanic 

cycle, has been partly filled by rhyolitic lava flows, and 

is surrounded by a ring of predominantly rhyolitic 

welded tuff (Christiansen and Blank, 1972). The ash 

flows cover several hundred km2

, and eolian ash deposits are widespread throughout the western U.S. (Snoke, 

1993). With the volume of expelled Quaternary volcanic material totalling about 8,000 km3

, Yellowstone 

may be the world’s largest center of active silicic volcanism (Smith and Braile, 1993). At the present time, the 

extremely high flow of heat from the Yellowstone 

Plateau (Smith and Braile, 1993) is evidence that magmas, partial melts, and extensive hydrothermal systems 

lie beneath the caldera.

A third volcanic area, the Sliderock Mountain 

(fig. 5) area, lies north of the Beartooth uplift and is 

underlain by mainly intrusive, volcanic, and volcaniclastic rocks that correlate with deposits of the Livingston Group that lie to the north and northwest (Elliott 

and others, 1993). The Livingston Group is a thick 

sequence of volcanic sedimentary rocks that was 

derived principally from a volcanic field to the northwest (Roberts, 1963) that lies well outside the YRB. 

However, within the Sliderock Mountain area the geology is dominated by the remains of a deeply eroded 

Upper Cretaceous stratovolcano composed of lahar 

deposits and andesite lava flows (Elliott and others, 

1993).

The Beartooth uplift, a broad fault-bounded Laramide uplift, includes all of the Beartooth Mountains and 

merges with the northern Absaroka Range along the 

south-central part of the uplift (Elliott and others, 1993). 

The Beartooth uplift is bounded on the east by the Bighorn Basin, on the west by the alluvium-filled valley of 

the Yellowstone River, and on the south by the Clarks 

Fork Yellowstone River (Elliott and others, 1983). The 

Beartooth uplift contains a core of Precambrian crystalline rocks—gneiss, granitics, and supracrustal rocks 

(Page and Zientek, 1985)—flanked by Paleozoic and 

younger rocks, and is bounded by thrust faults (Casella 

and others, 1982). An upper plate of metasedimentary 

and plutonic rocks was displaced about 15 km toward 

the north-northeast along a low-angle thrust fault dipping approximately 30o to the southwest (Brown, 1993; 

Page and Zientek, 1985). Along the southwest marginal 

thrust fault, Precambrian rocks crop out from beneath 

the Absaroka volcanic field and Paleozoic sedimentary 

rocks; the western section of the uplift features metasedimentary rocks intruded by granitics; and the eastern 

two-thirds of the uplift contains mainly gneisses 

(Casella and others, 1982; Mueller and others, 1985).

Along the northern edge of the Beartooth Mountains, the Stillwater Complex (fig. 5) of Precambrian 

age, a mafic to ultramafic, layered igneous intrusion
10 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 11
12 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 13

containing mineralized areas, crops out over an area 

about 50 km long and up to 8 km wide (Page, 1977). 

The parent magma of the complex was intruded as a 

horizontal sill into sedimentary rocks that were metamorphosed to hornfels. After crystallizing, the complex was tilted, beveled by erosion, later deformed 

during the Laramide orogeny, and today stands nearly 

vertical (Simons and others, 1979). As a result of the 

tilting, a 5,500-m thick section of layered igneous 

rocks is exposed (Elliott and others, 1993, p. 9). 

The Bighorn Mountains (fig. 5) represent a 

major structural arch whose crystalline core is bounded 

and transected by faults. The northern part of the structural arch plunges northwesterly at a low angle (Van 

Gosen and others, 1996). The Bighorn Mountains are 

bounded on the north by a narrow, northwest-trending 

zone composed of a series of northeast-trending, normal faults (Blackstone, 1975). Along their eastern 

margin, the Bighorns have been displaced eastward 

along low-angle thrust faults (Brown, 1993). The 

northern part of the Bighorn Mountains lies on the 

upthrown side of an east-west trending thrust fault, and 

the southern part lies on the upthrown side of a highangle fault trending northeast-southwest (Blackstone, 

1993). The crystalline rocks remain buried beneath 

Paleozoic sedimentary rocks throughout much of the 

northern Bighorn uplift, including the Pryor Mountains.

Although they are topographically distinct from 

the Bighorn Mountains, the Pryor Mountains (fig. 2) 

are the northwest extension of the Bighorn structural 

uplift (Blackstone, 1975). Five tilted fault blocks compose the Pryor Mountains, with each block being 

uplifted highest at its northeast corner (Blackstone, 

1975). Block boundaries include faults and faulted or 

unfaulted folds (Van Gosen and others, 1996).

The Owl Creek Mountains (fig. 2) are a narrow, 

east-west trending, asymmetrical anticline. The north 

flank of the uplift is overlain by northward-dipping 

Paleozoic sedimentary rocks, and the southern flank is 

overlain by Tertiary strata. Concealed by the Tertiary 

deposits are Paleozoic and Mesozoic rocks that are 

overthrust by Precambrian crystalline rocks that form 

the core of the uplift (Hausel, 1989).

The Wind River Range (fig. 5) is an asymmetrical anticline having a Precambrian core of high-grade 

metamorphic and igneous rock. Along its eastern 

flank, Paleozoic and Mesozoic strata dip eastward, but 

along the northern and northeastern flanks the strata are 

folded and faulted. Major, moderately dipping faults 

bound the range on the south and west and overthrust 

the Precambrian igneous and metamorphic rocks above 

basin-filling sedimentary strata. The crystalline core of 

the range is a complex of migmatite, felsic orthogneiss, 

and paragneiss intruded by quartz diorite to granite plutons (Hausel, 1989). The southern margin of the early 

crystalline complex is intruded by a granodiorite 

batholith that also intrudes the gneiss complex adjacent 

to the South Pass greenstone belt (Hausel, 1991).

Structural Basins

The names of several structural basins are identical with names of surface drainage basins. In most 

cases the geographic extent of both same-named basins 

is roughly equivalent. However, the reader is alerted to 

a substantial difference in the geographic extent of the 

hydrologic Powder River Basin (entirely within the 

study unit) and the structural basin of that same name 

(extending well beyond the study unit on its southeastern margin).

The Bighorn Basin is asymmetric, with its western flank being the steepest and its axis trending northwest-southeast; sedimentary rocks of the structural 

basin exceed a maximum thickness of 7,200 m along 

the basin axis southeast of Cody (Blackstone, 1993). 

On the east, the Bighorn Mountains are thrust westward onto basin sedimentary rocks at the northern and 

southern ends of the mountain range (Lageson and 

Spearing, 1988). Major deformation produced a zone 

of folded rocks around the flanks of the basin (Blackstone, 1988); some of these structural features are 

important reservoirs of oil and gas. 

The Crazy Mountains Basin (or Syncline) (fig. 5) 

is a northwesterly trending structural basin filled with 

sedimentary, intrusive, volcanic, and volcaniclastic 

rocks (Elliott and others, 1993). Subsidence and deposition were greater in the western part of the basin 

(Roberts, 1963). The deposits attain a maximum thickness of about 9,000 m.

The Powder River Basin (fig. 5) is a gentle syncline between the Bighorn Mountains on the west and 

the Black Hills of South Dakota on the east (not shown 

on fig. 5) (Berryhill and others, 1950). The Miles City 

Arch separates the Powder River and Williston structural basins (fig. 5). The southern and southeast parts 

of the structural basin lie outside the study unit. The 

Powder River basin’s axis trends northwest-southeast 

and lies near its more steeply dipping western flank 
14 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

along the Bighorn Mountains and the Casper Arch 

(Blackstone, 1993). The maximum thickness of sedimentary rocks in the Powder River Basin exceeds 

5,400 m along the basin axis northeast of Casper.

The Williston Basin (fig. 5) is approximately 

oval shaped with the axis oriented north-northwest to 

south-southeast. The deepest part of the structural 

basin is in northwestern North Dakota, about 50 km 

east of the study unit. The maximum thickness of sedimentary rocks is about 5,200 m (Busby and others, 

1995). The dip of the sedimentary strata varies, but is 

typically about 0.3 percent (Crosby and Klausing, 

1984). The Cedar Creek anticline protrudes into the 

southwestern part of the Williston Basin and is the most 

pronounced positive structural feature in the northeastern YRB.

The Wind River Basin (fig. 5) is an east-west 

trending asymmetrical basin bounded by the Owl 

Creek Mountains to the north, the Wind River Range to 

the west, the Casper Arch to the east, and the Sweetwater uplift to the south. It is surrounded by folded and 

faulted Paleozoic and Mesozoic rocks on the flanks of 

the uplifts. On its northern and eastern margins, Precambrian blocks have been thrust south and west, 

respectively, to overhang the Paleozoic, Mesozoic, and 

Cenozoic sediments filling the basin (Brown, 1993). 

The deepest parts of the structural basin lie in the north, 

adjacent to the Owl Creek Mountains, where sedimentary rocks reach a thickness of about 7,600 m (Blackstone, 1993).

Stratigraphy

Sedimentary rocks in the study unit range from 

the Cambrian to Quaternary Systems, and include both 

continental and marine deposits. The outcrops of most 

units are shown on the individual State geologic maps 

for Montana (Ross and others, 1955; Raines and 

Johnson, 1996), North Dakota (Clayton, 1980), and 

Wyoming (Love and Christiansen, 1985; Green and 

Drouillard, 1994). A generalized composite of the outcrop maps is shown in figure 6. 

The distribution and thickness of the geologic 

units are controlled largely by the tectonic events that 

produced the basins, uplifts, and associated faults 

(Whitehead, 1996). Diagrammatic sections illustrating 

the association between generalized stratigraphy and 

the principal structural features are shown in figure 7. 

A generalized correlation chart for Paleozoic, 

Mesozoic, and Cenozoic rocks (fig. 8) shows that the 

distribution of geologic units varies considerably 

across the YRB. Differences in nomenclature between 

parts of the YRB further complicate the discussion of 

stratigraphy. The following sections provide an overview of the stratigraphy of the study unit, but for individual description of many of the geologic units shown 

in figure 8, the reader is referred to the sources from 

which that figure was compiled.

Paleozoic Rocks

The Pre-Cretaceous rocks (Cambrian through 

Jurassic Systems) frequently are not differentiated with 

respect to series, groups, or formations on the State 

geologic maps, and are exposed in only 8.3 percent of 

the study unit. Where deeply buried, as they are in 

structural basins, these rocks are beyond the scope of 

the NAWQA study-unit investigations that focus on 

shallow ground water and surface water. Paleozoic 

rocks are exposed at the land surface in discontinuous, 

often small, and irregular areas within the YRB. These 

outcrops generally are on the flanks of uplifts or where 

erosion has exposed the folds of an anticline (Whitehead, 1996).

The Cambrian sequence is exposed in less than 

1.4 percent of the study unit. A general decrease in carbonate rocks and an increase in sandstone and conglomerate exists from west to east across the YRB 

(Lochman-Balk, 1972). 

Ordovician strata are exposed in less than 1 percent of the study unit and reach a maximum thickness 

greater than 240 m in the northeastern extremity of the 

YRB. Directly overlying Cambrian rocks in much of 

the YRB, is the Bighorn Dolomite that forms nearly all 

of the 150 m of Ordovician rocks present in much of 

northern Wyoming (Boyd, 1993). All Ordovician Formations are absent in the southeastern Powder River 

Basin (Boyd, 1993; Foster, 1972). 

Silurian strata reach a maximum thickness 

within the study unit of about 220 m at the YRB outlet 

area, and thin steadily to the southwest (Gibbs, 1972, 

p. 87). Nowhere in the YRB are Silurian strata exposed 

at the surface. Devonian rocks are exposed over less 

than 1 percent of the study unit. The Devonian System 

is absent in the southeastern part of the study unit, generally thickens toward the northwest and northeast, and 

reaches a maximum thickness of about 430 m at the 

YRB outlet area (Baars, 1972, p. 94). 
ENVIRONMENTAL SETTING 15
16 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 17

A thick carbonate sequence of Mississippian 

strata covers the structural basins of the study unit, 

ranging in thickness from about 60 m in the southeast 

to about 760 m in the YRB outlet area. In the Montana 

part of the YRB, thickness increases northward, 

whereas in Wyoming, thickness increases northward 

and westward (Craig, 1972, p. 103). Mississippian 

rocks are exposed over less than 3.5 percent of the 

study unit. Lower Mississippian rocks include lower 

and middle sections of the widespread Madison Limestone, or equivalently, the Lodgepole and Mission Canyon Limestones of the Madison Group (Boyd, 1993). 

The Pennsylvanian System is less than 100 m 

thick in the northwestern YRB, thickens gradually to 

the south and southeast to an average thickness of 

about 150 m over the southern half of the study unit, 

and exceeds 210 m of thickness in the southern Powder 

River Basin (Mallory, 1972, p. 115). Pennsylvanian 

rocks are exposed over less than 0.5 percent of the 

study unit.

Permian rocks are exposed over less than 2.5 percent of the study unit. Lower Permian rocks are present 

only in the southern part of the Powder River Basin and 

northeastward from there along the eastern edge of the 

YRB (Boyd, 1993; Rascoe and Baars, 1972). In total, 

the Permian sequence is thickest in the southeastern 

part of the study unit, where it exceeds 160 m, and thins 

to the northwest, wedging out altogether along most of 

the northwestern margin of the YRB (Rascoe and 

Baars, 1972, p. 146).

Triassic and Jurassic Systems

Like the Paleozoic rocks that underlie them, the 

Triassic and Jurassic Systems are downwarped and 

deeply buried in the structural basins (Whitehead, 

1996). The Triassic System is an eastward-thinning 

wedge that covers most of the YRB, has a maximum 

thickness of about 600 m in the southwestern Wind 

River Basin, and is absent along the northwestern margin of the study unit (MacLachlan, 1972, p. 169). Triassic rocks are exposed over less than 2 percent of the 

study unit. 

The Nugget Sandstone (fig. 8) overlies the Upper 

Triassic rocks of the Wind River Basin to a maximum 

depth of about 120 m, but is absent elsewhere in the 

study unit (MacLachlan, 1972, p. 174). These coarsegrained, crossbedded eolian sandstones may be Upper 

Triassic in age but, according to Picard (1993), are 

probably Lower Jurassic. 

Jurassic rocks are exposed over less than 1 percent of the study unit. The Jurassic sequence covers all 

the structural basins of the study unit, is thickest (about 

380 m) near the YRB outlet and at the northwest end of 

the Wind River Range, and thins toward the southeastern and northwestern parts of the YRB (Peterson, 1972, 

p. 180).

Cretaceous System

Sedimentary rocks of the Cretaceous System 

extend over most of the plains and basins of the YRB, 

but are deeply buried at many locations. Where these 

rock units crop out, as they do in about 23 percent of the 

study unit, they usually are tilted and often beveled by 

erosion. The Lower Cretaceous rocks are thickest in 

the west and northwest (about 240 m), thin to generally 

less than 120 m in the east and southeast, and are absent 

only where removed by erosion on uplifts (McGookey 

and others, 1972; Steidtmann, 1993). Lower Cretaceous rocks generally outcrop in the YRB only on the 

flanks of uplifts and anticlines, or where displaced 

upward along faults. 

The Upper Cretaceous rocks exceed 2,400 m in 

thickness in the southern Powder River and eastern 

Wind River basins, generally range between 1,200 and 

2,400 m elsewhere, but are thinner in the southwestern 

and northeastern corners of the study unit (McGookey 

and others, 1972, p. 207). In ascending order, the 

Upper Cretaceous stratigraphic sequence in much of 

the YRB is the Mowry Shale, Frontier Formation, 

Cody Shale, Mesaverde Formation, Lewis Shale, Fox 

Hills Sandstone, and the Lance Formation. In the Bighorn and Wind River Basins, the marine Lewis Shale 

and nonmarine Meeteetse Formation interfinger and 

together occupy the interval between the Mesaverde 

and Lance Formations (fig. 8).

In the northwestern YRB is a thick Upper Cretaceous sequence of rocks consisting chiefly of volcanic 

debris that is named the Livingston Group (Roberts, 

1972). The group is composed of four nonmarine formations: a basal unit of siltstone and sandstone, overlain by a unit of alternating beds of siltstone and 

sandstone, a unit of largely mudstone, and an upper unit 

of sandstone and conglomerate (Roberts, 1972).
18 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 19
20 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Tertiary System

A thick sequence of sedimentary rocks of Tertiary age unconformably overlies the eroded Cretaceous surface in most of the plains and basins of the 

study unit. Tertiary sedimentary rocks are exposed 

over about 43 percent of the study unit, and Tertiary 

volcanic rocks are exposed over nearly 8 percent. The 

Paleocene Fort Union Formation, Eocene Wasatch, 

Willwood, or Wind River Formations, and Oligocene 

White River Formation are widely found in the Tertiary 

stratigraphic sequence. Units other than the Fort Union 

Formation or equivalents are absent in many places, 

and Tertiary volcanic rocks are present chiefly in the 

vicinity of the Absaroka volcanic field. 

Paleocene and Eocene sedimentary rocks consist 

chiefly of sandstone, siltstone, claystone, with interbedded coal and lignite (Whitehead, 1996). The Paleocene Fort Union Formation is widespread in three of 

the major structural basins of the study unit: the Bighorn, Powder River, and Wind River Basins. However, 

in the Eocene interval the different basins of the study 

unit display contrasting stratigraphy (fig. 8). 

The Eocene rocks of the Absaroka volcanic field 

were named the Absaroka Volcanic Supergroup 

(Smedes and Prostka, 1972) because three stratigraphic 

groups are included: in ascending order, the Washburn 

Group, Sunlight Group, and Thorofare Creek Group. 

No column for this volcanic field is included in the generalized correlation chart (fig. 8); the reader is referred 

to Sundell (1993) for a recent overview. The Washburn 

Group is the oldest part of the volcanic deposits and 

makes up much of the northern Absaroka Range. Farther south, it is overlain by rocks of the Sunlight Group. 

Due to the lack of regional marker beds, correlations 

between different areas of the Absaroka volcanic field 

are not well established. The stratigraphy is greatly 

complicated by multiple source areas, reworking, and 

post-depositional deformation including regional tectonism, volcanism, and mass movements (Sundell, 

1993).

Oligocene and Miocene sedimentary rocks consist mostly of semiconsolidated claystone, siltstone, 

sandstone, with some conglomerate (Whitehead, 

1996). The Oligocene White River Formation is widespread in the south and eastern parts of the study unit, 

but is absent in the Bighorn Basin. According to Love 

and others (1963), at the conclusion of the period of 

Miocene deposition, only scattered knobs of Precambrian basement rocks protruded from the thick blanket 

of sedimentary rocks. Within the study unit, the 

Miocene is represented primarily by the Split Rock 

Formation in the Wind River Basin and the Colter Formation on the Yellowstone Plateau.

Quaternary System

A great variety of Quaternary deposits occur 

within the study unit, including eolian, fluvial, glacial, 

landslide, and volcanic deposits. Valley-fill deposits 

consisting of unconsolidated gravel, sand, silt, and clay 

occur adjacent to most of the larger streams of the study 

unit (Whitehead, 1996). Although unconsolidated 

deposits, commonly being quite permeable and often 

located along streams, are likely to be important for 

understanding hydrology and water quality, they are 

not further discussed in this section because their spatial heterogeneity is so great that brief treatment is of 

minimal value. Of the consolidated Quaternary deposits, particular attention is given to the Quaternary volcanic rocks composing the Yellowstone Plateau. 

The rocks of the Yellowstone Plateau are almost 

exclusively rhyolites and basalts, with the rhyolites 

being greatly predominant (Christiansen and Blank, 

1972). The welded ash-flow tuffs that largely compose 

the outer part of the plateau are separated by unconformities into three major ash-flow sheets. The tuffs overlie rocks from the Precambrian through Quaternary 

Systems, but in the eastern plateau they chiefly overlie 

Eocene rocks of the Absaroka Volcanic Supergroup. 

At the center of the plateau, rhyolitic lava flows cover 

and partly fill the enormous elliptical caldera formed as 

a result of the eruption of the oldest ash-flow sheet.

Glaciation

The northeastern part of the study unit was subjected to three periods of glaciation that have not been 

radiometrically dated, but most likely the early Wisconsinan advance more than 20,000 years ago was the 

most recent glaciation (Clayton and others, 1980). 

Along the Yellowstone River valley, a high terrace surface (119 m above the river near Glendive, Mont.) converges with the present floodplain in the upstream 

direction. The two surfaces are only 38 m apart east of 

Billings, Mont. The upstream convergence of the two 

surfaces suggests that downcutting of the Yellowstone 

River was the result of base-level lowering following 

retreat of the ice sheet that had blocked the northeastflowing rivers of Montana (Johns and others, 1982). 
ENVIRONMENTAL SETTING 21

During one of the glacial advances, the Missouri and 

Yellowstone Rivers apparently flowed into North 

Dakota at a point about 18 km south of where the 

present Yellowstone River crosses that boundary 

(Clayton and others, 1980). The transport of water and 

sediment from the upper Missouri River through that 

relict valley potentially could be reflected in distinctive 

alluvial ground-water characteristics. 

Glacial sediments, other than scattered lag boulders, are found only behind the terminus of the most 

recent glacial advance into the study unit; there, lag 

boulders abound and glacial sediment is widely preserved (Clayton and others, 1980). Although about 

2.5 percent of the YRB was subjected to continental 

glaciation (Fenneman and Johnson, 1946), less than 

0.5 percent is presently overlain by sediments deposited by continental glaciation (Clayton, 1980).

Alpine glaciation was widespread in the high 

mountainous areas of the YRB during the Pleistocene 

Bull Lake and Pinedale Glaciations. During the 

Pinedale Glaciation, about 13,000 to 30,000 years ago, 

major ice streams from four source areas converged to 

form the northern Yellowstone outlet glacier that 

flowed 60 km down the Yellowstone valley (Pierce, 

1979). Ice from the Pinedale icecap on the Yellowstone 

Plateau also flowed into adjacent basins and out to 

other glacial termini, some of which were outside the 

study unit. During the late Pleistocene, mountain glaciers in places extended several kilometers beyond canyon mouths onto basin floors (Mears, 1987). The 

icecap on the Beartooth uplift was a source area for 

numerous glaciers, the northern Yellowstone outlet glacier among them (Pierce, 1979).

Although their size and elevation were probably 

the most important factors affecting the glacial productivity of the source areas, geographic location with 

respect to incoming storms was also very significant 

(Pierce, 1979). Many storms probably moved eastward 

from the Pacific Northwest and dropped great amounts 

of snow when forced to rise over the icecaps on the Yellowstone Plateau (Pierce, 1979), Beartooth Mountains, 

and Wind River Range.

Today, only small remnant glaciers are present in 

the study unit, mostly concentrated in the Beartooth 

and Wind River Mountains. But glacial and periglacial 

landforms are quite extensive. Many of the mountain 

streams flow through U-shaped valleys characteristic 

of alpine glaciation. Pinedale Till, and Bull Lake Till 

at locations beyond the maximum limit of Pinedale 

Glaciation, dominate the lower hillslopes of many 

mountain valleys. Glacial-outwash terraces stand 

higher than Holocene stream terraces and present-day 

floodplains in many stream valleys. 

Seismicity 

Seismicity of the region, based on earthquake 

frequency and intensity, was described by Simon 

(1972). The westernmost section of the study unit has 

been an area of moderate seismic activity, whereas seismicity in the Bighorn and Wind River Basins has been 

substantially less (Simon, 1972, p. 50). Substantial 

areas where no recorded earthquakes have occurred 

include the Powder River Basin and the Beartooth and 

Bighorn Mountains. Most of the strong earthquakes in 

the study area have occurred in the vicinity of Yellowstone National Park, where the Yellowstone caldera 

lies at the apex of a parabola-shaped zone of seismicity 

with the axis centered on the eastern Snake River Plain 

of Idaho (Smith and Braile, 1993). As early as 1871, 

F.V. Hayden recognized that Yellowstone was at a great 

volcanic center and, after experiencing several earthquake shocks while camped along Yellowstone Lake, 

remarked that, “I have no doubt that if this part of the 

country should ever be settled and careful observations 

made, it will be found that earthquake shocks are of 

very common occurrence” (Hayden, 1872). Yellowstone Lake has persisted as a notable area of earthquake 

swarms (Smith and Braile, 1993). 

The significance of seismic activity as a factor 

potentially affecting water quality is illustrated by the 

1959 earthquake and associated landslide in the Madison River valley of Montana (just west of the YRB). 

That event deposited 32 million m3 of colluvium on the 

canyon floor, impounding water behind a 60-m high 

rock dam it created (Dunne and Leopold, 1978).

Coal and Hydrocarbon Deposits

Most of the coal deposits in the YRB are located 

in the nearly horizontal Tertiary beds in the structural 

basins of the Northern Rocky Mountains and Northern 

Great Plains coal regions. Coal beds within the Paleocene Fort Union Formation are as much as 43 m thick 

(Flores, 1996). Vast amounts of subbituminous coal 

are found in extensive beds that are thick enough and 

near enough to the surface to be considered strippable 

deposits.
22 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

“Coals from the Powder River Basin analyzed for Hazardous Air Pollutants named in 

the 1990 Clean Air Act Amendments indicate 

lower * * * [pollutant] content than other coals 

from within this region and other regions in the 

United States. Thus, the unique chemical 

characteristics and thickness of these Fort 

Union coals make them an important resource 

for continued and expanded use within current 

and future environmental constraints” (Flores, 

1996).

In the Bighorn Basin, the coal-bearing rocks are 

exposed around the rim of the basin in a belt 5 to 24 km 

wide, and consist of the Upper Cretaceous Mesaverde, 

Meeteetse, and Lance Formations and the Paleocene 

Fort Union Formation (Berryhill and others, 1950). 

Coal-bearing formations in the Wind River Basin crop 

out only around the basin rim and are overlain by thick 

units of younger, non-coal-bearing rock in the central 

part (Berryhill and others, 1950). Because coal beds in 

the Wind River Basin are relatively thin and steeply 

dipping, they are economically insignificant compared 

to those in the Bighorn Basin, which are small in comparison with those in the Powder and Tongue River 

Basins (Peterson and others, 1987). The aggregate 

thickness of coal in the Fort Union Formation exceeds 

90 m in the Powder River Basin (Barlow and others, 

1993), but many coal beds are discontinuous, “podshaped” deposits (Flores, 1997).

Coal beds also are important reservoirs of methane-rich gas, only recently recognized to represent an 

enormous “unconventional” energy resource (U.S. 

Geological Survey, Energy Resource Surveys Program, 1997). Large quantities of water, sometimes 

saline, are unavoidably produced along with the methane gas. Fractures that permeate coal beds are usually 

filled with water; the deeper the coal bed, the less water 

usually present, but the more saline it becomes (U.S. 

Geological Survey, Energy Resource Surveys Program, 1997). 

Hydrocarbon minerals include natural gas and 

oil, in addition to the coal-bed methane just discussed. 

Productive reservoirs of hydrocarbon minerals in the 

Rocky Mountains and Northern Great Plains region are 

found in strata from the Cambrian through Tertiary 

Systems (Spencer, 1996). An estimated 99 billion m3

of gas and 2.5 billion barrels of oil are yet undiscovered, technically recoverable resources in the Bighorn, 

Powder River, and Wind River Basins (U.S. Geological 

Survey, 1996a). 

In the Powder River Basin, hydrocarbon reservoirs are located mostly in Cretaceous or Paleozoic 

rocks; reservoirs in the Bighorn Basin are typically in 

Paleozoic rocks; and in the Wind River Basin, reservoirs in Paleozoic, Cretaceous, and Tertiary rocks are 

common (DeBruin and Boyd, 1991). 

“In general, Paleozoic reservoirs are 

marine and eolian sandstone and marine carbonate and produce mostly oil and associated 

gas. Mesozoic reservoirs are mostly marine 

and fluvial sandstone; the Cretaceous is by 

far the dominant age of Mesozoic producing 

rocks. The Cretaceous produces oil and 

associated gas and non-associated gas. Tertiary reservoirs are dominantly continental, 

lenticular sandstone and * * * produce mostly 

gas” (Spencer, 1996).

Paleozoic hydrocarbon reservoirs include a few 

stratigraphic traps, such as at Cottonwood Creek in the 

Bighorn Basin, where the Permian Phosphoria Formation and related rocks are the reservoir rocks. The 

Pennsylvanian Minnelusa Formation also contains 

many stratigraphically trapped oil reservoirs in the 

Powder River Basin (DeBruin, 1993). Most of the 

many important Paleozoic reservoirs in the Bighorn 

Basin are associated with anticlines around the basin 

margins. Sandstone reservoirs of the Pennsylvanian 

Tensleep Sandstone, and carbonate reservoirs of the 

Phosphoria Formation and Mississippian Madison 

Limestone are among the major reservoirs of oil and 

gas (DeBruin, 1993).

In the Lower Cretaceous Series, the Muddy 

Sandstone hydrocarbon reservoirs are predominantly 

stratigraphic traps in the Powder River and Wind River 

Basins (Mullen and Barlow & Haun, Inc., 1993). The 

Frontier Formation is the major Upper Cretaceous 

hydrocarbon reservoir in the study unit. Within the 

YRB, major oil and gas reserves in the Frontier Formation are associated with nearshore marine sandstones 

and reworked marine sandstones (Doelger and others, 

1993). The nearshore marine sandstone of the Frontier 

Formation is particularly important at the Salt Creek 

and Teapot fields north of Casper.

In the Tertiary System, the Paleocene Fort Union 

Formation produces gas in two separate areas of the 

YRB. Several reservoirs are located in the interior of 

the Wind River Basin, and sandstone reservoirs associated with coal beds are found in the northeastern Powder River Basin (DeBruin and Boyd, 1991).
ENVIRONMENTAL SETTING 23

Metallic Mineral Deposits

Mineral deposits are the result of specific geologic processes and thus occur only in those geologic 

environments where those processes have been active 

(Anderson and others, 1993), or where mineralenriched material eroded from those deposits has been 

re-deposited in sufficient concentration to produce secondary enrichment. Only a small fraction of any area is 

likely to contain recoverable mineral deposits, and such 

is the case for the study unit, where about 2 percent of 

the total area lies in the principal areas of metallic mineral deposits (fig. 9). Known, potentially important 

metallic mineral deposits are found in each of the major 

structural units of the YRB, except the Williston Basin. 

Uranium mineralization is particularly widespread, 

occurring in most of the stratigraphic units (Harris and 

King, 1993). Important or substantial mineral-deposit 

areas are discussed separately by structural unit in this 

section of the report.

Absaroka Volcanic Field

In the Absaroka volcanic field (fig. 9), the geologic environment associated with metallic mineral 

deposits consists of deeply eroded intrusive centers 

(volcanic-plutonic complexes), where the plutonic 

component represents the solidified magma chamber 

beneath the former volcano (Hammarstrom and others, 

1993). Mineralization associated with Absaroka intrusive centers generally occurs as disseminations and 

stockworks in intensely altered rock and in fractures 

and veins. Hydrothermal alteration typically produced 

crudely zoned mineralization around the core of the 

intrusive complex. Porphyry copper deposits (veinlets 

and disseminated grains of copper-bearing minerals in 

plutonic rocks) may occur in the center of the complex 

(Hammarstrom and others, 1993), often associated with 

molybdenum, and sometimes traces of gold. Coppergold skarns and replacement deposits form in limestone 

when adjacent to the complex; and zinc-lead-silver vein 

deposits (sulfide minerals containing large concentrations of these, and sometimes gold or other, minerals) 

occur laterally away from the center along faults and 

shear zones (Hammarstrom and others, 1993). In addition to the porphyry copper deposits, heavy minerals 

such as gold may also be found in placer deposits downstream from porphyry districts (Hausel, 1989). 

Due to the presence of several large, porphyry 

copper deposits, the Absaroka Range has definite 

potential for becoming a metals mining district (Hausel, 

1989). Within the Absaroka Range there are four substantially mineralized areas: the Kirwin, New World, 

Stinkingwater, and Sunlight districts (Nelson and 

others, 1980). 

Several intrusive complexes penetrate the Eocene 

volcanic rocks of the Kirwin area (fig. 9). The layered, 

vent-facies volcanic rocks have been domed, hydrothermally altered, and radially fractured (Hausel, 1989). 

Copper, silver, molybdenum, and gold deposits are 

known.

The New World district (fig. 9), also known as the 

Cooke City district, was discovered in the 1860s. It is 

located in an eruptive center, but uplift and erosion have 

removed most of the volcanic material and exposed Precambrian gneiss, Paleozoic rocks, and Tertiary intrusive 

rocks throughout much of the district (Elliott, 1980b). 

With few exceptions, major ore bodies are localized 

adjacent to the stocks and mineralization extends outward in irregular metallogenic zones, from coppergold-silver skarn and replacement deposits near intrusions to distal lead-zinc-silver vein deposits (Hammarstrom and others, 1993b; Hausel, 1989). Many of the 

copper-gold-silver deposits are gold-bearing skarns that 

formed in carbonate rocks, in breccia, and along faults 

(Hammarstrom and others, 1993b). Silver-lead-zinc 

ores occur in fissure-replacement deposits intruding 

into Paleozoic limestone south of Cooke City, more 

than 4 km south of the intrusive centers (Lovering, 

1930). Since 1990, considerable exploration and development have focused on five copper-gold-silver deposits in the New World district (Hammarstrom and others, 

1993a).

The Stinkingwater district (fig. 9) includes several mineralized porphyries. Mineral deposits in the 

Stinkingwater area include disseminated sulfide minerals in altered rocks adjacent to granodiorite and dacite 

intrusive stocks, and fracture-filling veins in adjacent, 

layered volcanic rocks (Hausel, 1989). Metal zonation 

is characterized by a central copper-molybdenum zone 

surrounded by a zone of veins bearing silver, gold, lead, 

zinc, arsenic, and possibly mercury (Fisher, 1972). 

Streambed sediments in the vicinity of this mineralized 

area contain small amounts of gold and mercury 

(Fisher, 1972).

Both disseminated deposits and vein mineralization occur in the Sunlight district (fig. 9). Disseminated 

deposits are localized around intrusives and include 

pyrite in altered zones, chalcopyrite in intrusive rocks, 

and copper-bearing stockworks (Hausel, 1989). Copper
24 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 25

enriched veins tend to be near the stocks, and lead-silver bearing veins are found at distance from the intrusions (Elliott, 1980a). Samples of vein material 

collected in 1970 from 40 locations in the Sunlight 

mining region were analyzed spectrographically and 

showed consistently minor amounts of gold but locally 

substantial amounts of silver, copper, and lead (Nelson 

and others, 1980). Results of sampling bedrock and 

streambed sediment indicate a copper-gold zone associated with syenite stocks in the south and east part of 

the Sunlight region, and a lead-silver zone in the north 

and east (Elliott, 1980a).

Beartooth Uplift and Cretaceous Volcanic Centers

Mineral deposits in the Beartooth uplift are of 

diverse types and metal suites (U.S. Geological Survey 

and U.S. Bureau of Mines, 1983). Six primary areas of 

mineral deposits are known in the western Beartooths: 

the Emigrant-Mill Creek area, Independence (Boulder) 

district, and Horseshoe Mountain area are associated 

with Tertiary intrusive rocks; the Goose Lake area is 

associated with a Cretaceous intrusion; and the JardineCrevice Mountain (Sheepeater) district and Stillwater 

Complex are in Precambrian rocks. In the eastern 

Beartooths, the Red Lodge chromite district is the primary known mineral area. North of the Beartooth 

Mountains front, is the Sliderock Mountain area, containing Cretaceous volcanic and intrusive rocks (Elliott 

and others, 1993).

In the Emigrant-Mill Creek area (fig. 9), numerous small deposits include such metals as copper, 

molybdenum, gold, silver, lead, zinc, tungsten, bismuth, manganese, and iron. The Emigrant stock and 

several smaller dikes, sills, and laccoliths intrude Precambrian gneiss and schist, Paleozoic sedimentary 

rocks, and Tertiary volcanic rocks (Elliott and others, 

1983). Hydrothermal alteration, including widespread 

pyritic alteration, decreased with intensity outward 

from the stock and produced a zonation of metallic 

mineralization. Copper and molybdenum are concentrated near the center of the stock, gold is near its 

periphery, and lead, zinc, and silver occur mostly near 

or outside the stock periphery (Elliott and others, 

1983). The placer gold deposits of Emigrant Creek 

were a particularly large and productive mineral 

resource (Hammarstrom and others, 1993).

In the Independence district (fig. 9), an Eocene 

intrusion into rocks similar to those at the Emigrant 

stock is associated with a suite of metals also similar to 

those in the Emigrant-Mill Creek area (Elliott and others, 1983). Zonation of the altered and mineralized 

rocks is again evident and is typical of porphyry copper 

systems (Hammarstrom and others, 1993b). The intrusive rocks of the area are more mafic than those of the 

Emigrant-Mill Creek area, and therefore considered to 

have less mineral potential for copper and molybdenum (Elliott and others, 1983). Placer gold is present 

in most of the surrounding streams (Hammarstrom and 

others, 1993b).

An altered dacite porphyry at Horseshoe Mountain (fig. 9), midway between the Cooke City and Independence districts, has been historically prospected for 

gold in both lode and placer deposits. Geochemical 

anomalies for molybdenum, tungsten, precious- and 

base-metals have been detected in sediments of streams 

draining this area (Hammarstrom and others, 1993b).

The Goose Lake area (fig. 9), located in the headwaters of the Stillwater River at the northeast corner of 

the New World district, contains mineralization associated with a locally altered Late Cretaceous syenite 

stock intruding Precambrian granitic gneiss. The 

deposit is considered an unusual occurrence of coppergold-platinum mineralization that is an example of a 

magmatic segregation deposit associated with alkaline 

gabbros and syenites (Hammarstrom and others, 

1993b). The principal minerals are chalcopyrite and 

bornite, occurring as disseminations and vein fillings. 

Shear zones away from the syenite stock also are mineralized (Simons and others, 1979). Copper, gold, silver, platinum, palladium, lead, and tungsten are 

present.

Gold discovered in placer deposits in 1862 led to 

discovery of gold-bearing quartz veins at Jardine in 

1870. Substantial deposits of gold, arsenic and tungsten are present, as confirmed by geochemical analyses 

of rock and stream-bed sediment samples (Wedow and 

others, 1975). Extensively folded Precambrian 

metasedimentary rocks contain quartz veins and shear 

zones that are concentrated in two distinct belts, one 

through Mineral Hill and the other through Crevice 

Mountain (Hammarstrom and others, 1993b). The 

geologic setting of the Jardine-Crevice Mountain 

deposits (fig. 9) is typical of the metasedimentary parts 

of ancient greenstone belts, and the deposits are similar 

to other lode gold systems associated with a banded 

iron formation (Hammarstrom and others, 1993b). 

Mineral deposits of the Stillwater Complex 

(fig. 9) include chromium, nickel, copper, and platinum-group elements (PGE). The deposits are stratiform 
26 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

and exhibit great lateral continuity (Elliott and others, 

1983). The nickel-copper deposits are concentrated at 

the base of the complex as sulfide-rich zones that have 

been identified as large tonnage, low-grade resources 

(Zientek, 1993). PGE occur near the base of the complex and also a few thousand meters above the base in a 

thin, laterally persistent interval of PGE-enriched disseminated sulfide mineralization (Zientek, 1993). The 

Stillwater Complex contains the largest identified 

resources of PGE and chromium in the U.S., in addition 

to major nickel and copper resources (Hammarstrom 

and others, 1993a). Undiscovered deposits of copper, 

nickel, chromium, and platinum-group elements are 

likely to be present in the complex (Zientek, 1993).

Chromite was discovered in the Red Lodge district (fig. 9) in 1916 (James, 1946). The Red Lodge 

chromite occurs as lenses in masses of serpentinite 

intruded into granitic gneiss and is exposed at the edges 

of plateaus at an elevation of about 3,000 m (Simons 

and others, 1979). The relatively low-grade chromite 

deposits have been highly metamorphosed, tectonically 

disrupted, and isolated (Hammarstrom and others, 

1993b). The deposits are dissimilar from the stratiform 

deposits in the Stillwater Complex (James, 1946). The 

mineral resources remaining in this district are relatively small, many deposits having been completely 

mined out.

The Sliderock Mountain area (fig. 9) features a 

diorite stock surrounded by lahar deposits, andesite 

flows, and flow breccias. Concentrically zoned hydrothermal alteration and geochemical anomaly patterns 

indicate the presence of a shallow subsurface porphyry 

copper-gold deposit (Hammarstrom and others, 1993b). 

Silver, lead, zinc, and molybdenum also are present.

Bighorn, Owl Creek, and Pryor Mountains

The Precambrian granites and gneisses of the 

Bighorn Mountains are not considered likely hosts for 

major precious metal occurrences, but the possibility 

cannot easily be dismissed (Hausel, 1989). The known 

precious metal deposits are associated with mafic dikes, 

veins, and shear zones in the Precambrian core and with 

core debris in placers in the overlying Cambrian Flathead Sandstone. In the Bald Mountain area (fig. 9), 

mineral-enriched conglomerates in the Flathead Sandstone contain gold and other heavy minerals, such as 

ilmenite, magnetite, zircon, and monazite. Other minor 

deposits of metals in the Bighorn Mountains contain 

tungsten, silver, manganese, copper, and gold in quartz 

veins (Hausel, 1989); and numerous small deposits of 

uranium and vanadium are known in the Little Mountain area (Van Gosen and others, 1996).

In the eastern Owl Creek Mountains (fig. 2), Precambrian metamorphic rocks are exposed in the Copper 

Mountain district (fig. 9), which includes several 

known mineralized areas. In the western end of the district, a vein bearing sporadic sulfides and gold represents a relatively small deposit. Near the center of the 

Copper Mountain district, quartz veins and a banded 

iron formation are known to contain copper, titanium, 

gold, and silver (Hausel, 1989). In the eastern end of 

the district, a mafic dike is fractured and quartz veins 

fill the fractures; both the veins and the dike are mineralized with copper, nickel, gold, and silver (Hausel, 

1989). Also at Copper Mountain, uranium mineralization occurs in the Eocene Tepee Trail Formation and in 

altered rocks, breccia zones, and fractures in the underlying Precambrian rocks (Yellich and others, 1978). 

Numerous uranium-vanadium deposits were discovered in the Pryor Mountains (fig. 2) during 1955-58 

(Van Gosen and others, 1996). These deposits are 

mostly small and are hosted by shallow collapse structures in the upper third of the Madison Limestone. The 

mineral deposits are found mostly within vugs, open 

fractures, and the most permeable cavern-fill materials 

(Van Gosen and others, 1996).

Bighorn and Powder River Basins

In the Bighorn Basin, titaniferous black sandstone deposits, hosted primarily in the Upper Cretaceous Mesaverde Formation, are found in several 

locations, with the Grass Creek deposit (fig. 9) being 

the largest high-grade deposit in Wyoming (Hausel, 

1993). These deposits contain abundant heavy minerals, including zircon, monazite, and gold, in addition to 

titanium-bearing minerals (Hausel, 1989).

Uranium deposits (fig. 9) of economic significance in the Powder River Basin occur primarily in the 

Wasatch Formation of Tertiary age (Hausel, 1982). 

Most economic uranium deposits in the YRB are found 

in arkosic Eocene and Paleocene sandstones and conglomerates of continental origin, in which uranium 

minerals coat the grains and partly fill the interstices 

(Butler, 1972). Mineralization dominantly occurs as 

“roll-front” deposits located at contacts between oxidized (altered) and reduced (unaltered) zones (Hausel, 

1982). Uranium, being soluble in the oxidized state but 

not in the reduced state, is readily leached from the oxi-
ENVIRONMENTAL SETTING 27

dized rocks and deposited in zones where geochemical 

conditions change from oxidizing to reducing (Lowry 

and others, 1993). Uraniferous tuffaceous units, such as 

the Oligocene White River Formation, are the probable 

source of the leached uranium in the Powder River 

Basin (Harris and King, 1993). 

Radium deposits are also found in Tertiary sediments, such as with uranium in concretionary occurrences in the Pumpkin Buttes (fig. 9) area (Harris and 

King, 1993). Radium is soluble in water, regardless of 

oxygen availability, but is strongly adsorbed by manganese and iron oxides, so its presence in an oxidizing 

environment varies with those of the manganese and 

iron oxides (Harris and King, 1993). There is no 

present economic interest in radium deposits, as it is 

readily available from production in nuclear reactors.

Wind River Basin and Wind River Range

In the Gas Hills (fig. 9) uranium district at the 

southern end of the Wind River Basin, substantial uranium deposits occur in the Wind River Formation of 

Tertiary age (Hausel, 1982) and may be derived from 

leached or eroded uranium in the Precambrian granites 

of nearby mountains (Harris and King, 1993). Wyoming contains more than half of the Nation’s uranium 

reserves (U.S. Dept. of Energy, Energy Information 

Administration, 1997), and a substantial percentage is 

located within the YRB in the Powder River and Wind 

River Basins.

Alluvial gravels along the Wind River, Little 

Wind River, and Popo Agie River (not labeled on 

figures) contain gold, commonly occurring as very fine 

particles. The largest concentrations known were found 

in Wind River placer deposits west of Riverton and in a 

downstream reach now mostly submerged in Boysen 

Reservoir (Hausel,1989). Other placer deposits of gold 

and monazite occur in terrace deposits and gravels of 

Warm Spring Creek (not shown in figures) in the northern Wind River Range (Hausel, 1989). Placer gold also 

has been found in gravels of several other eastwardflowing streams draining the Wind River Range to the 

south.

The South Pass-Atlantic City (fig. 9) greenstone 

belt is known for rich deposits of gold and iron ore. The 

complexly folded rocks of the greenstone belt extend 

into the study unit at the southeast end of the Wind 

River Range (Hausel, 1989). Substantial lode deposits 

of silver, gold, and copper occur in shear-zone structures and quartz veins. Also, banded iron formation, or 

taconite, deposits are located along the northwestern 

flank of the greenstone belt. 

Near the greenstone belt, precious-metal bearing 

Oligocene conglomerates were deposited as a giant 

alluvial fan at the mountain front. The source of the 

metals in the Oligocene conglomerate is believed to be 

a small area of metamorphic rocks cut by veins and 

probably now buried by strata of the Oligocene White 

River Formation (Antweiler and others, 1980). Other 

Tertiary conglomerates in this same general area are 

known to contain precious metals (Hausel, 1991).

Vegetation

Vegetation is a sensitive indicator of multiple 

environmental factors, including precipitation, temperature, soil, geology, and wind; in turn, vegetation affects 

a number of important processes, including snow accumulation, soil moisture depletion, surface runoff, infiltration, and erosion (Knight, 1990). Vegetation input to 

streams (coarse- particulate organic material) and 

inputs of chemicals applied to vegetation are additional 

factors affecting stream quality.

Potential natural vegetation is defined as the ultimate successional stage of the native flora of an area, 

under the current climatic regime and in the absence of 

further human modification (Kuchler, 1970). Thus 

defined, the potential vegetation is partly determined by 

previous human practices. For example, since settlement, semidesert shrubs seem to have expanded their 

range into large areas that formerly were steppe (shortgrass prairie), due to overgrazing and trampling by livestock (Bailey, 1995). Plant successional processes 

alone may not be sufficient to potentially reestablish 

grasslands in all affected areas. Kuchler (1964) mapped 

the potential natural vegetation of the U.S. 

(1:3,168,000 scale), and the part of that map relevant to 

this report is shown in figure 10. Many of the plant 

communities generally are distributed in zones corresponding to differences in elevation. This pattern provides the organization for the remainder of this section.

In the alpine meadows that compose about 3 percent of the study unit, tundra vegetation is typical, and 

trees are nearly absent or stunted where present. Alpine 

meadows occur mainly above 3,100 m elevation in the 

YRB (Despain, 1990; Thilenius and Smith, 1985) and 

are most extensive on the plateaus of the Beartooth 

uplift. Common plant types in the alpine meadows are 
28 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 29

forbs, grasses, sedges, dwarf willows, and prostrate 

shrubs (Thilenius and Smith, 1985; Marston and 

Anderson, 1991). These plants are small and close to 

the ground, where they are protected from the high 

winds and warmed by longwave reradiation from soil 

and rocks (Despain, 1990).

The spruce-fir forest is found in the subalpine 

zone, typically dominated by Engelmann spruce and 

subalpine fir (Bailey, 1995), but at the upper end of this 

zone, whitebark pine is dominant (Despain, 1990). 

Below the subalpine zone lies the montane zone corresponding to Kuchler’s (1964) Douglas fir forest 

(fig. 10). The montane zone characteristically features 

alternating stands of Douglas fir and ponderosa pine, 

with the pines dominating on drier, more exposed slopes 

(Bailey, 1995). Lodgepole pine often is a codominant 

species at middle elevations (Marston and Anderson, 

1991). The distribution of coniferous forest by elevation 

zone is shown in figure 11. Coniferous forest begins to 

be the predominant land cover at elevations greater than 

about 2,100 m. Marston and Anderson (1991) state that 

an elevation of about 2,130 m defines the boundary of 

the Greater Yellowstone Ecosystem, because this factor 

exerts such strong control on the structure and function 

of both aquatic and terrestrial ecosystems in that area.
30 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

In addition to elevation, moisture gradients and 

disturbance history influence forest types. For example, 

in subalpine forests of Greater Yellowstone, subalpine 

fir is more common in moist locations, whereas lodgepole pine and Douglas fir dominate on drier slopes 

(Despain, 1990). Mountain vegetation zones extend to 

lower altitudes on wetter east- and north-facing slopes 

and steppe vegetation extends farther upward on southand west-facing ones (Bailey, 1995; Marston and 

Anderson, 1991). 

A major disturbance such as fire restarts the long 

successional process by which vegetation progresses 

through a series of plant communities toward the climax 

community (Despain, 1990). Under a natural fire 

regime, the wildfire return interval in most of the mountain forests in western parts of the YRB is between 200 

and 400 years (Despain, 1990; Romme, 1980). Such 

intervals do not necessarily allow the successional process to culminate. In addition to vegetation disturbance, 

forest fires produce increased stream discharge, erosion, 

and sediment transport from severely burned watersheds (Tiedemann and others, 1979). Also, loss of 

riparian vegetation and woody debris jams increases the 

risk of serious washouts of organic matter, nutrients, 

and sediment stored in channels if high-flow events 

shortly follow the burning of much of a watershed 

(McIntyre and Minshall, 1996). The large fires of 1988 

in the Greater Yellowstone Area burned more than 

3,300 km2

 of mostly forested land in the YRB (Minshall 

and Brock, 1991; Greater Yellowstone Post-Fire 

Resource Assessment Committee, 1988). The firerelated changes were expected to cause substantial 

impacts to extensively burned watersheds, but large 

watersheds (greater than 500 km2) were only partially 

burned and principal streams were little affected (Minshall and others, 1989; Minshall and Brock, 1991, 

p. 126).

Soil and rock types also affect forest communities 

through nutrient and moisture availability. For example, in the Greater Yellowstone Area, soils formed from 

andesite have good water-holding capacity and mineral 

nutrient content, whereas soils formed from rhyolite are 

coarse textured, with poor water-holding capacity and 

only one-third the nutrient level of andesitic soils 

(Despain, 1990). As a result, distinctly different forest 

communities are found on these contrasting soils: 

lodgepole pine is persistently dominant in areas underlain by rhyolite flows, whereas spruce-fir communities 

are usually dominant in old-growth forests on soils 

formed from andesite (Marston and Anderson, 1991).

In the lower-elevation, open woodlands of the 

Great Plains Province, ponderosa pine is the dominant 

species. Such forest lands cover about 6 percent of the 

study unit, and correspond to the peak in coniferforested area at about 1,200 m elevation (fig. 11). 

Grasses, with some sagebrush, typically compose the 

ground cover and understory in the open woodlands 

(Bailey, 1995).

Coniferous forests actually cover about 19 percent of the study unit, whereas they potentially would 

cover about 28 percent under natural conditions 

(Kuchler, 1964). The major forest types and percentage 

of YRB forested land they occupy are lodgepole pine 

(34 percent), ponderosa pine (22 percent), spruce-fir 

(19 percent), Douglas fir (14 percent), broadleaf hardwoods (6 percent), and pinyon-juniper (4 percent) forests (U.S. Department of Agriculture, Forest Service, 

1970).

Steppe is the most widespread vegetation class in 

the YRB, covering about 44 percent of the study unit, as 

estimated by comparing data from Bailey (1995) and 

U.S. Geological Survey (1986). Kuchler’s map (1964; 

fig. 10) shows three categories corresponding to steppe: 

foothills prairie, grama-needlegrass-wheatgrass grassland, and wheatgrass-needlegrass grassland. The short, 

typically bunched grasses of the steppes include predominantly grama, needlegrass, and wheatgrass 

(Bailey, 1995; Kuchler, 1964; Marston and Anderson, 

1991). Numerous species of wildflowers bloom during 

spring and summer, with sunflower among the most 

prominent (Bailey, 1995). Sagebrush is an important 

component of the vegetation mosaic in many grassland 

areas. Prickly-pear cactus apparently was abundant in 

the early 1800s, as it is today (U.S. Department of the 

Interior, 1965). Invasive exotic species, such as Russian 

thistle, are abundant in some locales. Except in moist 

hollows and valley floors, steppe grasses are usually 

sparse, with bare soil exposed between the bunches 

(Bailey, 1995).

Sagebrush steppe or semidesert shrub is the dominant vegetation type in about 21 percent of the study 

unit (compare Bailey, 1995, and U.S. Geological Survey, 1986). Kuchler’s map (1964; fig. 10) indicates that 

these shrub lands (wheatgrass-needlegrass shrub 

steppe, sagebrush steppe, and saltbush-greasewood 

shrubland) would potentially cover about 25 percent of 

the study unit. Typically, this vegetation type is a mixture of sagebrush with short grasses, but greasewood is 

prominent in extensive alkaline areas of the Bighorn 

Basin (Bailey, 1995). Sagebrush is typically found on 
ENVIRONMENTAL SETTING 31

well drained sites where most of the precipitation 

occurs as snowfall, whereas grasslands tend to be found 

where summer rainfall predominates (Knight, 1990). 

Other common shrubs include rabbitbrush, bitterbrush, 

and serviceberry (Marston and Anderson, 1991).

In the YRB, typically narrow riparian zones lie 

between the aquatic and terrestrial ecosystems and 

include several fluvial surfaces: channel islands and 

bars, channel banks, floodplains, and lower terraces on 

which the vegetation is dominated by deeply rooted 

plants capable of extracting water from the alluvial 

aquifer (Goodwin and others, 1997). The riparian vegetation community along a stream controls or influences several important ecologic characteristics, such 

as availability of habitat, canopy closure above the 

channel, water temperature, photosynthetic productivity, input of allochthonous resources, benthic invertebrate community composition, stream bank stability, 

density of coarse woody debris elements, and step size 

in step-pool channels (Hupp and Simon, 1986; Mihuc 

and others, 1996; Sweeney, 1992). Riparian vegetation 

may additionally benefit water quality by filtering sediments and utilizing nutrients (Correll and others, 1992; 

Knight, 1994). Riparian vegetation communities are 

highly productive and support an abundance of wildlife 

(Marston and Anderson, 1991), and domestic livestock 

also congregate there (Knight, 1994). Concentrations 

of large ungulates may trample banks and contribute to 

increased erosion and embeddedness. In contrast, beaver often produce a beneficial effect on water quality 

through dam building (Knight, 1994). Dams arrest streambed degradation, sedimentation behind the dams 

allows for greater stream clarity downstream, and 

enhanced bank storage of water results in higher latesummer streamflow that is beneficial to fish and wildlife.

Riparian vegetation is highly variable in the YRB 

and may include marsh, meadow, shrublands, and forest 

communities (Knight, 1994). At higher elevations, 

sedges, grasses, and short willows dominate the community typically found along headwater streams, but 

alders and tall willows become more conspicuous along 

lower mountain streams (Knight, 1994). In the foothills 

and intermontane basins, the riparian communities generally are dominated by cottonwoods, with aspens, 

lodgepole pine, and spruces also common (Knight, 

1994; Marston and Anderson, 1991). Along many 

streams of the eastern plains, grasses, rushes, and 

sedges are dominant plants in herbaceous riparian communities; and black greasewood, common chokecherry, 

coyote willow, silver buffaloberry, silver sagebrush, 

and western snowberry are dominant species in shrub 

riparian communities (Jones and Walford, 1995). The 

riparian zone along many other lowland streams is typically a woodland community with plains cottonwood 

being most abundant, and ash, boxelder, willows, and 

the exotic Russian olive all being common locally 

(Jones and Walford, 1995; Knight, 1994). The abundance of old-growth cottonwoods appears to be declining in some areas due to flood suppression (Knight, 

1994). Cottonwood regeneration is related to flooding 

and associated creation of bare sand and gravel bars that 

this pioneer species colonizes. Northern floodplain 

deciduous forests (fig. 10) occupy about 1 percent of 

the YRB, mostly as riparian communities along several 

of the principal streams (not only the Yellowstone 

River).

Other prominent vegetation in the study unit 

includes native hay, alfalfa, and seasonal coverage by 

crops such as small grains, beans, sugar beets, and corn. 

Agricultural lands are discussed in more detail in a separate section of this report.

Surface Water

The surface-water hydrology of the Yellowstone 

River Basin is characterized by the physiography of the 

basin and the flow and quality of its streams. Physiographic characteristics of the basin are important influences on the hydrology. Streamflows are characterized 

according to the origin of the streams as well as by the 

time of year. Characteristics of general stream-quality 

indicators illustrate the overall condition and variability 

in the quality of the basin surface-water.

Basin Characteristics

The diverse physiography of the YRB is an 

important factor in determining the surface-water 

hydrology. From its headwaters at the Continental 

Divide in northwestern Wyoming, the Yellowstone 

River flows out of the mountains and across the plains 

of south-central and eastern Montana to the confluence 

with the Missouri River in western North Dakota 

(fig. 12). The mainstem flows through the northwestern and northern parts of the study area only, resulting 

in an asymmetric drainage network. Most inflow to the 

mainstem is from the south from four major tributary 

basins: the Clarks Fork Yellowstone, Wind/Bighorn, 
32 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Tongue, and Powder Rivers. The higher elevations of 

the Beartooth Mountains, Wind River Range, Absaroka 

Range, and Bighorn Mountains are the headwaters of 

most of the perennial streamflow in the basin (Wahl, 

1970). Tributary to the major streams, as well as the 

mainstem, are many streams that originate in the lower 

elevations of the basins and plains areas.

Basin relief is large; headwaters elevations 

exceed 3,900 m above sea level for streams originating 

in the mountains, while the mouth of the mainstem Yellowstone River is only 564 m above sea level. The 

mainstem is more than 1,130 km long. The combination of large relief in the mountainous areas and long 

stream lengths across the basins and plains results in a 

large range in slopes for the major tributaries. Grading 

downstream from steep to relatively flat, channel slope 

of the mainstem ranges from more than 8 m/km in the 

upper reaches to about 0.6 m/km for the lower 400 km 

of the river. Mean depth and width increase with distance downstream for many streams in the basin. 

Leopold and Maddock (1953) demonstrated that for a 

discharge of given frequency (or an approximation 

thereof, such as mean annual discharge), those channel 

dimensions can be approximated as power functions of 

that discharge at sites along the Powder and Wind/Bighorn River Basins and downstream along the mainstem 

Yellowstone River. 

Streamflow Characteristics

Streamflow characteristics in the YRB (table 1) 

vary by geographic location, time of year, and degree of 

human influence. For most streams in the basin with little or no flow modifications, streamflow characteristics 

can be described by annual streamflow and flow duration at representative locations. For streams where 

human activities have modified the natural drainage, 

regulation, diversion, and return flows affect streamflow characteristics to varying degrees. Variations in 

geography and weather cause severe floods and 

droughts in the basin. Flows resulting from these 

extreme hydrologic events affect water quality.

Annual Streamflow

Annual streamflow can be characterized by measures of central tendency, such as the mean, and of variability, such as the coefficient of variation (COV), 

defined as the standard deviation divided by the mean. 

Most of the annual streamflow from the YRB originates 

in the mountainous areas of the upper Yellowstone 

River and the Clarks Fork Yellowstone River and Wind/

Bighorn River Basins. Representing about half of the 

study unit’s total drainage area, the combined mean 

annual flows from the upper Yellowstone, Clarks Fork 

Yellowstone, and Wind/Bighorn Rivers equal about 

86 percent of the mean annual flow at the mouth of the 

basin (table 2). Runoff amounts from the basins and 

plains areas are lower than those from the mountainous 

areas. Annual runoff in the YRB ranges from less than 

0.25 cm (0.1 in.) in the lower-elevation parts of the 

Wind/Bighorn River Basin to more than 75 cm (30 in.) 

in the higher elevations of the Wind River Range and 

Beartooth Mountains (U.S. Geological Survey, 1970a). 

Mean annual runoff from the Powder River Basin is less 

than 2 cm (0.8 in.), even though the drainage area for 

this major tributary is about 20 percent of the YRB 

study unit (table 2).

Streams in the mountainous areas of the YRB 

generally are perennial (Wahl, 1970; Lowham, 1988). 

Most of the flow in mountain streams is from snowmelt 

runoff. Annual streamflows in the mountainous areas 

are dominated by a single snowmelt peak of moderate 

duration during late spring/early summer with low variability in daily mean discharge throughout the year 

(figs. 13 A and F). Variabilities in annual flows in 

streams in the mountainous areas of the basin are generally small. Relative to the intense localized convective rainstorms of the basins and plains areas, mountain 

snow accumulations are less variable in aerial extent 

and between years. The coefficients of variation in 

annual discharge at sites 1-4, 9, 16, and 18 (table 1) are 

about 0.2. Coefficients of variation are slightly larger 

(0.3 to 0.4) at sites on the northern and eastern flanks of 

the Bighorn Mountains (sites 7, 24, 25, 37, and 38).

Most streams originating in the basins or plains 

areas of the YRB are ephemeral, flowing only as a result 

of local snowmelt or intense rainstorms (Wahl, 1970; 

Omang, 1992). Intense localized convective rainstorms 

can produce most of the total flow for any given year in 

these watersheds. The distribution and occurrence of 

these events vary between years (Lowham, 1988, 

p. 18). Because of the localized extent and annual variability of these storms, the resulting flows in any given 

watershed are variable between years. Annual flows of 

streams originating in the basin or plains areas often 

consist of multiple peaks: a lowland snowmelt peak of 

moderate duration occurring late winter/early spring 

and several rainstorm peaks of short duration occurring 

late spring through late summer (fig. 13 B). 
ENVIRONMENTAL SETTING 33
34 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Table 1. Basin characteristics and annual streamflow statistics at selected sites, Yellowstone River Basin Reference number (fig. 12) Station number Streamflow-gaging station Elevation (m above sea level) Drainage area (km2

)

Annual s

Mean 

annual

 (m3/sec)

Mean 

annual

 (ft3/sec)

1 06186500 Yellowstone River at Yellowstone Lake Outlet, YNP 2,356 2,606 37.6 1,328 2 06191500 Yellowstone River at Corwin Springs, MT 1,548 6,794 87.9 3,104 3 06192500 Yellowstone River near Livingston, MT 1,385 9,197 105 3,731 

4 06200000 Boulder River at Big Timber, MT 1,237 1,350 16.3 575 5 06207500 Clarks Fork Yellowstone River near Belfry, MT 1,215 2,989 26.5 934 

6 06214500 Yellowstone River at Billings, MT 939 30,549 198 6,988 

7 06216000 Pryor Creek at Pryor, MT 1,221 303 .973 34.4 8 06220500 East Fork Wind River near Dubois, WY 1,960 1,110 6.94 245 9 06224000 Bull Lake Creek above Bull Lake, WY 1,790 484 8.29 293 

10 06225000 Bull Lake Creek near Lenore, WY 1,723 552 7.76 274 

11 06225500 Wind River near Crowheart, WY 1,718 4,898 34.0 1,201 

12 06228000 Wind River at Riverton, WY 1,494 5,980 23.1 815 13 06235500 Little Wind River near Riverton, WY 1,494 4,931 16.3 575 14 06244500 Fivemile Creek above Wyoming Canal, near Pavillion, WY 1,680 306 .0937 3.30 15 06253000 Fivemile Creek near Shoshoni, WY 1,450 1,080 4.50 159 16 06278300 Shell Creek above Shell Reservoir, WY 2,760 59.8 .988 34.9 17 06279500 Bighorn River at Kane, WY 1,120 40,831 62.9 2,222 18 06280300 South Fork Shoshone River near Valley, WY 1,890 769 11.6 411 19 06284500 Bitter Creek near Garland, WY 1,240 208 4.05 143 20 06285100 Shoshone River near Lovell, WY 1,170 6,090 25.6 903 21 06287000 Bighorn River near St. Xavier, MT 963 50,938 98.8 3,490 22 06288200 Beauvais Creek near St. Xavier, MT 1,020 259 .668 23.6 23 06289000 Little Bighorn River at State Line, near Wyola, MT 1,330 500 4.25 150 24 06290500 Little Bighorn River below Pass Creek, near Wyola, MT 1,100 1,110 5.89 208 25 06291500 Lodge Grass Creek above Willow Creek Diversion, near Wyola, 

MT

1,270 209 1.37 48.4 

26 06294000 Little Bighorn River near Hardin, MT 879 3,351 8.23 290 27 06294500 Bighorn River above Tullock Creek, near Bighorn, MT 823 58,052 108 3,810 

Table 1. Basin characteristics and annual streamflow statistics at selected sites, Yellowstone River Basin[Number of years in analyses refers to complete years used in computation of annual streamflow statistics; km2, square kilometer; m3/sec, cubic meter per second; ft3/sec

Yellowstone River mainstem stations shaded; YNP, Yellowstone National Park; MT, Montana, WY, Wyoming; NR, not relevant]
ENVIRONMENTAL SETTING 35

28 06294700 Bighorn River at Bighorn, MT NR 59,272 a a 

29 06294995 Armells Creek near Forsyth, MT 780 958 .163 5.75 30 06296003 Rosebud Creek at mouth, near Rosebud, MT 756 3,372 .892 31.5 31 06306300 Tongue River at State Line, near Decker, MT 1,045 3,825 13.1 462 32 06307500 Tongue River at Tongue River Dam, near Decker, MT 1,019 4,580 12.6 446 33 06307600 Hanging Women Creek near Birney, MT 960 1,220 .0997 3.52 34 06307740 Otter Creek at Ashland, MT 889 1,830 .134 4.72 35 06308500 Tongue River at Miles City, MT 719 13,980 11.9 421 

36 06309000 Yellowstone River at Miles City, MT 711 124,980 324 11,440 

37 06309200 Middle Fork Powder River near Barnum, WY 2,200 117 .849 30.0 38 06311000 North Fork Powder River near Hazelton, WY 2,490 63.5 .423 14.9 39 06313000 South Fork Powder River near Kaycee, WY 1,400 2,980b 1.03 36.5 

40 06313400 Salt Creek near Sussex, WY 1,370 1,990 1.28 45.1 41 06313500 Powder River at Sussex, WY 1,331 8,000 5.76 204 42 06317000 Powder River at Arvada, WY 1,100 15,700 7.85 277 43 06318500 Clear Creek near Buffalo, WY 1,580 311 1.75 61.9 44 06324500 Powder River at Moorhead, MT 1,016 20,950 12.8 452 45 06324970 Little Powder River above Dry Creek, near Weston, WY 1,040 3,199 .622 22.0 46 06326500 Powder River near Locate, MT 727 34,160 16.6 586 

47 06329500 Yellowstone River near Sidney, MT 573 178,980 361 12,750 

aFlow equivalent to USGS 06294500 (site 27).bEstimated.

Table 1. Basin characteristics and annual streamflow statistics at selected sites, Yellowstone River Basin (Continued)

Reference 

number 

(fig. 12)

Station 

number Streamflow-gaging station

Elevation 

(m above 

sea level)

Drainage 

area 

(km2)

Annual s

Mean 

annual

 (m3/sec)

Mean 

annual

 (ft3/sec)
36 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Several streams that originate in the basins and plains 

areas of the YRB are characterized by large variability 

in annual streamflow. Coefficients of variation in 

annual mean discharge at representative sites are about 

1.0 or larger (sites 29, 30, 33, 34, 40, and 45) (table 1).

Larger streams with headwaters in mountainous 

areas of the YRB that flow across the basins and plains 

exhibit a combination of annual streamflow characteristics of mountain, basin, and plains streams. Annual 

flows consist of a lowland snowmelt peak during late 

winter/early spring followed by a peak from the mountain snowmelt during late spring/early summer with 

several short to moderate duration rainstorm peaks 

superimposed on the snowmelt peak and throughout 

the remainder of the summer (figs. 13 C and G). Coefficients of variation are moderate at representative sites 

on the Tongue, Powder, and Yellowstone Rivers, ranging from more than 0.2 to about 0.5 (sites 35, 36, 41, 

42, 44, 46, and 47) (table 1).

Flows in some streams in the YRB are affected 

by various flow modifications. In some watersheds 

where irrigated agriculture is a major land use, most of 

the streamflow results from agricultural return flows 

and sustained base flows. Formerly intermittent or 

periodically ephemeral streams often flow perennially 

upon the development of irrigation (Colby and others, 

1956; Plafcan and others, 1993). Annual flows generally are uniform throughout the irrigation season, with 

a gradual decrease to a sustained base flow following 

the end of the growing season (fig. 13 D). Relative to 

the event-based annual flows in undeveloped ephemeral drainages, these flows are generally less variable 

between years. Variabilities in annual flows for two 

representative watersheds in the basin are very low; 

coefficient of variation at site 15 is 0.21 and at 19 is 

0.11 (table 1). Streamflow characteristics of perennial 

streams below major diversions or reservoirs sometimes exhibit large variations during high flows 

(fig. 13E). Coefficients of variations at representative 

sites in the Wind/Bighorn River Basins are moderate 

(sites 12 and 20) (table 1).

Flow Duration

Streamflow duration is the time over which a 

given discharge is recorded at a site. For all discharges 

at a site during the period of record or interest, flow 

durations can be summarized graphically as a curve 

derived by plotting each discharge with the cumulative 

exceedance probability (in percent) for that discharge. 

Table 2. Mean annual runoff for major tributaries and the mainstem as measured at selected sites, Yellowstone River Basin

(Modified from Shields and others, 1997)

[km2, square kilometer; m3/sec, cubic meter per second; ft3/s, cubic feet per second; cm, centimeter; in., inches]

Site

Mean annual 

flow

(m3/sec

and (ft3/s))

Mean 

annual 

runoff

(cm

and (in.))

Period 

represented

Reference 

number 

(fig. 12)

Station 

numbera

a

For tributaries, station represents site closest to the tributary mouth.

Tributary and/or part of mainstem 

represented by station

Drainage 

area

(km2)

b

bDrainage area at site is less than total area of river basin.

6 06214500 Yellowstone headwaters/Upper Yellowstone/

Clarks Fork Yellowstone River

30,549 198

 (6,988)

20.4

(8.04)

1929-96

27 06294500 Wind/Bighorn River 58,052 111 

(3,910)

6.03

(2.37)

1967-96c

c

Period after completion of Yellowtail Dam (Bighorn Lake).

35 06308500 Tongue River 13,980 11.9

 (421)

2.68

(1.06)

1938-96

46 06326500 Powder River 34,160 16.6

 (586)

1.51

(.60)

1939-96

47 06329500 Yellowstone River Basin 178,980 359

(12,660)

6.33

(2.49)

1967-96c
ENVIRONMENTAL SETTING 37
38 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

The resulting flow-duration curve shows the percentage 

of time during which a range of flows were equaled or 

exceeded during the period of interest without consideration for the chronology of the individual flows. Thus, 

the curve is a graphical representation of the variability 

of streamflow at a site over an entire period of interest. 

The shape of the flow-duration curve is a function of 

the basin hydrological and physical characteristics. If 

flow-duration curves are based on representative data, 

the curves are useful for predicting flow distributions 

for water-quality assessments (Searcy, 1959).

Most of the flow in perennial streams that originate in the mountainous areas of the YRB is from snowmelt (see previous section on Annual Streamflow). Site 

9 (fig. 12) is representative of streams originating in the 

mountainous areas of the basin. The flow-duration 

curve for site 9 (fig. 14 A) is generally flat for high and 

low flows, illustrating the small variability in flows typical of perennial streams. The flatness of the curve for 

both small and large exceedances is typical of a perennial stream with consistent high flows and sustained low 

flows from snowmelt.

Ephemeral streams that originate in the basins or 

plains areas of the YRB flow only as a result of snowmelt or rainstorms. As described previously (see Annual 

Streamflow section), these snowmelt and rainstorm 

events vary greatly in intensity, duration, frequency, and 

extent. Because flows in the basins and plains areas are 

in response to these discrete events, the variability in 

streamflows depends on the variability of the events. 

Similarly, periods of no flow are common; low or base 

flows from surface- or ground-water storage, if present, 

generally do not persist throughout the year nor throughout the stream reaches (Omang, 1983; Parrett, 1983; 

Rankl and Armentrout, 1986). Site 45 (fig. 12) is representative of streams, with few effects from flow modifications, that originate in the basin and plains areas of the 

YRB. The flow-duration curve for site 45 (fig. 14 C) is 

generally steep throughout most of the range of flows, 

illustrating the large variability in flows typical of 

ephemeral streams. The steepness of the curve for large 

exceedances also is typical of ephemeral streams, illustrating the lack of sustained low or base flows.

Streams that originate in the mountains and flow 

across the basins and plains areas generally are perennial. Larger drainage areas, snowmelt runoff, and 

ground-water sources minimize variability in high and 

low flows (Omang, 1986). The flatness of flowduration curves for sites 6, 36, and 47 (fig. 14 G) on the 

mainstem Yellowstone River illustrates the low variabilities in flows in these types of streams. Some streams, 

however, lose flow in the plains areas of the basin, 

resulting in intermittent flow some years. Ringen and 

Daddow (1990) attributed net losses in streamflow 

along the Powder River during 1978-80 at sites 42 and 

44 (fig. 12) to seepage and evapotranspiration. Such 

losses result in more variability in base flows, a condition illustrated by the moderately steep flow-duration 

curve for site 46 on the Powder River (fig. 14 D).

Flow modifications have affected flow durations 

of some streams in the YRB. In some watersheds, landuse practices augment natural flows to sites downstream. Inflows from irrigation drainage below site 14 

have resulted in lower streamflow variability and sustained base flows at site 15 (fig. 12). The resulting flowduration curve is similar to that of a perennial stream 

(fig. 14 B). Ground water is pumped and discharged as 

surface water during the production of oil and gas in 

some watersheds. Such produced water is discharged 

upstream from site 40, resulting in perennial flow in 

what was once an ephemeral stream (Lindner-Lunsford 

and others, 1992). The resulting flow-duration curve is 

flat for large exceedances, illustrating the sustained 

base-flows (fig. 14 C, site 40). Flows are completely 

regulated by reservoirs in some watersheds in the YRB. 

Variations in flow are low at site 21 (fig. 12) downstream of Bighorn Lake (Yellowtail Dam). The flowduration curves for the periods after the completion of 

Boysen Reservoir (1952-61 and 1967-96) are flat 

throughout the range of all flows, illustrating that the 

regulated flows are sustained for most exceedances 

(fig. 14 E, site 21).

The composite effects of flow modifications in 

the YRB decrease streamflow variability. At the mouth 

of the basin, the decrease in streamflow variability over 

time is illustrated in flow-duration curves for site 47 

(fig. 14 F). The curves represent periods between completion of major reservoirs in the basin. The cumulative 

effect of sustained regulated flows from reservoir 

releases results in the flattening of the flow-duration 

curve, indicating less variability in streamflows. In 

addition to flow modifications, climate changes influence the overall variability in streamflows in the basin. 

The period 1952-61 represents the time after which 

operation of Boysen Reservoir began and before which 

construction of Yellowtail Dam (Bighorn Lake) was 

completed (fig. 14 E and F). Overlapping this period, 

the drought of 1948-62 had severe effects on most of 

Wyoming and Montana (see following section).
ENVIRONMENTAL SETTING 39
40 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Floods and Droughts

Diverse geographic features and variable moisture systems combine to cause severe floods and 

droughts in the YRB (Druse, 1991; Merritt and others, 

1991). Floods and droughts are extreme hydrologic 

events that can degrade ambient surface-water quality. 

The highest constituent loads from nonpoint sources, 

such as overland flow, occur during flooding (Novotny 

and Olem, 1994). Floodwaters can scour gravels and 

deposit sediment, processes that are detrimental to 

spawning beds for some fish species (Merritt and others, 1991). Decreased flows during drought periods 

lessen the diluting effect of streams on inflows (including surface and ground water), potentially resulting in 

increased concentrations of dissolved solids. Lower 

dissolved-oxygen levels and higher stream temperatures also may occur during extended periods of low 

flows, adversely affecting aquatic life (Matthai, 1979).

Floods

Flooding in the YRB can occur as a result of 

snowmelt, widespread rainfall, or intense thunderstorms. In mountainous areas of the basin, most flooding occurs during spring and early summer from rapid 

snowmelt. Flooding in the basins and plains areas 

occurs during winter and early spring from lowland 

snowmelt, during spring from large regional rainstorms, and during summer and fall from intense localized thunderstorms. High antecedent soil moisture, 

frozen ground, and rainfall on melting snowpacks contribute to the most severe floods (Craig and Rankl, 

1978; Lowry and others, 1986; Lowham, 1988; Druse, 

1991; Omang, 1992; Holnbeck and Parrett, 1996). 

Flooding occurred during 1923 in parts of the 

YRB during July 23-25 and again during September 

27-30 as a result of widespread thunderstorms and rainfall. Referred to by long-term residents as “the big 

floods of 1923,” these were the most severe floods 

since 1880 and were during the period 1918-27 in 

which more large floods occurred in Wyoming than 

any other decade (Cooley, 1990). Peak flow recurrence 

intervals exceeded 100 years at several USGS stations 

in the Wind, Bighorn, and Powder River Basins (Druse, 

1991). The 1923 peak discharge at site 42 (Powder 

River at Arvada, Wyoming; fig. 12) of 2,830 m3

/sec 

(Smalley and others, 1997) is twice the next largest 

peak of record (period of record 1919-96).

Severe flooding occurred during May 1978 in 

southeastern and south-central Montana and northeastern Wyoming (Merritt and others, 1991). Watersheds 

affected in the basin included the Yellowstone River 

from near Billings, Montana, to Miles City, Montana; 

the Bighorn, Tongue, and Powder Rivers; and many 

smaller tributary watersheds (Parrett and others, 1984). 

Widespread rain on saturated soils combined with 

bankfull snowmelt runoff conditions in most streams to 

cause the flooding. Flood recurrence intervals 

exceeded 50 years for most of the affected area (Merritt 

and others, 1991). Sediment transport is often very 

high during floods. Record maximum daily suspended-sediment loads at four sites were measured on 

the Powder River (table 3). Other major floods 

occurred in the YRB during 1918, 1962, 1963, and 

1981 (Druse, 1991; Merritt and others, 1991).

Droughts

Severe droughts of several-years duration have 

occurred in the YRB. Droughts with recurrence intervals greater than 25 years occurred during the periods 

1929-42 and 1948-62 over most of Wyoming and Montana (Druse, 1991; Merritt and others, 1991), including 

nearly all of the YRB. Such regional drought conditions are common in the upper parts of the Missouri 

River Basin (Matthai, 1979). Droughts with recurrence 

intervals of 10 to 25 years occurred in the YRB 

between 1976 and 1982. Recent data for representative 

streamflow-gaging stations in the basin used in previous drought analyses by Druse (1991) and Merritt and 

others (1991) indicate lower than normal flows from 

about 1987 to about 1994 (table 4).

The drought of 1977 affected most of the United 

States west of the Mississippi River (Matthai, 1979, 

fig. 2). In the YRB, the 1977 drought was part of the 

1976-82 drought period; the most severely affected 

areas were the upper parts of the Wind and Yellowstone 

River watersheds (Druse, 1991; Merritt and others, 

1991). Decreased streamflows during droughts can 

result in higher dissolved-solids concentrations. 

Median concentrations of total dissolved solids in samples collected during September through the following 

February of each year (that is, the period of year during 

which flows are lower) were significantly higher (Wilcoxon rank-sum test; p < 0.05) during the drought 

period 1976-82 than during non-drought years at sites 

3 and 6 on the upper Yellowstone River (table 5). 

Dissolved-solids concentrations were higher during the 
ENVIRONMENTAL SETTING 41

1976-82 drought than during non-drought periods at 

site 12 on the Wind River; however, the difference was 

not significant (table 5). Site 12 might not be representative of drought conditions in the upper parts of 

the Wind River watershed because of flow modifications upstream of the site.

Water-Quality Characteristics

Concentrations of suspended sediments and dissolved solids in streams are indicators of water quality 

that are useful for characterizing the overall waterquality conditions of the YRB. Suspended-sediment 

and dissolved-solids concentrations are smaller in 

mountain streams overlying older rocks than in streams 

crossing the younger rocks of the basins and plains 

areas. Human activities that disturb the land contribute 

to suspended sediment and dissolved solids in surface 

water. Concentrations of suspended sediment and dissolved solids in streams in the basin vary with streamflow and season.

Table 3. Maximum daily suspended-sediment discharge, flood of May 1978, Powder River, Wyoming and Montana

(From Parrett and others, 1984)

[m3/s, cubic meters per second, ft3/s, cubic feet per second]

Reference 

number 

(fig. 12)

Powder River streamflow-gaging station

Date

Maximum flood 

discharge

(m3/s and (ft3/s))

Suspendedsediment 

discharge

Number Location (megagrams)

42 06317000 at Arvada, Wyoming May 20, 1978 a920 (32,500)

a

Approximate measurement.

2,550,000

44 06324500 at Moorhead, Montana May 20, 1978 935 (33,000) 2,020,000

b

b

Location not shown on figure 12.

06324710 at Broadus, Montana May 21, 1978 850 (30,000) 1,420,000

46 06326500 near Locate, Montana c

May 22, 1978

c

Maximum flood discharge occurred on May 23.

776 (27,400) 670,000

Table 4. Annual departure from mean annual discharge at selected streamflow-gaging stations, Yellowstone River 

Basin, water years 1987-96

(Modified from U.S. Geological Survey, 1997c)

[06191500 (2), station number and (reference number, fig. 12); --, no data]

Water year

Annual departure from mean annual discharge, in percent

06191500

(2)

06214500

(6)

06225500

(11)

06317000

(42)

06318500

(43)

06324500

(44)

 1987 -32.5 -30.3 -11.4 21.7 -2.6 7.2

 1988 -36.4 -36.4 -31.2 -43.8 -33.5 -40.2

 1989 -9.8 -11.1 -22.1 -53.5 -22.5 -64.7

 1990 -3.6 -10.3 -14.2 -31.6 6.3 -22.0

 1991 3.0 4.2 10.2 -12.9 31.9 8.4

 1992 -11.1 -9.5 -32.9 -39.5 31.7 -23.0

 1993 11.9 9.0 -16.3 36.3 -- 29.1

 1994 -22.6 -30.3 -34.4 -34.1 -- -31.2

 1995 6.9 12.6 4.7 130 -- 75.9

 1996 45.4 38.6 12.5 -8.8 -- 2.1
42 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Suspended Sediment

The concentration of sediment in suspension in a 

stream is an indicator of water quality. Suspendedsediment data are useful in describing general basin 

water-quality conditions. Suspended sediment 

includes fragments of rock and soil that are transported 

in suspension by streams. Overland flow and channel 

scour are important erosional sources of suspended 

sediment in watersheds with little development; mass 

wasting contributes substantially to sediment loads in 

some systems (Colby and others, 1956). While much 

of the suspended sediment in streams is the result of 

natural factors, some human activities also contribute 

to sediment loads. Large suspended-sediment concentrations in streams tend to occur in range and agricultural land-use areas because of the high erodibility of 

soils in these areas (Smith and others, 1993).

Concentrations of suspended sediment in the 

YRB generally are lower in mountain streams than in 

streams draining the basins and plains areas. Most of 

the fluvial sediment in the basin is derived from the 

basins and plains areas (Hembree and others, 1952; 

Colby and others, 1956). About 30 percent of the 

annual sediment load at site 47 (fig. 12) is from the 

Powder River Basin, in spite of the fact it accounts for 

less than 5 percent of the annual streamflow (Knapton 

and Bahls, 1993). Statistical summaries of data from 

selected sites on the Wind and Powder Rivers illustrate 

the large concentrations of suspended sediment in the 

basins and plains streams (fig. 15). Suspendedsediment concentrations increase downstream in the 

Tongue River Basin (Litke and Knapton, 1983). A 

decrease in suspended-sediment concentrations 

between sites 42 and 46 (fig. 12) on the Powder River 

might be attributed to significant sediment deposition 

through this reach (fig. 15; also, Martinson and Meade, 

1983; and, Ringen, 1986).

Suspended-sediment concentrations in streams in 

the YRB generally are lower in watersheds where older 

rocks are exposed, such as resistant Precambrian and 

Paleozoic rocks. Concentrations of suspended sediment generally are larger in streams in contact with less 

resistant rocks of Mesozoic age and younger. Important 

sources of suspended sediment in the basins and plains 

watersheds include actively eroding gullies as well as 

erosion of the channel alluvium (Hembree and others, 

1952; Colby and others, 1956).

Fluvial sediment is contributed by land-use activities in some watersheds in the YRB. Irrigation practices in the Clarks Fork Yellowstone River Basin, as 

well as natural factors, contribute a substantial part of 

the suspended sediment measured at site 6 (Knapton 

and Bahls, 1993). Prior to the completion of Boysen 

Reservoir, over half of the suspended-sediment load in 

the Bighorn River upstream of site 17 was contributed 

by Fivemile Creek (fig. 12, sites 14 and 15; see also 

Flow Duration section in this report). Colby and others 

(1956) determined 87 percent of the sediment load in 

Fivemile Creek originated within the irrigated area.

Table 5. Statistics for total dissolved-solids concentration at selected sites, September-February, during nondrought and drought periods, upper Wind and Yellowstone Rivers, 1969-94

[n, number of analyses; mg/L, milligrams per liter; Y, yes; N, no]

Reference 

number

(fig. 12)

Station 

number

Non-drought 

periods 1976-82 drought

n

Median 

concentration

(mg/L) n

Median 

concentration

(mg/L)

Significance

(p-value)a

a

Wilcoxon rank-sum test for significance; one-sided p-value. Higher concentrations of total dissolved solids

during the 1976-82 drought are significant at the 95-percent confidence level if p < 0.05.

3 06192500 60 172 33 183 Y (p=0.0239)

6 06214500 89 272 38 282 Y (p=0.0363)

12 06228000 93 277 37 287 N (p=0.1752)
ENVIRONMENTAL SETTING 43
44 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Suspended-sediment concentrations in alluvial 

streams with little or no flow modifications generally 

increase with increasing streamflow (Leopold and 

Maddock, 1953). In the YRB, the positive correlation 

between suspended-sediment concentrations and discharge is strong because of relatively high suspendedsediment concentrations and large seasonal and annual 

variability in streamflows. The largest concentrations 

in basins and plains streams usually occur during periods of direct runoff when erosion from overland flow 

and channel scour contribute the most sediment 

(Lambing, 1986). The correlation of suspendedsediment concentrations with discharge is generally 

poor in watersheds where natural flows are altered or 

where sources of sediment are limited.

Suspended-sediment concentration generally 

correlates well with discharge at selected sites in the 

YRB where suspended-sediment concentrations are 

relatively high. Suspended-sediment concentrations 

are the highest in the Powder River Basin; correlation 

coefficients range from about 0.7 to nearly 0.8 at 

selected sites (table 6; sites 41, 45, and 46). The good 

correlation between suspended-sediment concentration 

and discharge at site 12 (Spearman rank correlation 

coefficient = 0.84) might be more a function of 

sediment-flushing flows from a diversion dam 

upstream from the site rather than one of natural 

streamflow and suspended-sediment conditions.

Correlation of suspended-sediment concentration with discharge generally diminishes at selected 

sites in watersheds with streamflow modifications. 

Sites 17, 20, 28, and 35 are downstream from major 

reservoirs and irrigation diversions and returns. 

Suspended-sediment concentrations do not correlate 

well with discharge at site 28, where more than 90 percent of the mean annual flow is controlled by releases 

from Bighorn Lake (Yellowtail Dam). Decreased 

stream velocities at reservoir inlets result in the deposition of suspended sediment; consequently, suspendedsediment concentrations are limited in flows from reservoir outlets.

Dissolved Solids

Dissolved-solids concentrations in surface 

waters are composite water-quality indicators that can 

be used to characterize the general water quality of a 

watershed. Total dissolved solids represents the sum of 

all dissolved constituents in a water sample. For watersheds without substantial development, the major dissolved constituents detected in streams result from the 

dissolution of minerals in soil and rock (Hem, 1985; 

Drever, 1988). Dissolved-solids concentrations are 

generally lower in forested watersheds and areas of 

higher precipitation; concentrations are generally 

higher in rangeland watersheds and regions of lower 

precipitation. Human activities that disturb the land, 

such as agriculture, mining, and other development, 

can increase dissolved solids in surface waters by 

exposing more minerals for dissolution (Smith and 

others, 1993). Concentrations of dissolved solids generally are inversely related to streamflow (Hem, 

1985); thus, dissolved-solids concentrations vary with 

seasonal variations in streamflow.

Table 6. Correlation of suspended-sediment and dissolvedsolids concentration with instantaneous discharge at 

selected sites, Yellowstone River Basin

[n, number of sample/discharge pairs; --, too few sample/discharge pairs: 

correlation not computed]

Reference 

number

(fig. 12)

Station 

number

Suspendedsediment 

concentration

Dissolved-solids 

concentration

 n

Spearman 

rank

correlation

coefficienta

a

See Helsel and Hirsch (1992) for description of Spearman rank 

correlation coefficient.

 n

Spearman 

rank

correlation

coefficienta

3 06192500 141 0.62 147 -0.97

5 06207500 -- .-- 124 -.86

6 06214500 170 .57 142 -.86

12 06228000 163 .84 131 -.57

13 06235500 92 .66 101 -.72

17 06279500 282 .56 137 -.81

20 06285100 114 .45 114 -.81

28 06294700 163 .28 149 -.26

35 06308500 156 .66 137 -.45

41 06313500 95 .79 170 -.58

42 06317000 -- .-- 176 -.69

45 06324970 119 .71 161 -.50

46 06326500 196 .78 145 -.69

47 06329500 296 .65 186 -.53
ENVIRONMENTAL SETTING 45

Concentrations of dissolved-solids in the YRB 

generally are lower in streams in mountainous areas 

than in those originating in the basins or plains areas 

(Knapton, 1983; Lambing, 1986; Larson, 1986; Peterson, 1987). Dissolved solids generally increase in 

streams flowing across the basins and plains areas of the 

YRB (Lowry, Smalley, and others, 1993; Peterson, 

1993; Susong and others, 1993; Plafcan and Ogle, 

1994). Statistical summaries of data from selected sites 

on streams in the Powder River Basin and tributaries to 

the Yellowstone and Tongue Rivers illustrate dissolvedsolids concentrations are relatively high in streams originating in the plains areas of the YRB (fig. 15, sites 29, 

30, 33, 34, and 45). Dissolved-solids increase downstream at representative sites on the Wind/Bighorn and 

Tongue Rivers, as well as on the mainstem Yellowstone 

River, which originate in the mountains and flow across 

the basins and plains areas (fig. 15, sites 12, 17, and 28; 

32 and 35; and 3, 6, and 47, respectively).

Dissolved-solids concentrations in streams in the 

YRB are generally lower in watersheds overlying most 

older and some younger rocks. Dissolved solids are low 

in watersheds where Precambrian igneous and metamorphic rocks or more resistant Paleozoic sedimentary 

rocks predominate. Watersheds mostly underlain by 

Tertiary volcanic rocks also are low in dissolved solids. 

Concentrations of dissolved solids are generally higher 

in streams in contact with less resistant younger rocks. 

Dissolved solids are relatively high in watersheds draining mostly Mesozoic sandstones, siltstones, and shales, 

in particular, the Cody Shale and other 

Cretaceous marine shales (Hembree and others, 1952; 

Colby and others, 1956). Thermal springs along the 

upper Yellowstone and Bighorn Rivers are other geologic sources of dissolved solids (Knapton and Bahls, 

1993; Peterson, 1993).

Various land-use practices contribute to dissolved 

solids in streams in some watersheds in the YRB. Irrigation practices contribute to dissolved solids in 

streams in the Clarks Fork Yellowstone, Wind/Bighorn, 

and Powder River Basins (Knapton and Bahls, 1993; 

Cary, 1991; Peterson and others, 1991; Lowry, Smalley, 

and others, 1993; Larson, 1986). Oil and gas development contributes to dissolved solids in the Wind/Bighorn (Lambing, 1986) and Powder River Basins. The 

Salt Creek watershed contributes approximately 25 to 

30 percent of the dissolved solids at site 41 (fig. 12) on 

the Powder River; most of this load is from saline oilproduction water (Lindner-Lunsford and other, 1992; 

Peterson, 1993).

Concentrations of dissolved solids generally are 

higher at lower discharges in most streams in the YRB. 

Ground water and other inflows contribute a substantial 

part of the lower flows in many streams of the basin and 

plains areas. Concentrations of dissolved solids in 

ground water, irrigation drainage, and wastewater discharge generally are higher than those in the receiving 

streams. Thus, the concentration of dissolved solids in 

streams increases during lower flows. Conversely, 

dissolved-solids concentrations are lower at higher discharges. Higher flows generally are the result of runoff 

from snowmelt or rainstorms, sources with very low 

concentrations of dissolved solids. Other inflows are 

diluted by the runoff, thereby decreasing the concentration of dissolved solids (Colby and others, 1956; Knapton, 1983; Lambing, 1986; Larson, 1986).

Correlation of dissolved-solids concentration 

with instantaneous discharge ranges from excellent to 

poor at selected sites in the YRB. Concentrations of 

dissolved solids correlate well with discharges at sites in 

the Clarks Fork Yellowstone and Wind/Bighorn Basins 

and on the upper mainstem Yellowstone River (table 6, 

sites 3, 5, 6, 13, 17, and 20). Dissolved-solids concentration does not correlate as well with discharge at 

site 12 (Spearman rank correlation coefficient = -0.57); 

upstream releases of very dilute reservoir waters contribute to a wide range of dissolved concentrations at 

lower flows (Colby and others, 1956, p. 123). 

Dissolved-solids concentrations do not correlate as well 

with discharges at selected sites in the Tongue and Powder River Basins (sites 35 and 45). Dissolved-solids 

concentrations increase initially during event-based 

flows in streams in the basins and plains areas from 

washoff of naturally occurring salts and larger ambient 

concentrations in tributary streams (Larson, 1986; Lindner-Lunsford and others, 1992; and, Lowry and others, 

1993). Dissolved-solids concentrations do not correlate 

well with discharges at site 28 (Spearman rank correlation coefficient = -0.26) downstream from Bighorn 

Lake. Reservoirs function as holding and mixing basins 

in watersheds, affecting the quality of surface waters 

downstream (Hem, 1985).

Concentrations of dissolved solids vary by season 

at some sites in the YRB. Because dissolved-solids 

concentrations generally are related to streamflows and 

much of the variation in streamflows is seasonal, sitespecific variation in dissolved-solids concentrations in 

the basin also is seasonal. Thus, dissolved-solids concentrations generally are higher during the fall and winter and lower during the spring and summer. 
46 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Concentrations of dissolved solids are significantly 

higher (table 7) during the winter (January through 

March) than during any other season at site 26 (fig. 12). 

Dissolved-solids concentrations are lowest during the 

spring; however, the difference is only significant 

(table 7) when compared to the fall and winter months.

Stream Ecology

Ecologists are concerned with interrelations 

between biological communities and their habitat. A 

variety of biological, chemical, and physical factors 

control the abundance, distribution, and productivity of 

aquatic organisms (Gordon and others, 1992, p. 17). 

Competition for preferred habitat, predation, water 

chemistry, nutrient inputs, the presence of waterfalls or 

dams, and streamflow variability are important factors 

for stream biota (Gordon and others, 1992, p. 17). The 

preceding section of this report introduced chemical 

water quality and summarized one of its constituents. 

In this section, selected physical and biological features 

of stream ecology in the YRB is discussed. 

Physical Stream Habitat

The physical character of rivers within the study 

unit changes markedly from headwater streams to lowland streams. Upstream reaches are characterized by 

turbulent flows, steep gradients, cold water temperatures, coarse substrates, and well-oxygenated water, 

whereas lowland reaches are typically characterized by 

warmer water temperatures (especially during summer), gentle gradients, turbidity, sediment deposition, 

fine substrates, and smaller concentrations of dissolved 

oxygen. 

The physical factors that are of greatest ecological significance include streamflow (discharge and 

velocity), channel shape, channel substrate, and water 

temperature (Gordon and others, 1992). Stream discharge, including the ecologically important periods of 

floods and droughts, was discussed in a previous section of this report. Streamflow velocities are a function 

of flow depth and gradient or slope in the downstream 

direction. To allow comparison of streamflow velocities among different sites or streams, a discharge of 

approximately constant frequency of occurrence is referenced at all sites; commonly, the mean annual discharge is used.

Leopold and Maddock (1953) tabulated mean 

streamflow velocities at mean annual discharge for 

46 streamflow-gaging stations in the YRB, and several 

summary observations may be made from those data, as 

follows. Average velocity increases in the downstream 

direction along the upper through middle reaches of the 

principal rivers, as it does typically due to increasing 

flow depths. But velocities decrease in the lower 

reaches of the Bighorn, Powder, and Yellowstone Rivers. The Wind/Bighorn River mainstem has an average 

velocity between 0.9 and 1.1 m/s for much of its length 

upstream from Bighorn Lake, but on the Great Plains 

the Bighorn and Little Bighorn Rivers have average 

velocities of about 0.7 m/s. Also on the Great Plains, 

average velocities in the several forks of the Powder 

River near their confluences with the mainstem are less 

than 0.6 m/s. In a middle section of the Powder River 

mainstem, average velocity increases to about 1.0 m/s, 

but then decreases to less than 0.9 m/s in its lower 

reaches. In contrast, average velocity in the Yellowstone River at Corwin Springs, Mont., is 1.1 m/s, and 

increases to 1.5 m/s at Billings, Mont., but then 

decreases to just less than 1.0 m/s at Sidney, Mont. 

(Leopold and Maddock, 1953). 

The streamflow velocities actually encountered 

by biota are more relevant than average velocity. But 

large local variability produces a mosaic of velocity patterns that support species with differing preferences 

(Gordon and others, 1992, p. 20). The velocity patterns, 

and related turbulence of flows, are difficult to quantify 

Table 7. Seasonal variability in total dissolved-solids 

concentration in stream samples collected at site 26, Little 

Bighorn River near Hardin, Montana, 1970-96

[Site 26 shown in fig. 12; mg/L, milligrams per liter; Fall, OctoberDecember; Winter, January-March; Spring, April-June; Summer, JulySeptember; Y, significant, N, not significant]

Season

Significance (p-value)a

a

Wilcoxon rank-sum test for significance; two-sided p-value (Helsel

and Hirsch, 1992). Difference in concentration between any two

seasons is significant at the 95-percent confidence level if 

p < 0.05.

Number 

of 

samples

Sample 

median

Summer Spring Winter (mg/L)

Fall Y

(p<0.0001)

Y

(p=0.0350)

Y

(p=0.0062)

45 492

Winter Y

(p<0.0001)

Y

(p=0.0014)

-- 47 556

Spring N

(p=0.4537)

-- 62 411

Summer -- 50 438
ENVIRONMENTAL SETTING 47

for an individual river, much less over a large drainage 

such as the YRB.

Channel shape is a function of the width and 

depths of streamflow. As related to flow depth, it influences water temperature, light penetration, and fish 

migration (where depth is too shallow for passage). As 

related to width, it influences the degree of shading by 

overhanging bank vegetation, instream photosynthesis, 

and input of organic matter (Gordon and others, 1992, 

p. 21). The width-to-depth ratio (W:D) commonly is 

used as an index of channel shape.

Osterkamp and Hedman (1982) presented channel geometry measurements, including active-channel 

width and average bankfull depth, for 21 stream stations in the YRB. For three streams with drainages primarily in the Wyoming Basin Province, W:D was 11.1 

or less, indicative of incised channels. Seven cobblebed streams with primarily mountainous drainages had 

W:D averaging 29.4 and ranging from about 20 to 51. 

On the Great Plains, six sand- or gravel-bed streams 

had W:D averaging 31.2 and ranging from 24 to 43 

(Osterkamp and Hedman, 1982). Both of the sand-bed 

sites were on the Powder River. For comparison, sandbed streams of some other Great Plains river systems 

(Platte and Republican Rivers) commonly have W:D 

greater than 100. The sand-bed channels of the Powder 

River contain more silt and clay in their beds than do 

those of the other two river systems, and such cohesive 

materials tend to provide greater resistance to erosion. 

The interrelations among physical habitat characteristics become apparent when seeking to understand the 

variation in these factors.

Stream substrate, or the particle-size distribution 

of bed material, is associated with streamflow velocity 

and, as implied above, influences channel shape. Substrate is an important factor controlling the occurrence 

of benthic (bottom-dwelling) fauna, and the complex 

mixture of coarse particles found in riffles provides the 

richest habitat for aquatic insects (Gordon and others, 

1992). Although both the average and range of particle 

sizes, and their arrangement on the streambed, are ecologically important, data are most readily available for 

the median particle size (d50).

In addition to substrate data for 19 stream stations in Osterkamp and Hedman (1982), the authors 

measured d50 for 15 stations while conducting a reconnaissance of YRB streams in 1997. These measurements were based on pebble counts performed using 

the half-largest-particle grid interval method of sampling (Wohl and others, 1996). Data from both sources 

were considered together to analyze the overall pattern 

of average particle sizes. 

Three stations draining the Wyoming Basin 

Province have an average d50 of 6 mm, but substrates 

range from silt to coarse gravel. The exposed soil area 

around sparse vegetation in drier parts of the Wyoming 

Basin allows sheet-and-rill erosion to wash fine sediment into the stream channels. 

Particle size generally decreases in the downstream direction, and data for the YRB streams are 

mostly consistent with this generalization. Among 

15 stations with predominantly mountainous drainages, the average d50 is 125 mm, with substrates ranging from coarse gravel to medium boulders. Five 

stations that are transitional between mountains and 

plains drainages have median particle diameters ranging from 38 to 58 mm. On the Great Plains, the average 

d50 for 11 YRB stations is 12 mm, with substrates ranging from very fine sand to very coarse gravel. An 

exception to the trend of downstream fining of bed 

material occurs in the lower Powder River between 

Arvada, Wyo., and Locate, Mont., where d50 coarsens 

from 0.16 to 0.35 mm (Osterkamp and Hedman, 1982).

Water temperature varies both seasonally and 

daily in concert with air temperature, but it generally 

increases in the downstream direction as the cooler, 

high-elevation climate is displaced by the lowland climate. This pattern is seen in YRB streams, particularly 

with respect to mean water temperature in the warmest 

months. Available USGS data for the 14 stream stations listed in table 6, and for the Gardiner River near 

Mammoth in Yellowstone National Park, were summarized by month. The period of record used generally 

was 1970-94, although the period was slightly different 

for a few stations; however, for the Gardiner River, the 

record extends from 1985-96. Mean water temperature 

is warmest in July, with five exceptions: August has 

warmer mean water temperature than July for sites 5, 6, 

28, 46 (fig. 12), and for the Gardiner River site. Site 46 

has equally warm mean water temperature in June and 

August; all other sites have a single peak on the curve 

of monthly mean temperatures. Mean water temperature in the warmest month ranges from about 16.5o

C at 

sites 3 and 5 to about 23.5o

C at sites 35, 41, and 42. 

Mean water temperature is coolest in January, 

with five exceptions: February has cooler mean water 

temperature than January for sites 6, 28, 42, and 45, and 

December has the coolest mean water temperature for 

the Gardiner River site. Mean water temperature in the 

coolest month generally has a small range, from 0o

C at 
48 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

sites 12, 13, 35, and 46 to about 1.3oC at sites 5, 6, and 

28. In contrast, the geothermally influenced Gardiner 

River has warmer mean water temperatures for December through February, ranging from 10.3oC to 11.3o

C. 

Cold winter water temperatures in the northeastern part 

of the YRB reflect a continental climatic gradient, 

whereas those in the Wind River Basin are indicative of 

its higher elevation.

Aquatic Biological Communities

Longitudinal Zonation

Longitudinal zonation of species occurs primarily with respect to altitude, water depth, and substrate 

composition in relation to stream gradient. Highgradient headwater streams are termed “salmonid” 

(after Family Salmonidae--Trouts, includes whitefishes, trouts, salmons, chars, and graylings) or 

rhithron zones, whereas lowland streams are termed 

“cyprinid” (after Family Cyprinidae--Minnows, 

includes minnows, shiners, dace, and chubs) or potamon zones. A transition zone, characterized by intermediate levels of both erosion and deposition, 

moderate gradients, alternating riffle-pool sequences, 

and sand-gravel substrates separates the two. 

Biotic zonation was originally defined by the 

apparent relation between a stream’s gradient and the 

fish species harbored therein. However, algae and 

invertebrate studies have since shown that the complete 

assemblage of fauna shows zonation, both in community composition and in numbers of species (Hynes, 

1970). For example, Newell (1977) sampled macroinvertebrate communities at 20 stations along the Yellowstone River. He defined the salmonid zone to 

extend downstream to about 680 km upstream of the 

mainstem mouth, and the transition zone to extend 

200 km further downstream to the confluence with the 

Bighorn River. Newell’s (1977) downstream boundary 

for the salmonid zone roughly corresponds to the physiographic transition from mountains to plains.

Downstream variations in aquatic biota also have 

been explained using the river continuum concept 

(Vannote and others, 1980), relating stream order 

(Strahler, 1957), size of particulate organic matter, and 

the ratio of photosynthetic production to community 

respiration. Low-order streams (for example, firstorder tributaries in the forested headwaters of the Yellowstone River) typically receive substantial terrestrial 

contributions of coarse particulate organic matter 

(CPOM) and have little photosynthetic production. 

Food chains in intermediate-sized streams are more 

dependent on photosynthetic production along with 

fine particulate organic matter (FPOM) from upstream 

shredding of CPOM by invertebrates (Newell, 1977). 

Greater turbidity in large rivers (such as downstream 

reaches of the Powder and Yellowstone Rivers) limits 

photosynthetic production such that FPOM becomes 

the principal base of the food chain.

Algae

Algae are ubiquitous, autotrophic, unicellular or 

multicellular organisms maintaining reproductive characteristics that distinguish them from liverworts, 

mosses, and vascular plants. Algae are central to 

aquatic ecosystems (Bahls and others, 1984) as producers of carbohydrates and proteins via photosynthetic 

processes and are an essential component of the riverine food chain (Round, 1973). Benthic species are 

most important when considering a stream’s primary 

production and plant diversity, and may be of epilithic 

(attached to stones), epipelic (attached to mud or sand), 

epiphytic (attached to plants), or epizoic (attached to 

animals) habits (Bold and Wynne, 1985). In flowing 

water, planktonic (open water) algal species are of 

lesser biological significance because their presence 

depends largely upon flow regime, catastrophic drift, 

and tributary inoculation. 

In a variety of ways, algae and other photosynthetic plants are indicative of stream water quality 

(Whitton, 1979). Historically, biologically based surveys of streams, as opposed to those of lakes and reservoirs, have used animals, both vertebrates and 

invertebrates, as indicators. However, as pollutant concentrations meeting standards formulated to protect 

fish may be lethal to other aquatic species, modern bioassays generally use at least three food-chain components: algae, invertebrates, and fish or amphibians 

(Whitton, 1979). Changes in algal community structure, population, biomass, photosynthetic and respiration rate, nitrogen-fixation rate, chemical composition, 

and morphology may all signify water-quality changes 

within a given reach. Similarly, water quality may be 

questionable if a widely distributed species is not found 

during sampling (Bahls and others, 1984). A stream’s 

general water quality may best be indicated by benthic 

organisms. 
ENVIRONMENTAL SETTING 49

Discharge, current velocity, substrate, turbidity, 

scour, suspended solids, nutrient status, dissolved oxygen, dissolved salts, light intensity, pH, hardness, toxicants, water temperature, and grazing animals are all 

factors influencing riverine algal communities. Community structure is strongly affected by the seasonal 

periodicity of species, with annuals achieving maximum development, abundance, and reproduction during specified seasons, perennials maintaining a 

continuous vegetative cycle, and ephemerals appearing, opportunistically, for short times during any season (Smith, 1950). Correspondingly, dispersal agents 

(such as wind, birds, amphibia) (Smith, 1950), catastrophic events, origin of water (spring, standing, or 

tributary), and riparian vegetation will vary community 

composition (Round, 1973; Bahls and others, 1984). 

Lastly, although limited algal species succession 

occurs, the traditional plant community concept as 

defined by terrestrial ecologists does not generally 

apply to streams; algal associations are difficult to 

delimit.

The divisions of algae most common in the YRB 

include:

• Bacillariophyta (diatoms) -- Diatoms are the predominant algae in streams draining the montane 

portions of the YRB (Robinson and others, 

1996). Diatoms are essentially cold-water 

organisms, common during spring and autumn, 

and live on substrates including rock, sand, mud, 

and water plants, or as epiphytic growth on other 

algae. Planktonic habits of these essentially unicellular algae also are known (Bold and Wynne, 

1985). 

• Charophyta (stoneworts) -- These algae are usually 

found in freshwater, anchored on muddy or 

sandy substrates or hard limestone stream beds 

(Bold and Wynne, 1985). Many species become 

encrusted with carbonates, mainly of calcium 

and magnesium; hence suggesting their common 

names (stoneworts or brittleworts). Some species prefer brackish water, but most thrive best in 

clear, hard water; aeration is not essential. 

• Chlorophycota (green algae) -- These algae are 

found in a variety of settings including standing, 

tranquil, and swiftly flowing water (cataracts, 

waterfalls, dam spillways). They inhabit water 

having a great range of salinity, and both benthic 

and planktonic species are known (Bold and 

Wynne, 1985).

• Chrysophyta (golden-brown algae) -- These algae 

typically are restricted to unpolluted, cold 

streams and springs. 

• Cyanophycota (blue-green algae) -- These algae are 

generally more abundant in neutral or slightly 

alkaline water, but are found in water having a 

great range of salinity and temperature (Bold and 

Wynne, 1985). Both planktonic and benthic species are included in this division. Blue-green 

algae inhabit alkaline hot springs in Yellowstone 

National Park having water temperatures as 

warm as 74o

C (Bold and Wynne, 1985).

• Euglenophycota (euglenoids) -- These single-celled 

algae are widely distributed, being found in 

freshwater, brackish water, and on moist soils 

and mud (Bold and Wynne, 1985). Euglenoids 

usually are abundant in small pools rich in 

organic matter, especially those to which livestock have access.

• Pyrrophycophyta (dinoflagellates) -- Although generally marine, freshwater dinoflagellates occur in 

pools, ditches, and small lakes with considerable 

vegetation. The unicellular algae of this division 

are widely known for the blooms known as “red 

tides” that they produce in some marine locations.

• Rhodophycota (red algae) -- Although generally 

marine, freshwater species are restricted to cold, 

well-aerated water of rapids, falls, and spillways 

(Bahls and others, 1984). Many species do not 

appear reddish at all, and a full range of pigmentation occurs among freshwater species (Bold 

and Wynne, 1985).

• Xanthophyta (yellow-green algae) -- In streams, 

habits of these algae are epiphytic; terrestrial 

members of this division may be found growing 

in dense stands on drying mud along stream 

banks. 

Algae found in cold, mountain streams within 

the YRB are more distinctive, and include a larger percentage of species restricted to a particular habitat, than 

those found in warmer, lowland streams (Smith, 1950). 

Algae found in rapids and waterfalls usually are 

attached to stones (Smith, 1950). Additionally, rocks 

continuously moistened by spray from rapids and 

waterfalls, or where there is a continuous trickle of 

water, develop extensive feltlike or gelatinous algal 

masses upon rocky cliffs (Smith, 1950). Certain spe-
50 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

cies of blue-green algae are the most frequently 

encountered algae on dripping cliffs (Smith, 1950). 

Algae of high-gradient streams are of two distinct morphological types: encrusting algae (diatoms, for example) and those in which the greater part of the thallus 

(undifferentiated body) trails in the current (Smith, 

1950). Certain red algae and river mosses (Cladophora) are restricted to cold, high-gradient streams 

at elevations higher than approximately 3600 m 

(Smith, 1950).

The algal flora of lowland streams with finegrained substrates are far less diverse than those of 

mountain streams (Smith, 1950). Most lowland flora 

are attached to mud or sand, and include diatoms, bluegreen algae, green algae, and euglenoids. In contrast to 

encrusting and trailing algae upstream, these species 

are often motile.

Invertebrates

As with algae, the composition of invertebrate 

communities varies longitudinally downstream within 

the YRB. Overall, factors influencing both distribution 

and abundance of aquatic invertebrates include current 

velocity, water temperature, substrate, stability of both 

aquatic and riparian vegetation, dissolved substances, 

competition, zoogeography, food, disturbance history, 

and human practices. Water temperature and chemistry 

exert great influence, but invertebrate morphological 

features and respiratory requirements, in relation to 

streamflow habitat, ultimately define macroinvertebrate community composition. Large, stable substrates 

such as boulders and cobbles support larger, more productive invertebrate populations than do unstable 

gravel and sand substrates. Consequently, a longitudinal decrease in substrate size and riffle frequency 

results in lower macroinvertebrate production and 

diversity downstream. Although deposition of organic 

sediment at slow current velocities may increase 

benthic production for midges (Chironomidae) and 

earthworms (Oligochaeta) that depend on allochthonous detritus (Baril and others, 1978), the filling of 

interstitial spaces with fine, inorganic sediment eliminates potential habitat. 

Within the YRB, invertebrate fauna are likely be 

dominated by mayflies (Ephemeroptera, includes both 

mountain and prairie varieties) whose current-velocity 

preferences vary; for example, Baetis prefer rapid currents whereas Tricorythodes prefer slow velocities. 

Other prominent taxa include caddisflies (Trichoptera), many of whom, such as Hydropsyche, 

depend on rapid currents for proper net functioning 

(Baril and others, 1978); true flies (Diptera) (Newell, 

1977); and stoneflies (Plecoptera), whose general preference is for rapid currents and whose fauna is diverse 

but not abundant (Newell, 1977). Another swift-current specialist is the riffle beetle (Elmidae). Invertebrates present in large numbers in transition-zone 

streams of the YRB generally are tolerant of a wide 

range of habitat conditions, whereas those abundant in 

prairie reaches are considered tolerant of turbid, silty 

conditions (Baril and others, 1978).

Macroinvertebrate distribution and abundance 

often vary widely through the year in response to seasonal flow variations. Dispersal is affected by floods 

through channel scouring and catastrophic drift that 

can transport invertebrates that would not drift otherwise (Peterson, 1990). Broadly, within each ecoregion 

of the YRB (described later and shown in fig. 23), 

invertebrate communities of all perennial streams will 

be similar; moreover, communities of ephemeral and 

intermittent streams will resemble one another (Peterson, 1990).

Fish

The distribution of fish within the YRB is influenced by elevation, drainage divides, and other physiographic features (Baxter and Stone, 1995). As with 

other aquatic communities, both abiotic and biotic factors control the occurrence, interspecific relationships, 

numbers, and growth rates of fish. Abiotic factors 

include dissolved oxygen, water temperature, current 

velocity, discharge fluctuation, dissolved salts, substrate, and turbidity. Information on the effect of biotic 

factors (numbers and growth rates of competitors) is 

much less available, although closely related species 

interact with one another in such a way as to divide 

habitat between them (Hynes, 1970). Of abiotic factors, water temperature is important to cold- and warmwater fish species and indirectly influences oxygen 

consumption; water chemistry seems to be of lesser 

importance in natural systems (Hynes, 1970).

Warm-water species considered native to the 

YRB include the goldeye, brassy minnow, fathead minnow, flathead chub, sturgeon chub, western silvery 

minnow, white sucker, shorthead redhorse, stonecat, 

burbot, channel catfish, and sauger. Of these, the stonecat may be found in high-gradient reaches while the 

goldeye, flathead chub, western silvery minnow, bur-
ENVIRONMENTAL SETTING 51

bot, and sauger all prefer larger, sluggish, deeply turbid 

streams. The channel catfish prefers larger rivers with 

turbid habitats. The shorthead redhorse, white sucker, 

brassy minnow, fathead minnow, and sturgeon chub 

may all be found in transition zones that generally have 

less turbidity, some vegetation, and gravel substrates.

Cold-water species native to the study unit but 

also found on the western side of the Continental 

Divide include the mountain whitefish, cutthroat trout, 

arctic grayling, and mottled sculpin. Species common 

to cool-water, transition-zone streams include the lake 

chub, longnose dace, longnose sucker, mountain sucker 

(Brown, 1971; Baxter and Stone, 1995). Of these, the 

lake chub is usually found in smaller streams, and the 

mountain sucker inhabits a wide variety of habitats, 

including large rivers and mountain streams. The longnose dace prefers the swift-water environment of riffles, and the longnose sucker prefers clear water.

In the YRB, several species of fish are accorded 

special concern by the Federal or State governments. 

The pallid sturgeon, a Federally listed Endangered Species, occurs in the lower Yellowstone River. The sturgeon chub and arctic grayling currently are designated 

as Candidate species for Federal Endangered or Threatened status (Fertig, 1997). The Yellowstone cutthroat 

trout is designated a Sensitive Species by the U.S. Forest Service. Species of special concern to State governments include the pallid sturgeon, paddlefish, 

Yellowstone cutthroat trout, arctic grayling, sturgeon 

chub, sicklefin chub, northern redbelly x finescale dace 

(hybrid), and blue sucker. Most of these are warmwater species inhabiting the lower Yellowstone River, 

but the Yellowstone cutthroat trout and the arctic grayling are cold-water species.

Ground Water

The hydrologic and water-quality characteristics 

of ground water in the study unit differ with location, 

elevation, and geologic unit. The physical and 

geochemical properties of the rock units comprising 

the aquifers to a large extent determine the quantity and 

quality of ground water available for use. The primary 

aquifers in the YRB are unconsolidated Quaternary 

deposits, lower Tertiary rocks, and rocks of Mesozoic 

and Paleozoic age. Aquifers are located throughout 

most of the study unit (fig. 16). The unconsolidated 

Quaternary deposits and lower Tertiary rocks are found 

primarily in the structural basins, whereas the Mesozoic and Paleozoic units are at or near the surface 

chiefly in the uplifted areas of the YRB. The lower 

Tertiary aquifers have the greatest surface extent in the 

study unit. The quality of ground water in shallow 

aquifers may be influenced by the quality of nearby 

surface water, such as lakes and streams. The areal distribution of the uppermost principal aquifers in semiconsolidated and consolidated rocks of the study unit is 

shown in figure 16.

The four major structural basins of the YRB--the 

Wind River, Bighorn, Powder River, and Williston 

Basins (fig. 5)--each contain substantial quantities of 

ground water and will be the focus of this section. The 

remainder of the study unit is composed of small 

basins, structural uplifts, and mountain ranges that contain smaller quantities of ground water. Ground-water 

resources in these regions will not be discussed in this 

report.

Wind River Basin

The Wind River Basin contains both confined 

and water-table aquifers. The Wind River Formation 

of Eocene age and older sedimentary rocks often yield 

water under confined conditions. In alluvial and eolian 

sand deposits of Quaternary age, unconfined (watertable) conditions often prevail (Whitcomb and Lowry, 

1968). The physical and water-bearing characteristics 

of the geologic formations in the Wind River Basin are 

summarized in table 16 (at end of report). The following text briefly summarizes the hydrologic characteristics of the rock units within the Wind River Basin. For 

a more complete description the reader is referred to 

Whitcomb and Lowry (1968), Richter (1981), Plafcan 

and others (1995), and Daddow (1996).

Quaternary Aquifers

Whitcomb and Lowry (1968) report that coarse 

sand and gravel beds in the valleys of perennial streams 

whose headwaters are in the Wind River and Owl 

Creek Mountains, are capable of yielding moderate to 

large quantities of water. Generally, the concentration 

of dissolved solids in the upper reaches of floodplain 

deposits of perennial streams is less than that in deposits further downstream. For example, a well drilled in 

the alluvium of the central Wind River valley yielded a 

calcium bicarbonate water that contained 272 ppm 

(parts per million) of dissolved solids, whereas a well 

about 16 miles downstream yielded a sodium sulfate
52 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 53

water that contained 1,230 ppm of dissolved solids 

(Whitcomb and Lowry, 1968). Large-scale development of this water resource has not occurred because 

surface-water supplies generally are adequate for 

present needs. 

According to Whitcomb and Lowry (1968), alluvial deposits located along ephemeral and intermittent 

streams that originate within the Wind River Basin 

generally consist of fine to coarse sand intermixed with 

silt and clay. Wells in these deposits normally only 

yield enough water for stock and domestic use. 

Recharge to these aquifers is mainly from infiltration of 

precipitation on the drainage area, and the amount of 

ground water available to wells fluctuates accordingly. 

Terrace deposits can yield adequate quantities of 

water for stock or domestic use in areas where surface 

water has been applied for irrigation. Yields fluctuate 

in response to irrigation, and some of the shallower 

wells go dry during the winter (Whitcomb and Lowry, 

1968).

In the northeastern part of the basin, Whitcomb 

and Lowry (1968) reported that deposits of eolian sand 

are an important source for stock and domestic supplies 

of ground water. The water in these deposits is generally derived from local infiltration of precipitation.

Tertiary and Cretaceous Aquifers

According to Whitcomb and Lowry (1968), the 

major Tertiary aquifers in the Wind River Basin are the 

Split Rock Formation of Oligocene and Miocene age 

and the Eocene Wind River Formation. The Oligocene 

White River Formation has similar hydrologic characteristics as the Split Rock Formation but has not been 

widely developed as a ground-water source because 

shallower aquifers usually provide adequate water for 

stock and domestic uses. The Tepee Trail, Aycross, and 

Indian Meadows Formations of Eocene age, together 

with undifferentiated rocks of Tertiary age, may yield 

moderate to large supplies of water, but these rocks 

have not been tested as a potential source of ground 

water.

Cretaceous aquifers generally are not used for 

ground-water supplies throughout the basin for several 

reasons. According to Whitcomb and Lowry (1968), 

although the water in these formations is usually under 

artesian pressure and piezometric surfaces are often 

near the land surface, the formations themselves lie at 

great depths except around the margins of the basin and 

in areas of local uplift. The formations principally consist of shale containing minor amounts of fine-grained 

silty sandstone and generally have low permeabilities. 

The Upper Cretaceous Lance Formation and the Lower 

Cretaceous Cloverly Formation have more suitable 

aquifer characteristics, but areas in which the formations lie within economical drilling depths are small. 

Water from the Mesaverde Formation, Cody Shale, and 

Frontier Formation is generally unsuitable for domestic 

use, and in some wells it is so unpalatable that stock 

will not drink it (Whitcomb and Lowry, 1968).

Pre-Cretaceous Aquifers

Pre-Cretaceous formations are used to a limited 

extent as supplies of ground water in the basin. Whitcomb and Lowry (1968) reported that many of the formations have suitable aquifer characteristics but occur 

below economical drilling depths in most of the Wind 

River Basin. The formations form narrow outcrops on 

the flanks of the Wind River, Owl Creek, and Bighorn 

Mountains and in some of the major anticlinal structures within the basin. The Jurassic Morrison and Sundance Formations, the Triassic Chugwater Formation, 

the Permian Phosphoria Formation and related rocks, 

the Pennsylvanian Tensleep Sandstone, and Mississippian Madison Limestone are used in small areas of the 

basin as ground-water supplies. Pre-Mississippian 

rocks in the basin are unused because of great burial 

depths and inaccessibility in outcrop areas.

Bighorn Basin

The Bighorn Basin contains both artesian and 

water-table aquifers. Aquifers in Quaternary unconsolidated deposits are generally unconfined, and the water 

table is generally at shallow depths. Water in bedrock 

aquifers occurs under either confined or unconfined 

conditions (Plafcan and others, 1993). The physical 

and water-bearing characteristics of the geologic formations in the Bighorn Basin are summarized in 

table 16 (at end of report). The hydrologic characteristics of the rock units within the basin are briefly summarized in the following two sections. For a more 

complete description the reader is referred to Lowry 

and others (1976), Libra and others (1981), Plafcan and 

others (1993), Susong and others (1993), and Plafcan 

and Ogle (1994).
54 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Quaternary and Tertiary Aquifers

Holocene and Pleistocene unconsolidated deposits in the Bighorn Basin include floodplain alluvium, 

terrace deposits, pediments, minor colluvium, and alluvial fan deposits. The floodplain alluvium is generally 

less than 10 m thick. Alluvial and colluvial deposits 

located along the larger streams, such as the Bighorn 

and Shoshone Rivers, are among the most productive 

and predictable sources of ground water in the basin 

(Plafcan and others, 1993; Susong and others, 1993).

Lower Tertiary aquifers in the Bighorn Basin are 

used primarily as a drinking-water source for residents 

without access to a public drinking-water supply (Libra 

and others, 1981). The areally extensive Willwood Formation of Eocene age is the principal Tertiary aquifer in 

the basin. The sandstone content of the formation 

ranges from 3 to 88 percent with the average being 

25 percent. Ground water occurs in sandstone lenses 

that are often small, making it difficult to locate productive wells for stock or domestic uses (Plafcan and Ogle, 

1994; Plafcan and others, 1993; Susong and others, 

1993).

The Paleocene Fort Union Formation is a minor 

aquifer in parts of the basin. The average sandstone 

content is 25 percent, but sandstone lenses are seldom 

continuous for more than a few hundred yards. The formation is used as an aquifer primarily where it crops out 

west of the Bighorn River (Plafcan and Ogle, 1994; 

Plafcan and others, 1993; Susong and others, 1993).

Mesozoic and Paleozoic Aquifers

Cretaceous aquifers have not been widely developed throughout the basin, although the Upper Cretaceous, Lance, Mesaverde, and Frontier Formations 

have potential for more extensive development (Plafcan and others, 1993). Jurassic and Triassic sandstones 

are locally used as aquifers. The Gypsum Spring Formation of Jurassic age consists of interbedded red shale, 

dolomite, and gypsum (table 16, at end of report). Solution of gypsum in this formation may provide enough 

secondary porosity to provide high yields to wells in 

some areas (Lowry and others, 1976). Mesozoic rocks 

generally do not occur at economical drilling depths 

except at the margins of the basin and along areas of 

structural uplift.

The major Paleozoic aquifers in the basin include 

the Pennsylvanian Tensleep Sandstone, the Mississippian Madison Limestone, the Ordovician Bighorn 

Dolomite, and the Cambrian Flathead Sandstone (Plafcan and Ogle, 1994; Plafcan and others, 1993; Susong 

and others, 1993). These aquifers are confined by overlying Mesozoic shales as well as by impermeable Paleozoic strata such as the Goose Egg Formation of 

Triassic and Permian age, the Amsden Formation of 

Pennsylvanian and Mississippian age, the Ordovician 

Gallatin Limestone, and the Cambrian Gros Ventre Formation (Plafcan and others, 1993). Paleozoic aquifers 

in the Bighorn Basin are recharged primarily in the 

mountains on the basin margins. Except near basin 

margins and other areas of structural uplift, Paleozoic 

rocks do not occur at economical drilling depths (Plafcan and Ogle, 1994; Plafcan and others, 1993; Susong 

and others, 1993).

Powder River Basin

The physical and hydrologic characteristics of the 

geologic formations in the Powder River Basin are summarized in tables 16 and 17 (at end of report). The following two sections briefly summarize the hydrologic 

characteristics of the aquifers within the basin. For a 

more complete description the reader is referred to Crist 

and Lowry (1972), Hodson and others (1973), Lewis 

and Hotchkiss (1981), and Slagle and others (1984).

Quaternary and Tertiary Hydrogeologic Units

According to Lewis and Hotchkiss (1981), the 

shallow aquifer system of the Powder River Basin is 

composed of five mappable hydrogeologic units 

located stratigraphically above the regionally persistent 

and essentially impermeable Upper Cretaceous Bearpaw Shale. The uppermost hydrogeologic unit in the 

shallow aquifer system is the Wasatch-Tongue River 

aquifer (table 17, at end of report) (Lewis and Hotchkiss, 1981). The aquifer is extensive, thick (up to 1,190 

m thick), and is exposed at the land surface in most of 

the basin. The depositional environments of the geologic units included in the aquifer are generally terrestrial. The average sand content of this unit is 

54 percent, indicating it could be an aquifer over most 

of the area.

The Lebo confining layer (up to 920 m thick) 

extends over most of the basin and underlies the 

Wasatch-Tongue River aquifer (table 17, at end of 

report). The unit generally correlates with the Lebo 

Shale Member of the Fort Union Formation. The sand 

content of the unit has a mean value of 31 percent, indi-
ENVIRONMENTAL SETTING 55

cating that it generally acts as a confining layer (Lewis 

and Hotchkiss, 1981).

The Tullock aquifer (up to 600 m thick) underlies 

the Lebo confining layer except near outcrop areas. 

With an average sand content of 53 percent the unit is 

considered an aquifer in most of the basin (Lewis and 

Hotchkiss, 1981).

Mesozoic and Paleozoic Hydrogeologic Units

The Cretaceous Upper Hell Creek confining layer 

(table 17, at end of report) underlies the Tullock aquifer 

(Lewis and Hotchkiss, 1981). The unit is a major confining layer throughout the basin, is up to 610 m thick, 

and has a mean sand content of 35 percent (Lewis and 

Hotchkiss, 1981).

The Upper Cretaceous Fox Hills-Lower Hell 

Creek aquifer (table 17, at end of report) underlies the 

Upper Hell Creek confining layer except at outcrop 

areas (Lewis and Hotchkiss, 1981). The aquifer is up to 

780 m thick, has a mean sand content of 50 percent, and 

yields water to wells in most areas. The base of the 

aquifer is the top of the Bearpaw Shale (equivalent to 

Lewis Shale). 

Several formations of the Lower Cretaceous 

Series in the basin consist of shales that are not usable 

as aquifers. However, the Cloverly, Fall River, and 

Lakota Formations contain sandstone aquifers, but are 

too deeply buried to be useful, except at the basin margins.

According to Hodson and others (1973), several 

Paleozoic formations, including the Minnelusa Formation and Tensleep Sandstone of Permian and Pennsylvanian age, and the Madison Limestone of Mississippian 

age, have great potential for ground-water development. The Bighorn Dolomite of Ordovician age and the 

Flathead Sandstone of Cambrian age also have potential. Although these Paleozoic formations would likely 

produce relatively high yields of ground water, they are 

largely unused because, except at the basin margins, 

they are deeply buried.

Williston Basin

Only a small, southwestern part of the Williston 

Basin is included in the study unit. The shallow hydrogeologic units of the Williston Basin included in the 

study unit are similar to the shallow hydrogeologic units 

of the Powder River Basin. 

Stoner and Lewis (1980) mapped the shallow 

hydrogeologic units in southeastern Montana, including 

the southwestern part of the Williston Basin. The shallow aquifer system was divided into mappable units 

(table 18, at end of report) that are similar to those 

mapped by Lewis and Hotchkiss (1981) in the Powder 

River Basin. The most notable difference between the 

aquifer units defined in the Williston Basin and those in 

the Powder River Basin occurs in the upper part of the 

shallow aquifer system. The aquifers lying above the 

Lebo confining layer in the Williston Basin are divided 

into four separate mappable units, whereas in the Powder River Basin they are grouped as a single unit. For a 

more complete description of the shallow aquifer system in the Williston Basin the reader is referred to 

Stoner and Lewis (1980), and Slagle and others (1984).

Anthropogenic Factors

The term “anthropogenic” means of, relating to, 

or involving the impact of humans on nature. Because 

human practices are the source of many water-quality 

contaminants in the YRB, they must be described as an 

integral part of the environmental setting. Agricultural 

runoff, urban runoff, point discharges of municipal and 

industrial wastes, mine drainage, septic-system effluent, landfill leachate, and contaminated atmospheric 

deposition are all sources of anthropogenic contamination in the study unit. These sources are directly related 

to population density, land cover and land use, water 

use, and waste disposal in the YRB. The major contaminants associated with human activities are summarized 

in table 8.

Population

The early human populations of the YRB 

included the Crow Nation, which had pushed westward 

as far as the mouth of the Bighorn River in the mid1700s, and by 1800 had displaced the Shoshone Tribe 

throughout the entire upper YRB (U.S. Department of 

the Interior, 1965). Fur trappers considered the YRB a 

prime area of production from 1805-35. However, permanent settlement by non-Native populations coincided 

with the mining booms, cattle drives, and westward railroad construction beginning in the 1860s. In 1874, 

commercial river boats began service on the 
56 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Table 8. Human activities contributing to contamination of surface and ground water 

(modified from Risser and Siwiec, 1996, table 16)

Activity

Contaminants frequently cited as 

results of activity Other remarks

Land-use activities

Agricultural activities: applications of 

commercial fertilizers, manure, and 

pesticides

Nitrate, phosphate, bacteria, and pesticides Surface erosion washes chemical residues 

into streams; infiltrating water transports 

dissolved contaminants to ground water

Mining activities: mining and spoil disposal Acids, iron, manganese, sulfate, uranium, 

thorium, radium, molybdenum, selenium, 

and other trace elements

Leachates from spoil piles of coal, metallic-, 

and nonmetallic-mineral mining contain 

a variety of contaminants

Urban activities Bacteria, hydrocarbons, dissolved solids, 

lead, cadmium, and other trace elements

Urban runoff washes contaminants into 

streams; infiltration from detention 

basins and drainage wells can reach 

ground water

Storage tanks Petroleum products, acids, metals, and 

organic compounds

Useful life of steel tanks is 15-20 years; 

leaks, spills, and overflows may 

contaminate ground water

Waste disposal

Septic systems Bacteria, viruses, nitrate, phosphate, 

chloride, and synthetic organic 

compounds

None

Landfills Dissolved solids, iron, manganese, trace 

elements, acids, organic compounds, and 

pesticides

Traditional disposal method used for 

municipal and industrial waste; number 

of abandoned landfills and dumps is 

unknown

Surface impoundments Manure, trace elements, and organic 

compounds

Used to store agricultural wastes, industrial 

liquid wastes, municipal sewage sludge, 

and other wastes

Injection wells Dissolved solids, bacteria, sodium, chloride, 

nitrate, phosphate, organic compounds, 

pesticides, and acids

Hazardous wastes also were injected in the 

past

Surface applications of waste Bacteria, nitrate, phosphate, trace metals, 

and organic compounds

Waste disposal from municipal sewagetreatment plants, septic systems, and 

drinking-water treatment plants

Discharges from sewage-treatment plants Bacteria, nitrate, ammonia, phosphate, 

organic compounds, and suspended 

solids

Base flow in receiving water body may be 

inadequate for proper dilution

Industrial discharges Organic compounds and trace elements Treated effluent in many cases; discharged 

to surface water
ENVIRONMENTAL SETTING 57

Yellowstone River as far upstream as present-day 

Billings, but were abandoned in 1884, 2 years after the 

railroad reached Billings (U.S. Department of the Interior, 1965). The frontier was considered closed by 

1890, and within another 20 years much of the YRB 

achieved its maximum population density to date 

(Missouri Basin Inter-Agency Committee, 1969). 

From 1900-90, the rate of population growth in 

this region has lagged behind the National average. 

The YRB is sparsely populated (fig. 17), and the population was relatively stable during the decades preceding 1990. Population density in 1960 was less than two 

persons per km2

 (Missouri Basin Inter-Agency Committee, 1969). From 1980-90 the population of the 

study unit decreased by about 2 percent (U.S. Bureau 

of the Census, 1980; 1991), and averaged less than two 

persons per km2

 in 1990.

According to the 1990 census, about 323,000 

people resided in the study unit on April 1, 1990. The 

population distribution by State is 206,000 people in 

Montana, 116,000 people in Wyoming, and 1,000 in 

North Dakota. The major population center of the 

study unit is the Billings, Mont., metropolitan area, 

with a population of 113,419 in 1990 (U.S. Bureau of 

the Census, 1997c). No other cities in the YRB had a 

1990 population greater than 14,000 residents (U.S. 

Bureau of the Census, 1997b). Gateway communities 

to Yellowstone National Park do not have a large resident population, but a substantial temporary population 

of visitors is typically present because of tourism and 

recreational opportunities. The summer seasonal peak 

of temporary population is pronounced.

Population within the study unit is estimated to 

have increased by 8.4 percent since 1990 to about 

350,000 by July 1, 1996. These population increases 

have been centered in several areas experiencing rapid 

growth, largely by the conversion of ranch land to lowdensity residential use. During 1990-96, the six county 

area generally corresponding to the northwestern YRB 

experienced an 11 percent increase in estimated population—an increase of about 19,000 residents, and 

about half of this growth occurred in Billings (U.S. 

Bureau of the Census, 1997a; 1997b). Several Wyoming communities along the eastern front of the Bighorn Mountains also have had 1990-96 population 

growth, averaging nearly 8 percent (U.S. Bureau of the 

Census, 1997b).

Land Cover and Land Use

Land-cover and land-use maps indicate the spatial distribution of management practices likely to 

affect the quality of water resources (Gallant and others, 1989). Activities such as grazing, logging, and 

mining on rangeland and forest land may affect water 

quality by increasing suspended-sediment loads and 

concentrations of metals or nutrients, and by altering 

hydrologic conditions. Pesticide and fertilizer applications, crop rotation, and tillage practices may affect 

both surface-water and ground-water quality on agricultural land. Water quality in urban areas is subject to 

runoff, pesticide and fertilizer usage, and waste disposal.

The available USGS data for land cover and land 

use (U.S. Geological Survey, 1986) are relatively 

detailed but also somewhat dated. The 1- by 2-degree 

quadrangles covering the YRB study unit were interpreted from aerial photography acquired during 1970-

84 (U.S. Geological Survey, 1976-85). The USGS data 

(1:250,000 scale) are not a large-scale source, yet were 

the most spatially detailed with consistent coverage of 

the entire study unit. At the most general level of classification (Anderson and others, 1976), the land cover 

and land use of the YRB are shown in figure 18.

Agricultural, urban, and mineral-related land 

uses are discussed in separate sections that follow. Silviculture is another potentially important land-use 

activity. Forests cover about 20 percent of the YRB 

(U.S. Geological Survey, 1986). Most of the timber 

resources of the YRB are contained within the National 

Forest System lands. Each national forest has developed a Forest Plan that specifies the maximum volume 

of timber that may be sold from the suitable timber base 

during the specified planning period. For example, the 

allowable sale quantity (ASQ) of timber established by 

the 1986 Forest Plan of the Shoshone National Forest 

(fig. 19) was 11.2 million board feet per year on a suitable timber base of about 350 km2 (U.S. Dept. of Agriculture, Forest Service, 1997) (One board foot equals 

0.0929 m2

 [one foot square] by 25.4 mm [1 inch] 

thick). Following the wildfires of 1988, ASQs of timber were revised for some areas due to extensive burning of the timber base. Shoshone National Forest is 

currently using an ASQ of about 4.5 million board feet 

per year, and an additional 2.3 million board feet of 

other products (fuelwood, posts, and poles) are sold 

annually (U.S. Dept. of Agriculture, Forest Service, 

1997).
58 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

60 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 61

Agriculture

Agricultural activities, primarily livestock production and irrigated and dryland crop production, are 

the most areally extensive land-use industries in the 

study unit. Rangeland is the dominant land cover, with 

about 43 percent of the study unit composed of subhumid grassland and semiarid grazing land, about 16 percent grazed desert shrubland, and about 5 percent 

grazed open woodland. Cropland and pasture compose 

only about 11 percent of the study unit (U.S. Geological Survey, 1986), with about 36 percent of the harvested cropland in 1987 having been irrigated 

(Battaglin and Goolsby, 1994). Most irrigated cropland is located in the principal stream valleys.

The effects of agriculture on sediment, nutrient, 

bacteria, and pesticide concentrations are water-quality 

issues in the study unit. Grazing, especially when pasture or rangeland are stocked near or above their carrying capacities, has been shown to increase runoff and 

soil erosion as vegetative land cover decreases and 

stream banks become trampled (Owens and others, 

1996). Even near-natural rangelands, such as those 

within national parks, may be subject to overgrazing, 

particularly when humans apply various management 

strategies to modify populations of herbivorous wildlife (Coughenour and Singer, 1991).

According to the 1987 Census of Agriculture, 

9,584 farms covered 117,367 km2 in the YRB, of 

which 4,023 km2 was irrigated land. Cropland 

accounted for 15,011 km2

 of this area, in which wheat 

(2,891 km2), alfalfa (2,231 km2), barley (1,249 km2), 

hay other than alfalfa (1,089 km2

), corn (268 km2), oats 

(214 km2

), and sugar beets (165 km2) were the principal crops harvested (Battaglin and Goolsby, 1994). A 

total of 8,360 km2

 of cropland was harvested in 1987. 

Pasture, along with 1,971 km2 of woodland, composed 

the remainder of the farmland. The predominant crop 

rotation in the Wyoming part of the YRB is barley and 

sugar beets, with some producers also including corn 

and alfalfa as part of their rotation (P. Shelton, USDA 

Natural Resources Conservation Service, Casper, 

Wyo., written commun., 1998).

Commercial fertilizers (see table 9) were applied 

to 4,758 km2 of cropland. Estimates of commercial 

fertilizer usage do not include manure applications. 

Nutrients from manure are a potential problem because 

animal densities are high in localized areas. Although 

records are not kept of manure applications, the domestic livestock in the study unit (table 10) are counted regularly. Cattle raised for beef are the most numerous 

group of domestic livestock, followed by sheep, hogs, 

and poultry. 

Chemical pesticides often are applied to agricultural lands to control weeds, insects, and fungus. 

Applications to field crops are the dominant agricultural use of pesticides, followed by applications to pastures. Many of the chemical compounds applied are 

soluble in water and may be leached from the soil into 

surface or ground water. 

Table 9. Estimated annual applications of fertilizers in the 

Yellowstone River Basin

(Modified from Battaglin and Goolsby, 1994)

Fertilizer used

Estimated 

application of active 

ingredient during 

1987, in kilograms

Nitrogen fertilizers:

Anhydrous ammonia 32,750,529

Urea 7,009,657

Miscellaneous forms 6,545,967

Nitrogen solutions 6,254,551

Ammonium nitrate 5,244,132

Total nitrogen fertilizer 57,807,019

Phosphate 19,309,058

Potash 3,338,697

Table 10. Livestock numbers in the Yellowstone River Basin, 

1987

(Modified from Battaglin and Goolsby, 1994)

Type of livestock Number raised

Cattle and calves 1,131,373

Beef cows 565,762

Steers, steer calves, bulls, and bull calves 266,136

Milk cows 6,846

Sheep and lambs 510,436

Horses and ponies 36,960

Hogs and pigs 35,810

Chickens (3 months or older) 20,583

Hens and pullets of laying age 14,182

Broilers and other meat-type chickens 4,459

Turkeys 1,247
62 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

The NAWQA program analyzed information on 

national and regional patterns of pesticide occurrence in 

surface water of the United States and the major influences on the sources and transport of pesticides (Larson 

and others, 1997). The following points are known in 

relation to surface water:

• Low levels of pesticides have been widespread in the 

Nation’s surface water for several decades.

• Pesticide concentrations in surface water follow 

strong seasonal patterns that result from the timing of pesticide applications and runoff conditions.

• Many pesticides are rarely detected in surface waters 

because of relatively low use, how they are 

applied, chemical properties, or relatively insensitive analytical methods.

• In many streams, some pesticides exceed waterquality criteria for seasonal periods each year, 

but annual average concentrations seldom exceed 

regulatory standards for drinking water.

• Potential effects of pesticides on humans and aquatic 

ecosystems are difficult to evaluate because of 

inadequate information on effects of low-level 

mixtures, transformation products, and seasonal 

exposure.

• Improved information is needed on long-term trends, 

pesticides and transformation products that have 

not been widely measured, and biological effects 

of typical exposure patterns.

NAWQA also analyzed information on national 

and regional patterns of pesticide occurrence in ground 

water of the United States and the major influences on 

the sources and transport of pesticides (Barbash and 

Resek, 1996). The following points are known in relation to ground water:

• Pesticides from every major chemical class have been 

detected in ground water.

• Pesticides are commonly present in low concentrations in ground water beneath agricultural areas, 

but seldom exceed water-quality standards.

• Frequencies of pesticide detection are almost always 

low in low-use areas, but vary widely in areas of 

high use.

• Pesticide levels in ground water show pronounced 

seasonal variability in agricultural areas, with 

maximum values often following spring applications.

• Factors most strongly associated with increased 

likelihood of pesticide occurrence in wells are 

high pesticide use, high recharge by infiltration 

of either precipitation or irrigation, and shallow, 

inadequately sealed, or older wells.

NAWQA also analyzed information on national 

and regional patterns of pesticide occurrence in the 

atmosphere and the major influences on the sources and 

transport of pesticides (Majewski, 1995). The following points are known in relation to the atmosphere:

• Existing data, gleaned from 132 studies, are unevenly 

distributed, with most of the data from the Great 

Lakes region and California.

• Most of the pesticides studied have been detected in 

rain or air, but many that are used have never been 

studied.

• Pesticides have been detected in the atmosphere in all 

areas of the Nation sampled.

• Concentrations of a pesticide in air and rain are most 

affected by its use and resistance to environmental 

degradation.

• The highest atmospheric concentrations of pesticides 

occur seasonally in high-use areas when applications are greatest.

• Low levels of long-lived pesticides are present in the 

atmosphere throughout the year.

• Atmospheric deposition of pesticides is most likely to 

affect stream-water quality during runoff events 

when precipitation and direct surface runoff are 

the major sources of streamflow, but the full significance to water quality is largely unknown. 

Within the study unit, the small area devoted to 

row crops (sugar beets, corn, and beans, primarily) is 

reflected by the types and amounts of herbicides that 

are most widely used in the YRB (table 11). The chlorinated phenoxy 2,4-D is used mainly as a postemergence herbicide on grassland, small grains, and fallow 

land (Meister, 1996). Dicamba is a benzoic acid used 

to control broadleaf weeds in asparagus, corn, grassland, fallow land, and small grains; picloram is used on 

broadleaf weeds in small grains and pastures, and for 

brush control on rangeland and utility rights-of-way; 

cycloate is a selective herbicide used for sugar beet production; EPTC is effective on grassy weeds and some 

broadleaf weeds in beans and potatoes; triallate is a 

selective herbicide for use in producing small grains 

and some beans and peas (Meister, 1996).
ENVIRONMENTAL SETTING 63

The largest concentrations of pesticides detected 

in air and rain occur in those areas where they are used 

most frequently and in the largest quantities. Patterns 

of atmospheric pesticide concentrations from the early 

1970s showed general correlations among atmospheric 

concentrations and regional use and cropping patterns 

(Majewski, 1995). Atmospheric pesticide concentrations within the YRB were low relative to areas of the 

Nation that have more intensive agricultural activity 

and more extensive cropland.

Urban Areas

Urban and residential areas affect surface- and 

ground-water quality by altering the physical hydrology 

and by adding waste products. Streamflow and groundwater flow are altered where rooftops, paved surfaces, 

and sewers accelerate the movement of storm runoff to 

streams at the expense of ground-water infiltration. 

Urban runoff frequently carries large nonpoint-source 

loads of sediment and inorganic and organic constituents from paved surfaces, parks, lawns, and golf 

courses. Point sources of contamination from the populated urban environment include sewage-treatment 

facilities, industrial discharges, landfills, and leaking 

storage tanks. Contamination of ground water by volatile organic compounds is a special concern because of 

its widespread occurrence in urban areas (Risser and 

Siwiec, 1996). 

Urban or built-up land accounts for only 0.3 percent of the study unit area. However, in this small area 

there are a large number of point sources having the 

potential to contaminate receiving waters. The U.S. 

Environmental Protection Agency (USEPA) regulates 

341 facilities in Billings, Montana. These facilities 

include National Pollutant Discharge Elimination System permit holders, hazardous waste handlers, Superfund sites, facilities releasing and transferring toxic 

chemicals, and sources of air pollution.

Herbicides, pesticides, and fertilizers are sometimes applied to urban land at greater rates than typically applied to agricultural land and can contribute to 

water-quality impairment. Lawns, gardens, parks, and 

golf courses are subject to intense pesticide applications. NAWQA determined that the most commonly 

detected insecticides in streams across the U.S. have 

substantial urban and suburban use (Larson and others, 

1997).

Recent population growth in urban and suburban 

areas of the YRB represents potential water-quality 

problems. As sparsely populated grazing land is converted to residential use, there is increased risk of water 

supplies becoming contaminated. Many housing 

developments rely on septic systems from which contaminants can leach into the ground-water supply.

Minerals Extraction

In addition to agriculture and urban uses, other 

important land uses in the study unit include metals and 

coal mining and hydrocarbon production. These mineral industries are discussed in the following sections.

Metals Mining--The study unit contains few of the 

historically most productive metals mines in either 

Montana or Wyoming. However, there are some substantial deposits of metallic minerals within the YRB 

(fig. 9), and several areas experienced mining-related 

impacts. Potential water-quality problems associated 

with metals mining are increased sedimentation and the 

discharge of mine drainage that often exhibits water 

chemistry substantially different from the natural 

chemistry of the receiving water body. Mine dumps 

and tailings ponds are additional sources of potential 

concern. Surface-water quality could be affected by 

detention ponds used at uranium mines and tailings 

ponds at mills, but if operated properly, adverse effects 

are not likely (Lowry and others, 1993).

Table 11. Estimated annual applications of the 12 most 

extensively used herbicides in the Yellowstone River Basin 

(Data from Battaglin and Goolsby, 1994)

Pesticide

Estimated application of active 

ingredient, in kilograms

2,4-D 308,184

dicamba 127,223

picloram 76,611

cycloate 64,885

EPTC 60,017

triallate 57,883

MCPA 17,634

2,4-DB 14,526

diethatyl-ethyla

a

Discontinued by manufacturer in 1993.

10,189

alachlor 9,566

ethofumesate 7,152 

cyanazine 6,156
64 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Metal deposits are known in some areas of the 

structural basins and at locations related to Cretaceous 

and Tertiary intrusions, but the Precambrian crystalline 

rocks have been the primary targets of prospectors and 

geologists seeking metallic ores. Early exploration for 

precious metals in the YRB led to the subsequent discovery of gold placer deposits in the vicinity of South 

Pass, Wyo. (fig. 9), in 1842, and in several tributaries 

of the upper Yellowstone River in the 1860s. However, 

lode deposits were not substantially developed until the 

1860s near South Pass (Hausel, 1991) and in the 1870s 

at the Jardine and New World districts (fig. 9) of Montana (Hammarstrom and others, 1993a). Nickel, copper, and chromite deposits in the Precambrian 

Stillwater Complex (fig. 9) of Montana were discovered during 1883-90 (Hammarstrom and others, 

1993a).

Historically, mines in the study unit have produced gold, silver, arsenic, tungsten, copper, lead, zinc, 

and chromium from lode deposits, as well as gold from 

placer deposits (Hammarstrom and others, 1993). But 

within the past 55 years, iron, chromium, uranium, and 

platinum have been the most important metallic mineral products of the YRB. The Atlantic City surface 

mine produced more than 80 million megagrams of 

iron ore from 1962 until operations ceased in 1983 

(Hausel, 1984). (Although the Atlantic City mine pit 

lies just outside the study unit, part of the mine dump 

extends into the YRB [Hausel, 1991, plate 1], and is a 

potential influence on water quality in the study unit.) 

Some chromite was mined from the Stillwater Complex during and after World War II to provide government stockpiles of critical minerals (Hammarstrom and 

others, 1993), but large-scale chromite mining 

occurred during 1953-61 (Hammarstrom and others, 

1993a). From the discovery of uranium in Tertiary sedimentary rocks in the early 1950s (Houston, 1979) until 

the rapid decline of the industry by the mid-1980s 

(Lowry and others, 1993), millions of megagrams of 

uranium ore were mined in the study unit. Development of the platinum-group elements (PGE) deposits of 

the Stillwater Complex began in the 1960s (Hammarstrom and others, 1993a). 

In recent years, only four commercial mines 

have produced metallic ores in the study unit: (1) the 

Stillwater mine is producing PGE, gold, copper, and 

nickel from the Stillwater Complex (fig. 9); (2) the 

Mineral Hill mine had been producing gold and silver 

from lode deposits in the Jardine district (Hammarstrom and others, 1993) (fig. 9); (3) the Christiansen 

Ranch operation, an in-situ uranium producer located 

about midway between Casper and Gillette, is active 

(Harris, 1997); and, (4) a few kilometers north, the Irrigary uranium mine has become inactive recently 

(Anderson and others, 1993; R.E. Harris, Wyoming 

State Geological Survey, oral commun., 1998). The 

underground Stillwater mine recently expanded operations in order to increase the production goal to 

1,800 megagrams per day. Closure of the Mineral Hill 

mine was recently announced (U.S. Geological Survey, 

1997a). In the New World district, a proposed new 

project (underground mining of gold, silver, and copper) was on hold as Federal officials and environmental 

groups negotiated an exchange of private land holdings 

and unpatented mineral rights for surplus Federal property (U.S. Geological Survey, 1997a). In the Gas Hills, 

plans have been announced to construct a uranium 

mine and recovery plant (U.S. Geological Survey, 

1997b).

Hydrocarbon Production--Located within the study 

unit are three of the largest oil fields of the Rocky 

Mountain region--the Salt Creek field in the Powder 

River Basin, and the Elk and Oregon Basins in the 

Bighorn Basin (fig. 20). Each field contained more 

than 500 million barrels of oil initially (Spencer, 1996). 

Two other YRB oil fields also rank among the top 120 

nationally in annual production--Hartzog Draw in the 

Powder River Basin and Pennel in the Williston Basin 

(U.S. Department of Energy, Energy Information 

Administration, 1996). One of the Nation’s largest natural gas fields lies within the study unit--the Madden 

field in the Wind River Basin. In addition to the well 

fields, two large oil refineries at Billings, Mont., and a 

third located 27 km west, process almost 50 million 

barrels of crude oil annually (Montana Oil and Gas 

Conservation Division, 1996).

The importance of oil fields for understanding 

water-quality conditions and aquatic biota lies in 

potential contamination by both hydrocarbons and 

associated brines. Large volumes of saline water typically are produced in conjunction with oil extraction 

and commonly are discharged to nearby streams. Also, 

evaporation pits located in oil fields sometimes contain 

a surface layer of floating oil, which has been blamed 

for the deaths of numerous migratory waterfowl each 

year (Marsden, 1997). Pipeline breaks and accidental 

discharges are other events that can cause water-quality 

impairment.

66 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Commercial oil production began in 1884 with 

the Dallas oil field in the Wind River Basin, and in 

1889 at what later became the Salt Creek field 

(DeBruin, 1993). From 1906 through 1928, about 10 

major fields within the YRB were discovered on anticlines along the margins of the Bighorn, Powder River, 

and Wind River Basins. Recent production estimates 

for several of the largest oil fields within each of the 

four major structural basins in the study unit are listed 

in table 12. 

Hydrocarbon production in the Bighorn, Powder 

River, and Wind River Basins is predominantly from 

Cretaceous reservoirs and often is associated with 

structural features. About 90 billion m3

 of natural gas 

has been produced from Rocky Mountain foreland 

structures in those three basins, with 97 percent of 

hydrocarbon production in the Bighorn Basin having 

come from reservoirs in foreland structures (Mullen 

and Barlow & Haun, 1993). In the Bighorn Basin, 

most hydrocarbon production has come from anticlines 

around the basin margins, predominantly in association 

with oil reservoirs (Broadhead and Robertson, 1993). 

In contrast, the most productive reservoirs in the Powder River Basin have substantial stratigraphic components (Mullen and Barlow & Haun, 1993). Gas 

production in the Wind River Basin is predominantly 

from structural traps on basin margins and from stratigraphic and structural/stratigraphic traps near the basin 

center, from gas reservoirs not substantially associated 

with oil (Broadhead and Robertson, 1993). Nearly half 

of the 34 billion m3

 of structurally trapped gas produced in the Wind River Basin has come from the Beaver Creek field (fig. 20) near Riverton (Mullen and 

Barlow & Haun, 1993). 

Foreland structures were primary targets for oil 

exploration through the 1950s, but early drilling in the 

Powder River Basin occurred near seeps that later were 

recognized as originating at stratigraphic traps (Mullen 

and Barlow & Haun, 1993). The large, shallow structural traps of the Bighorn Basin discovered prior to 

1920 have produced more than half of its gas and threefourths of the 2.2 billion barrels of oil produced 

through 1990 (Mullen and Barlow & Haun, 1993). In 

the Wind River Basin, many structural traps are related 

to basement block-fault adjustments and may have no 

surface expression (Mullen and Barlow & Haun, 

1993). 

The Frontier Formation of the Upper Cretaceous 

Series is the major hydrocarbon reservoir in the study 

unit. The nearshore marine sandstone of the Frontier 

Formation is particularly productive at the Salt Creek 

and Teapot fields north of Casper, Wyoming. From this 

114 km2

 area, about 375 million barrels of oil and 

21 billion m3 of associated gas had been produced 

through 1990 using more than 2,000 wells (Doelger 

and others, 1993).

Contamination of streams or aquifers due to 

coal-bed methane development represents another 

hydrocarbon-related environmental concern (U.S. 

Geological Survey, Energy Resource Surveys Program, 1997). Partial pressure of the gas must be 

reduced to release it from the coal; this is achieved by 

removing water from the coal bed. The produced water 

is either discharged to the surface or injected underground. The salinity of the produced water is often 

greater than that of the receiving water body (U.S. Geological Survey, Energy Resource Surveys Program, 

1997).

Coal Mining--In 1994, the United States produced 

909 million megagrams and consumed 844 million 

megagrams of coal, with 88 percent used by electric 

utilities (U.S. Dept. of Energy, Energy Information 

Administration, 1995). About 30 percent (270 million 

megagrams) of the domestic coal production comes 

from beds in the Paleocene Fort Union Formation, 

which contains vast amounts of strippable coal (Flores, 

1996), much of it located within the YRB. Production 

from the thick, sub-bituminous low-sulfur coal beds in 

the Powder River Basin is increasing rapidly in 

response to the demand for low-sulfur steam coal by 

electric utility consumers, primarily outside the YRB 

(U.S. Geological Survey, 1996b). 

Coal was produced commercially in the YRB in 

the early 1880s near Red Lodge, Mont., in the 1890s in 

the Powder River Basin, and at several other locations 

by 1915-20 when early production peaked. Largescale surface mining began in the 1920s from the Rosebud coal bed (fig. 21), but most of the YRB coal production prior to 1940 continued to be from underground mines (U.S. Department of Energy, Energy 

Information Administration, 1994). Another peak in 

coal production coincided with the war years of 1942-

45, but most of the mines were closed prior to 1960 

(Slagle and others, 1983; 1986). The present period of 

increased coal production began in 1962 with the opening of the Big Horn Mine (fig. 21) north of Sheridan, 

Wyo. (Slagle and others, 1983). 
ENVIRONMENTAL SETTING 67

Table 12. Principal oil fields in the Yellowstone River Basin (Data from Wyoming Oil and Gas Conservation Commission, 

1997; and Montana Oil and Gas Conservation Division, 1996) 

[Montana data are for 1995; Wyoming data are for 1996]

Structural basin Field Year discovered

Annual oil 

production, 1995-96, 

in million barrels

Cumulative oil 

production through 

1995-96, in million 

barrels

Bighorn Elk Basin 1915 2.4 536

do. Oregon Basin 1912 4.5 437

do. Hamilton Dome 1918 1.8 248

do. Grass Creek 1914 1.5 200

do. Garland 1906 2.0 181

do. Little Buffalo Basin 1914 .9 130

do. Byron 1918 .7 126

do. Frannie 1928 .3 116

do. Cottonwood 1953 1.6 61

Powder River Salt Creek 1889 2.4 653

do. Bell Creek 1967 .1 132

do. Hartzog Draw 1976 2.2 94

do. Sussex-Sussex West 1948 .3 90

do. Teapot 1922 .6 26

Williston Pine 1952 .9 113

do. Cabin Creek 1953 1.5 100

do. Pennel 1955 2.3 91

do. Lookout Butte 1961 1.0 29

Wind River Winkleman 1917 .3 91

do. Steamboat Butte 1943 .3 89

do. Big Sand Draw 1918 .1 57

do. Beaver Creek 1938 .3 57

do. Circle Ridge 1923 .7 32

Since 1975 coal production from the study unit 

has increased dramatically, particularly in the Powder 

River Basin (U.S. Geological Survey, 1996b). The 

approximately ten-fold increase in production is due 

both to rising demand for fuel for electric power generation and to the unsuitability of coal from other regions 

for attaining environmental-quality goals. More than 

25 percent of the Nation’s present coal production is 

from 25 mines developing the Wyodak-Anderson, 

Anderson-Dietz, and Rosebud coal beds or zones in the 

Powder River Basin (Flores, 1996). Some individual 

mines in the Powder River Basin produce more coal 

than some of the major coal-producing States to the 

east (Milici, 1996). All of the active coal mines in the 

study unit are surface (strip) mines. Despite tremendous current production, most fields in this coal region 

remain in early stages of development because transportation networks and markets remain less than fully 

developed (Milici, 1996; U.S. Geological Survey, 

1996b). 

More than 10 large surface coal mines are active, 

or temporarily idled, in the YRB: six of these strip coal 

from the Wyodak-Anderson bed in the mining area 

near Gillette, Wyo. (fig. 21), and another four 
68 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 69

mine the Anderson-Dietz bed near or upstream of 

Tongue River Reservoir (Jones, 1990; Lowry and others, 1986; Slagle and others, 1983); four mines (fig. 21; 

not all active) developed in the Rosebud bed are located 

40-50 km south of the Yellowstone River in Montana 

(Bergantino and others, 1980). The Wyodak-Anderson 

coal bed has an average thickness of 15-30 m with less 

than 15 m of overburden, and is the most important of 

several minable coal beds within the Tongue River 

Member of the Paleocene Fort Union Formation (Glass 

and Jones, 1991). The Anderson-Dietz coal bed is 

about 14 m thick north of Sheridan, Wyo. (Glass and 

Jones, 1991). The Rosebud coal bed is up to about 9 m 

thick with 6-15 m of overburden (U.S. Department of 

Energy, Energy Information Administration, 1994). 

Although not yet extensively developed, a stratigraphically lower coal bed, the Knobloch, is about 23 m 

thick in a core area along the Tongue River valley about 

50 km northeast from Tongue River Reservoir (Sholes 

and Daniel, 1992). At least one mine is actively stripmining lignite beds about 45 km northeast of Glendive, 

Mont., but production is essentially negligible (U.S. 

Department of Energy, Energy Information Administration, 1994).

Surface mining may increase the potential for 

water-quality problems. Surface mining alters the configuration of the land surface and subsurface strata, and 

long-term detrimental effects may result if the area is not 

properly reclaimed. Mining operations, which include 

vegetation removal, excavation, and production of large 

volumes of unconsolidated spoil material, increase the 

potential for erosion and sedimentation. Channel filling 

by sedimentation can decrease the water-carrying 

capacity of the stream and may lead to increased flooding. The habitat of aquatic organisms may be altered 

substantially through burial and decreased dissolvedoxygen concentration of the water, owing to decreased 

depth and increased temperature. Dissolution of minerals contained in sediment derived from recently excavated overburden material may result in increased 

dissolved-solids and trace-element concentrations and 

decreased pH values.

Acid mine drainage from coal beds is virtually 

unknown in the Northern Great Plains and Rocky 

Mountain Provinces. Coal in these provinces has fewer 

sulfide minerals, and the semiarid climate results in less 

water to transport the acid, should it form. In addition, 

the natural alkalinity of water and soil in the Western 

United States generally neutralizes acid, should it form.

Erosion and sediment deposition, potential issues 

in any watershed affected by mining, generally are controlled at all active mine sites as required by State and 

Federal surface-mining regulations. Erosion is controlled by proper grading and revegetation of areas 

affected by mining, and sediment runoff is controlled 

through the use of settling ponds. However, channel 

scour can modify aquatic habitat downstream from settling ponds if the discharged water has suspendedsediment concentrations that are substantially smaller 

than the natural, or premining concentrations (Britton 

and others, 1989).

The potential effects of mining on ground water 

have caused some concern. Regionally, however, there 

probably will be no measurable effect on the quantity of 

flow because flow is dominantly local, rather than 

regional, in the upper several hundred feet of the coalbearing rocks. The possible effects on quality of water 

have not been fully assessed (Lowry and others, 1986), 

although results from geochemical modeling in the 

Powder River Basin indicate potential for improved 

postmining water quality (Martin and others, 1988).

“Ground-water levels may be affected by 

surface coal mining. Mines located above 

water-yielding zones have little, if any, effect 

on water levels. Ground-water levels can 

decline in and near surface-mined areas 

where excavation intersects a water-yielding 

zone. Water-level declines may cause a 

decrease or loss of production or flow in wells 

and springs. These effects generally will be 

temporary and occur only during and for a limited time after active mining. The areal extent 

of mining effects on water levels is largely 

dependent on the geologic and hydrologic 

setting of the mine and the duration of mine 

dewatering. Coal beds are characterized by 

fracture systems that provide limited paths for 

the movement of water. Consequently, each 

sand lens or fracture system not contiguous 

with another can be considered to be an individual and relatively isolated aquifer. 

Recharge available to this individual aquifer is 

limited to leakage through the surrounding 

confining layers; thus, the areal extent of 

water-level changes resulting from mining can 

be relatively local. In most instances, upon 

completion of mining, water levels will rise 

until premining equilibrium conditions are 

approximated” (Slagle and others, 1984, 

p. 22).
70 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

The quality of ground water in the vicinity of surface mines may be affected by the replacement of overburden material after the coal is removed. Replacement 

of overburden results in the exposure of fresh mineral 

surfaces and provides the opportunity for renewed 

chemical reactions. The actions of sulfate-reducing bacteria can decrease sulfate concentrations. Chemical 

analyses of spoil-derived water from the Powder River 

Basin (Rahn, 1975; Van Voast and others, 1978) have 

indicated that the median dissolved-solids concentration 

of water in spoils can be 160 to 173 percent of that in 

stock and domestic wells. Computer modeling designed 

to assess potential increases in dissolved solids in 

streams as a result of leaching of spoil materials (Woods, 

1981) indicates that large increases in dissolved-solids 

concentration are local and dilution occurs downstream. 

Simulation of a hypothetical plan to simultaneously 

mine all Federally owned coal potentially available for 

mining in the Montana part of the Tongue River Basin 

resulted in a maximum increase of 4.7 percent in the 

average annual dissolved-solids concentration of the 

Tongue River at Miles City. 

Cooperative and individual studies of effects of 

existing mines by the Montana Bureau of Mines and 

Geology and U.S. Geological Survey (1978) have 

shown that: 

• Ground-water inflow to mine pits generally has been 

small. 

• Mine effluents have not created serious water-quality 

problems.

• Local water-level declines can be substantial during 

mining.

• Water levels generally will recover toward premining 

positions after mining ceases.

• Mine spoils generally transmit water as well as or 

better than the natural aquifers.

• The problem of mineralization of water is small 

regionally.

• Deeper aquifers are available to replace water 

supplies that are permanently lost.

“The largest impact on the hydrology 

probably will be caused by the increases in 

population and land use, rather than actual 

mining activities” (Lowry and others, 1986, 

p. 6).

Water Use

An awareness of how water is used in the YRB is 

necessary to understand hydrologic conditions. In general, water use refers to a set of interactions between 

human activities and the hydrologic environment. 

Within the YRB, agriculture, mining, thermoelectric 

power generation, other industries, municipal and 

domestic supply are the chief purposes for which water 

is withdrawn, delivered, and consumed or returned.

Withdrawals are quantities of water removed 

from ground- or surface-water sources and are considered to be self supplied. Delivery is water withdrawn 

from a ground- or surface-water source by a municipal 

water supply and conveyed to customers. Consumptive 

use is water evaporated, transpired, combined into products or crops, or consumed by humans or livestock and 

not immediately available for reuse. Water that reaches 

a ground- or surface- water source after leaving the point 

of use is considered a return (Solley and others, 1993). 

Return flows commonly affect the temperature and 

suspended-solids, nutrient, and major-ion concentrations of the receiving water body.

About 98 percent of all water used in the study 

unit in 1990 was surface water (table 13). Most of this 

water—about 99 percent—was used for agricultural 

crop and livestock production. However, ground water 

is an important resource in the study unit. About 97 percent of the domestic supply and about 27 percent of the 

public supply is from ground water. About 61 percent of 

the ground-water use in 1990 was for agricultural purposes. Mining, power generation, and industry made up 

most of the remaining uses of surface and ground water 

in 1990 (fig. 22).

Recreational uses, although important, are difficult to quantify. Many streams and lakes are heavily 

used for recreation, and the demand is increasing—

affecting the water quality and aquatic organisms. The 

cold-water fisheries program stocked several million 

trout. Most of these were placed into high-quality coldwater streams.

There are no storage reservoirs on the mainstem 

of the Yellowstone River, though a proposal to build one 

at the narrows, just south of Livingston, was defeated in 

the 1970s (Chapple, 1997). However, hundreds of small 

impoundments for water supply, recreation, power, and 

flood control have been constructed within the study 

unit, in addition to several large dams on the major tributaries to the Yellowstone River. The six largest storage 

reservoirs in the study unit are listed in table 14.
ENVIRONMENTAL SETTING 71

Reservoirs often have a dramatic effect on water 

quality and resident biota. The typically nutrient-rich, 

stilled water allows aquatic vegetation to flourish, 

which can greatly diminish dissolved-oxygen concentrations through respiration and decomposition (Cole, 

1983). Reservoir construction causes a loss of riparian 

habitat through inundation, and alters the aquatic and 

riparian habitats downstream (Knight, 1994). Reservoirs can be very effective sediment traps; flowing 

water loses its transport capacity as it enters a calm 

water body and deposits its sediment load. In addition, 

reservoir water levels typically fluctuate considerably—

high in the late spring to store snowmelt runoff for flood 

control and for the onset of the agricultural irrigation 

season, but low by the end of the growing season. Fluctuating water levels create nearly barren shores around 

such reservoirs when the water level is low, because 

most riparian plants cannot tolerate the drastic fluctuations of water levels (Knight, 1994).

Waste Disposal

Waste materials deposited into water, land, air, 

and underground disposal sites can become a source of 

surface- and ground-water contamination. Although 

substantial improvements have been made in recent 

years in their disposal, wastes have not always been 

properly handled. Hundreds of active or abandoned 

Table 13. Reported surface- and ground-water use by 

category, 1990, Yellowstone River Basin 

(Modified from Miller and Quinn, 1997)

[m3

/d, cubic meters per day; Mgal/d, million gallons per day]

Category

Surface-water use

Ground-water 

use

m3/d Mgal/d m3/d Mgal/d

Public supply 150,000 40 57,000 15

Domestic

 (self-supplied)

800 .21 24,000 6.3

Commercial 680 .18 950 .25

Industrial 61,000 16 5,700 1.5

Thermoelectric power120,000 33 0 0

Mining 34,000 9.0 140,000 37

Agriculture 26,000,000 6,900 350,000 93

a

Total

a

Values expressed using two significant figures.

26,000,000 7,000 570,000 150

waste sites exist in the YRB, including two sites from 

the USEPA’s Comprehensive Environmental Response, 

Compensation, and Liability Act (CERCLA) National 

Priorities List. 

The USEPA’s Toxics Release Inventory (TRI) is a 

publicly available database containing specific toxicchemical release and transfer information from industrial facilities (table 15). Facilities that have the equivalent of 10 or more full-time employees and meet the 

established thresholds for manufacturing, processing, or 

otherwise using listed chemicals must report their 

releases and transfers. The TRI does not account for 

toxic emissions from automobiles and many other nonindustrial sources. 

Disposal to Streams

Sewage and industrial wastes can cause problems 

associated with bacteria, nutrients, dissolved oxygen, 

suspended solids, organic compounds, and trace elements in streams. Many bacteria are pathogens. Nutrients, such as nitrogen and phosphorus, can stimulate 

growth of aquatic plants and contribute to algal blooms. 

An algal bloom is the rapid growth of one or more species of suspended (phytoplankton) or attached (periphyton) algae that leads to an accumulation of algal 

biomass. As the algae die, decomposition can consume 

more oxygen than the living algae produce; hence, the 

dissolved-oxygen concentration of the water can 

decrease below the level needed for fish to survive. 

Suspended solids decrease light penetration, can be 

harmful to fish in large concentrations, and, if deposited, decrease the water-carrying capacity of the stream 

and may lead to increased flooding. Organic compounds and trace elements are ubiquitous contaminants 

having the potential to bioaccumulate in living organisms.

 The Federal Water Pollution Control Act of 1972 

(commonly known as the Clean Water Act) requires 

wastewater dischargers to have a permit establishing 

pollution limits, and specifying monitoring and reporting requirements. National Pollutant Discharge Elimination System (NPDES) permits regulate household 

and industrial wastes that are collected in sewers and 

treated at municipal wastewater treatment plants. Permits also regulate industrial point sources and concentrated animal feeding operations that discharge into 

other wastewater collection systems, or that discharge 

directly into receiving water bodies. More than 200,000 

sources are regulated by NPDES permits nationwide. 
72 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 73

One-hundred thirty-two NPDES permitted facilities 

were listed in the YRB in 1997. Of these, 26 facilities 

are identified as major dischargers (12 municipalities, 

4 beet-sugar manufacturers, 4 mines, 3 petroleum 

refineries, 2 electric power-generating plants, and 

1 railroad yard).

The USEPA’s TRI database listed about 

78,000 kg of toxic releases to water in 1993 from manufacturing facilities in the YRB. About 65 percent of 

this total is attributable to petroleum refineries, with the 

remainder resulting from other manufacturing operations.

Table 14. Names and descriptions of major reservoirs in the 

Yellowstone River Basin

(Data from Brosz and Hasfurther, 1990; Harza Engineering and others, 

1983; Shields and others, 1997)

[km, kilometer; km3

, cubic kilometer]

Reservoir name 

(fig. 1)

Year 

completed

Storage capacity, 

in km3 (acre-feet)

Bull Lakea

a

Not shown in fig. 1; located 58 km west-northwest of Riverton,

Wyo.

1938 .189 (153,000)

Boysen Reservoir 1951 .989 (802,000)

Buffalo Bill Reservoirb

bNot shown in fig. 1; located 11 km west of Cody, Wyo.

c

1909

c

Dam height increased by 7.6 meters, effective 1992; previous storage capacity was 0.746 km3 (424,000 acre-feet)

.857 (695,300)

Bighorn Lake 1967 1.695 (1,375,000)

Tongue River Reservoir 1939 .084 (68,040)

Lake De Smetd

dNot shown in fig. 1; located 39 km south-southeast of Sheridan,

Wyo.

e

1921

e

Dams enlarging the storage capacity of this natural lake were completed in 1921, 1971, and 1978.

.290 (235,000)

Table 15. Toxic Release Inventory emissions in the 

Yellowstone River Basin, 1993 

(Data from U.S. Environmental Protection Agency, 1995)

[All values in thousands of kilograms]

Industry type Water Land Air

Underground Total

Petroleum refineries 51 0.8 266 0 317

Chemical manufacturing 0 0 20 0 20

Mining and extraction 0 0 0.2 0 0.2

Other manufacturing 27 12.6 245 0 285

Totala

a

Values expressed using three significant figures.

78 13.4 531 0 622

Disposal on Land

Wastes are disposed on land within the YRB by 

several methods including septic systems, landfills, 

surface impoundments, and land application of treated 

substances. These methods generally have a more 

immediate potential to contaminate shallow groundwater resources than to affect stream water.

Septic systems contribute filtered sewage effluent 

directly into the ground. The septic effluent contains 

bacteria, nutrients, organic carbon, chlorides, and contaminants from household wastes. Septic systems are 

designed to use the soil to filter bacteria and attenuate as 

many of the other constituents as possible. However, 

where soil is thin, or where permeable rocks underlie 

the septic system, ground-water contamination is likely. 

A cesspool pit can pose a serious contamination threat 

to the nearby well-water supply (Risser and Siwiec, 

1996). As ranch land is converted to rural residential 

use there is a greater risk of contamination of the water 

supply.

 The USEPA’s TRI database listed about 

13,400 kg of toxic releases to land in 1993 from industrial facilities in the YRB. It is estimated that 90 percent 

of the industrial wastes that are considered to be hazardous are landfilled, primarily because it is the least 

expensive waste management option (Freeze and 

Cherry, 1979). The USEPA’s Office of Solid Waste 

listed 41 active municipal solid-waste landfills in the 

YRB in 1995 (U.S. Environmental Protection Agency, 

1996b). At a landfill, solid waste is reduced in volume 

by compaction and then is covered with earth. In North 

America a large number of the older sites that receive 

municipal wastes are open dumps or poorly operated 

landfills. Newer sites are generally better situated and 

better operated. Nevertheless, contamination of surface 

and ground water from landfills is possible from storm 

runoff and infiltration of leachate. Not only do the 

leachates contain contaminants derived from the solids, 

but many leachates contain toxic constituents from liquid industrial wastes placed in the landfill. 

Surface impoundments are used primarily to 

store wastes. Surface- and ground-water contamination 

can result if the impoundment leaks. Nutrient contamination—nitrates in particular—of ground water is a 

concern arising from the storage of animal manure.

Land application is widely used to dispose of stabilized municipal sewage sludge, septic-tank wastes, 

sludge from drinking-water treatment plants, and composted leaves. Treated wastes are commonly applied 
74 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

directly to the land surface in the study unit. This disposal method employs natural properties of soil and 

biota to remove bacteria and excess nutrients in the 

waste (Metcalf and Eddy, Inc., 1972). Care, however, 

must be exercised to assure that the ability of the soil to 

assimilate wastes is not overloaded; otherwise, groundwater contamination will result.

Disposal in Air

In the Western United States, available data indicate that precipitation at individual sites has been acidified by anthropogenic emissions. The acidification 

generally has been attributable to localized sources and 

the time of initial acidification is undefined (Turk, 

1983). 

The USEPA’s TRI database listed 531,250 kg of 

toxic releases to the air from industrial facilities in the 

YRB in 1993. About 50 percent of this total is attributable to petroleum refineries and another 46 percent 

resulted from other manufacturing operations. These 

amounts do not include small generators and non-point 

sources such as automobiles and agricultural-chemical 

applicators. Total emissions would likely be much 

greater if these other sources were included.

Precipitation cleans the atmosphere of airborne 

toxic vapors and particles and deposits them onto the 

earth’s surface, including lakes, rivers, and streams. In 

addition, dry atmospheric deposition in the form of 

vapors and particulate matter also returns airborne compounds to the earth’s surface. Toxic compounds in both 

precipitation and dry deposition reach surface water as 

direct deposition and in surface runoff, and can reach 

ground water by infiltration through the soil (Majewski, 

1995).

Disposal Underground

Injection of liquid wastes, mainly of industrial 

origin, has been widely adopted as a waste disposal 

practice in North America (Freeze and Cherry, 1979). 

The purpose of this procedure is to isolate hazardous 

substances from the biosphere. As the discharge of pollutants to rivers and lakes has become increasingly 

objectionable, and as legislation for protection of surface water resources has become more stringent, the use 

of deep permeable zones for liquid waste disposal has 

become an increasingly attractive waste management 

option for many industries. Chemical, petrochemical, 

and pharmaceutical companies are the largest users of 

waste-injection wells. Other important users are petroleum refineries, gas plants, steel mills, potash mines, 

uranium mills, and processing plants (Freeze and 

Cherry, 1979). Large septic systems used for disposal 

of sanitary waste in establishments serving more than 

20 people per day, or any septic system used for industrial wastewater, qualify as “underground injection 

wells” regulated by USEPA’s Underground Injection 

Control program. The Wyoming Department of Environmental Quality lists several hundred sites of underground injection in the YRB (Bob Lucht, Wyoming 

Department of Environmental Quality, oral commun., 

1998). 

Integrated Environmental Settings

Several sources of information are available that 

show the geographic distribution of environmental settings at national or continental scales as analyzed by 

previous investigators. Such settings integrate many 

interrelated characteristics, such as climatic, physiographic, geologic, biotic, and land-use features. One of 

the benefits of developing a regional framework of 

environmental settings is its use in selecting regional 

reference sites (Gallant and others, 1989). Such sites 

are used within the NAWQA Program to represent 

unimpaired or relatively unimpaired water-quality conditions for a specific environmental setting. Waterquality data collected from impaired sites can then be 

compared with those from environmentally similar reference sites.

Ecoregions

Ecosystems of regional extent, also known as 

ecoregions, have been defined and mapped by a number 

of investigators. Two of the more widely used classification systems for ecoregions of the U.S. are those 

developed by R.G. Bailey and J.M. Omernik.

Bailey’s ecoregions (1:7,500,000 scale) (Bailey 

and others, 1994) are based on associations of environmental factors that directly affect or indirectly express 

energy, moisture, and nutrient gradients that control the 

structure and function of ecosystems; these factors 

include climate, landforms, vegetation, and soil (Bailey, 

1996). In Bailey’s hierarchical system, developed to 

support ecosystem management on land administered 

by the U.S. Forest Service, the YRB lies in the dry climate domain, a subcontinental region of broad climatic 
ENVIRONMENTAL SETTING 75

similarity. Within the dry domain, several divisions are 

defined by vegetation differences corresponding to the 

levels of water deficit and winter temperatures (Bailey, 

1995). Two of these divisions occur in the YRB. 

Eighty-two percent of the study unit is classified as temperate steppes, areas having shortgrass prairie or semidesert vegetation that is generally sparse, leaving much 

exposed soil (Bailey, 1995). Bailey maps the remainder 

of the YRB as temperate deserts, where vegetation is 

typically semidesert shrubs such as sagebrush. 

Bailey’s delineation places 53 percent of the 

study unit in the Great Plains Dry Steppe Province, 

characterized by grasslands with scattered trees and 

shrubs. Another 29 percent of the YRB lies in the 

Southern and Middle Rocky Mountain Steppe Provinces where local vegetation zones are controlled by a 

combination of altitude, latitude, prevailing winds, and 

slope exposure (Bailey, 1995). The Intermountain 

Semidesert Province occupies about 18 percent of the 

study unit and features sagebrush mixed with short 

grasses. On moist, alkaline soils, greasewood is an 

abundant shrub.

At the same scale of 1:7,500,000, Omernik 

(1987) mapped the principal ecoregions of the U.S. 

based on integrated patterns of a combination of factors 

including land use, morphology, potential natural vegetation, and soil. Omernik’s map was developed specifically in support of water-resources management and is 

based on the premise that regional patterns of these 

environmental factors would be reflected in waterquality patterns (Gallant and others, 1989). Planned 

applications for the ecoregion maps include development of regional biological criteria and water-quality 

standards, and establishment of management goals for 

nonpoint-source water pollution (U.S. Environmental 

Protection Agency, 1996a).

The authors have attempted to refine the current 

version of Omernik’s ecoregions (U.S. Environmental 

Protection Agency, 1996a) within the YRB to more 

closely follow changes in land cover and geology seen 

on larger-scale sources of map data. Additional rationale for modification to the national-scale ecoregion 

map is found in the results from an adjacent NAWQA 

study unit, where patterns in fish-species distributions 

and assemblages did not correspond well with those 

ecoregion boundaries (Maret and others, 1997). On our 

modified map of Omernik’s ecoregions (fig. 23), 55 percent of the study unit lies within the Northwestern Great 

Plains. This ecoregion has plains with open hills of 

varying height and tablelands of moderate relief; and 

predominant land cover is subhumid grasses used for 

grazing (Omernik, 1987). Twenty-one percent of the 

YRB lies in each of two other ecoregions, the Middle 

Rocky Mountains and the Wyoming Basin. The Middle 

Rocky Mountains ecoregion features high mountains 

covered by Douglas fir, western spruce-fir forests, and 

alpine meadows (Omernik, 1987); land use includes 

grazing and silviculture. The Wyoming Basin has 

plains with hills or low mountains, some irrigated agriculture, and potential natural vegetation is shrub steppe, 

desert shrubland, and juniper-pinyon woodland (Omernik, 1987). The remainder of the YRB lies in the Montana Valley and Foothill Prairies ecoregion, 

characterized as subhumid grassland used for grazing, 

and some irrigated land (Omernik, 1987).

Environmental Stratification

Natural and anthropogenic factors, such as geology and land use, provide a framework for making comparative assessments of water quality within and among 

hydrologic systems at a range of scales and in different 

parts of the Nation (Gilliom and others, 1995). Characterizing this environmental framework is an important 

element in each study-unit investigation of the 

NAWQA Program. The environmental setting of a 

study unit is characterized by dividing it into several 

subareas (not necessarily contiguous) that have relatively homogeneous combinations of those natural and 

anthropogenic factors believed to be relevant to water 

quality (Gilliom and others, 1995). In terms of a design 

for a scientific study, this process is called stratification.

Six environmental data layers were included in a 

digital map overlay analysis to identify relatively homogeneous subareas for use in designing the YRB studyunit investigation. Three of the data layers have complete geographic coverage for the YRB: ecoregions, 

geologic units, and land cover or land use. The other 

three data layers are composed of localized areas where 

mineral deposits and (or) extraction activities occur or 

have a substantial likelihood of occurring: coal lease 

areas, metallic mineral deposits, and oil fields.

A three-tiered approach was used to stratify the 

study unit on the basis of environmental settings. 

Ecoregions represent an integration of multiple environmental controls and were distinguished by Omernik 

(1987) partly based on expected influence on water 

quality. Therefore, the four ecoregions (fig. 23) were 

selected as the first tier for the environmental stratification process. 
76 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 77

Geologic groups were selected as the second tier. 

The areas where mineral deposits and (or) extraction 

activities occur or may be likely were combined to 

form one geologic group, called mineral resource 

areas. The remainder of the study unit was assigned to 

geologic groups on the basis of generalized geologic 

units (fig. 6). Igneous rocks were grouped into Precambrian crystalline rocks and Cretaceous through 

Quaternary volcanic and intrusive rocks. The crystalline rocks crop out mainly at the core of three of the 

mountain uplifts in the YRB and typically produce 

water-quality characteristics distinct from either volcanic or sedimentary rocks (see Water-Quality Characteristics section of report). Sedimentary rocks were 

grouped into a “Paleozoics” group that also includes 

Mesozoic strata (Jurassic and Triassic rocks), a Cretaceous group, and a Tertiary group. Paleozoic units 

include numerous carbonate strata that exert a substantial influence on both water chemistry and hydrology. 

Water in Cretaceous aquifers typically contains considerably larger concentrations of dissolved solids than do 

many Tertiary aquifers. The areas of Quaternary 

unconsolidated deposits were assigned to the nearest 

neighboring geologic unit, and grouped accordingly.

For the third tier, land cover and land use, the categories used are those designated as Level I categories 

in the Anderson (1976) classification system. These 

major groupings of land cover and land use are associated with different, though generalized, expected 

impacts on water-quality characteristics.

The results of the digital map overlay analysis 

were a large number of types of relatively homogeneous areas—many more than could be considered as 

separate environmental settings for the study design. 

Therefore, selected types of relatively homogeneous 

areas were designated as environmental settings. 

Selections were made first based on NAWQA Program 

priorities, and then based on relative areal extent within 

the study unit. 

Specific environmental settings have been prioritized by the NAWQA Program for sampling by studyunit investigations, such as urban, agricultural, and 

mining areas. However, NAWQA is targeting large 

urban areas that have populations greater than 250,000 

(Lopes and Price, 1997). In the YRB, no urban areas 

are large enough to qualify as targets for study. Mineral 

resource areas were given priority. The environmental 

factor combinations of mineral resource areas that were 

most areally extensive were selected as environmental 

settings, whereas less extensive factor combinations 

were left unassigned to any setting. For example, 91 

percent of the metallic mineral deposits were located in 

five types of relatively homogeneous areas occurring in 

three ecoregions; consequently, those five types of relatively homogeneous areas were assigned to an environmental setting according to ecoregion, but the other 

9 percent of the metallic mineral deposits were left 

unassigned. Areas of coal and oil resources were prioritized similarly.

After the higher priority environmental-setting 

assignments had been made, areal extent was used to 

rank the remaining combinations of environmental factors. Generally, factor combinations that covered at 

least 1.5 percent of the study unit were assigned to 

environmental settings. The notable exception to this 

generalization is that alpine tundra areas were not 

assigned to any setting even though two combinations 

(combinations of tundra with the Precambrian and volcanic geologic groups) do cover about 1.5 percent of 

the study unit. Alpine tundra areas do not carry sufficient priority within the NAWQA Program to warrant 

specific consideration for study. 

A total of 18 environmental settings (fig. 24 A) 

were thus defined for the YRB. Relatively homogeneous areas from different ecoregions were not 

assigned to the same environmental setting, except in 

one case. Rangeland underlain by the Paleozoic geologic group in both the Middle Rocky Mountains and 

Wyoming Basin ecoregions were assigned to the same 

environmental setting because this geologic group 

mainly occurs in the transitional areas along the boundary between these two ecoregions.

In figure 24 B, colors applied to environmental 

settings are unique within each ecoregion, but the same 

color is sometimes used for similar environmental settings located in different ecoregions. About 11 percent 

of the study unit is composed of environmental-factor 

combinations that were not assigned to any environmental setting. If this initial environmental stratification of the study unit is adopted in the final study 

design, those combinations will not be targeted for specific study, though some of those areas will unavoidably be included within watersheds or aquifer areas 

that will be selected for study. The 18 environmental 

settings are considered initial candidates for specific 

study components, but resource constraints will not 

allow all 18 settings to be studied during the current 

phase of the YRB study-unit investigation. However, 

the spatial heterogeneity of selected study areas with 

respect to the included environmental factors will be 
78 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
ENVIRONMENTAL SETTING 79
80 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

readily quantified using the results from the stratification analysis. The selected factors were included 

because of their perceived influence on water quality 

or aquatic ecology, so differences in these conditions 

measured during the study-unit investigation are 

expected to correspond to differences in environmental setting.

SUMMARY

This report describes the natural and anthropogenic factors believed to influence water-quality conditions in the Yellowstone River Basin (YRB). Major 

water-quality issues in the YRB are sediment, trace elements, toxic compounds, salinity, and stream-habitat 

degradation. Land and water uses that relate to these 

issues include grazing, agriculture, mining, oil production, and residential expansion into rural areas.

The physiography parallels the structural geologic setting that is generally composed of several 

uplifts and structural basins. Contrasts in climate and 

vegetation reflect topographic controls and the midcontinental location of the study unit. Surface-water 

hydrology reflects water surpluses in mountainous 

areas dominated by snowmelt runoff, and water deficits 

in the remainder of the study unit. Principal groundwater aquifers are Tertiary sandstones and unconsolidated Quaternary deposits.

Human population, though sparsely distributed 

in general, is growing most rapidly in a few urban centers and resort areas, mostly in the northwestern part of 

the YRB. Land use is areally dominated by grazing in 

the basins, and economically dominated by mineralextraction activities. Forests are the dominant land 

cover in mountainous areas. Cropland is a major land 

use in principal stream valleys. Water use is dominated 

by agriculture overall, but mining and public-supply 

facilities are major users of ground water. Coal and 

hydrocarbon production and reserves distinguish the 

YRB as a principal energy-minerals resources region. 

Current metallic-ore production or reserves are nationally significant for platinum-group elements and chromium.

Other investigators integrated multiple environmental factors to subdivide the YRB into ecological 

regions that serve as a starting point for an initial environmental stratification of the study unit. Ecoregions, 

geologic groups, and general land-cover and land-use 

categories were used in combination to define 18 environmental settings in the YRB that collectively 

describe about 89 percent of the study unit. These 

18 settings are initial candidates for targeted study 

components that will be used to assess and compare 

water-quality conditions.

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_____[1976], Land use and land cover, 1972, White Sulphur 

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Land Use Series 76-634-1, scale 1:250,000. 

_____1979a, Land use and land cover, 1974-76, Gillette, 

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_____1979b, Land use and land cover, 1970-76, Hardin, 

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_____1979c, Land use and land cover, 1974, Miles City, 

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_____1979d, Land use and land cover, 1976, Newcastle, 

Wyoming; South Dakota; Nebraska: U.S. Geological 

Survey Land Use Series L-81, scale 1:250,000. 

_____1980a, Land use and land cover, 1972, Bozeman, 

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_____1980b, Land use and land cover, 1976, Ekalaka, Montana; North Dakota; South Dakota: U.S. Geological 

Survey Land Use Series L-137, scale 1:250,000. 

_____1980c, Land use and land cover, 1973, Glendive, 

Montana; North Dakota: U.S. Geological Survey Land 

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L-138, scale 1:250,000. 

_____[1983a], Land use and land cover, 1980, Arminto, 

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Series 

83-571-1, scale 1:250,000. 

_____[1983b], Land use and land cover, 1981, Casper, Wyoming: U.S. Geological Survey Open-File Land Use 

Series 

83-572-1, scale 1:250,000. 

_____[1983c], Land use and land cover, 1980-81, Sheridan, 

Wyoming; Montana: U.S. Geological Survey OpenFile Land Use Series 83-112-1, scale 1:250,000. 

_____[1984a], Land use and land cover, 1980, 1982, Ashton, 

Idaho; Mont.; Wyo.: U.S. Geological Survey Open-File 

Land Use Series 84-0324-1, scale 1:250,000. 

_____[1984b], Land use and land cover, 1976-1981, Billings, Montana: U.S. Geological Survey Open-File Land 

Use Series 84-0313-1, scale 1:250,000. 

_____[1984c], Land use and land cover, 1980, Cody, Wyoming: U.S. Geological Survey Open-File Land Use 

Series 

84-878-1, scale 1:250,000. 

_____[1984d], Land use and land cover, 1980, 1982, Driggs, 

Idaho; Wyoming: U.S. Geological Survey Open-File 

Land Use Series 84-0325-1, scale 1:250,000. 

_____[1984e], Land use and land cover, 1980, Forsyth, 

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Series 

84-039-1, scale 1:250,000. 

_____[1984f], Land use and land cover, 1982, Lander, Wyoming: U.S. Geological Survey Open-File Land Use 

Series 

84-0547-1, scale 1:250,000. 

_____[1984g], Land use and land cover, 1981-1983, Thermopolis, Wyoming: U.S. Geological Survey Open-File 

Land Use Series 84-0548-1, scale 1:250,000. 

_____[1985], Land use and land cover, 1982, 1984, 

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_____1986, Land use and land cover digital data from 

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38 p.

_____1996a, 1995 National assessment of United States oil 

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Series DDS-30, compact disc.
92 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

_____1996b, Assessing the coal resources of the United 

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online version <http://energy.usgs.gov/factsheets/nca/

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[10/1/97].

_____1997b, Minerals yearbook, volume II--Area reports--

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1996 Wyoming oil and gas statistics: Casper, Wyoming 

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GLOSSARY 93

GLOSSARY

Allochthonous refers to materials, such as organic sediments, that originate outside a given ecosystem and 

thus provide external inputs of food energy or nutrients.

Alluvium consists of clay, silt, sand, gravel, or other unconsolidated material deposited by a stream or other body 

of running water as a sediment in the bed of a stream or 

on a floodplain or delta, or as a fan at the base of a 

mountain.

Anticline is an arched fold in which the rock layers dip away 

from the axis of the fold. 

Aquifer is a body of rock that contains sufficient saturated, 

permeable material to yield substantial quantities of 

water to wells and springs. 

Armored streambed is one having a coarser assemblage of 

particle sizes in the surface layer of bed material as 

compared to the subsurface composition. This condition results when the sediment-transport capacity of the 

stream over a wide range of flows is sufficient to move 

only the finer particle sizes of bed material.

Badlands refers to areas having intricately dissected topography resulting from fluvial erosion and characterized 

by high drainage density and steep, mostly barren sideslopes with narrow interfluvial ridges. 

Bedrock is a general term for the consolidated (solid) rock 

that underlies soil or other unconsolidated surficial 

material. 

Board foot is a unit of lumber measurement equal to 0.305 

meter (1 foot) square by 25.4 millimeter (1 inch) thick.

Breccia refers to a coarse-grained clastic rock, composed of 

angular fragments bound by mineral cement or in a 

fine-grained matrix. The sharp edges and unworn 

corners of the fragments composing breccia differentiate it from conglomerate. 

Caldera is a large, basin-shaped volcanic depression having 

a diameter many times greater than that of the included 

vent or vents.

Clastic rocks are composed principally of broken rock fragments that are derived from pre-existing rocks or 

minerals and have been transported from their place of 

origin. The most common clastic rocks are sandstone 

and shale. 

Climax community refers to the final or stable biological 

community in a developmental series that is selfperpetuating and in equilibrium with the physical 

habitat.

Colluvium is heterogeneous, unconsolidated, incoherent 

soil or rock material. 

Confined aquifer is bounded above and below by impermeable beds or by beds of distinctly lower permeability 

than that of the aquifer itself. In addition, the water 

level in a well open to a confined aquifer rises above the 

top of the aquifer.

Cubic meter per second (m3

/s) is the rate of discharge 

representing a volume of 1 cubic meter passing a given 

point during 1 second and is equivalent to about 

264 gallons per second, or 35.31 cubic feet per second. 

Discharge is the volume of water (or generally, the volume 

of liquid plus suspended material) that passes a given 

point within a given period. Discharge also is called 

flow.

Disseminated refers to a mineral deposit in which the desirable mineral occurs as scattered particles in the rock, 

but in sufficient quantity to make the deposit an ore.

Dissolved refers to a substance present in true chemical solution. In practice, however, the term includes all forms 

of substances that will pass through a 0.45-micrometer 

membrane filter, and thus may include some very small 

(colloidal) suspended particles. 

Domestic water use is water for household purposes, such 

as drinking, preparing food, bathing, washing clothes 

and dishes, flushing toilets, and watering lawns and 

gardens. This water use is also called residential water 

use.

Drainage basin (drainage) is the total area drained by a 

stream and its tributaries. The area of a drainage basin 

is called drainage area and usually is determined planimetrically from topographic maps.

Ephemeral stream is a stream or reach of a stream that 

flows briefly only in direct response to precipitation in 

the immediate locality and whose channel is at all times 

above the water table. 

Fault is a fracture in bedrock along which movement of the 

bedrock has occurred. 

Flood plain is the lowland that borders a river, usually dry 

but subject to flooding when the stream overflows its 

banks.

Foreland structure is a geologic structural feature located 

along the edge of a stable continental platform and 

marginal to a belt of folded mountains.

Formation is a body of rock identified by unique physical 

characteristics and relative position.

Gaging station is a particular site on a stream, canal, lake, or 

reservoir where systematic observations of hydrologic 

data are obtained. 

Greenstone belt refers to elongate areas within Precambrian 

shields that are characterized by altered or metamorphosed basic igneous rocks and sedimentary rocks 

forming one or more metamorphosed volcanosedimentary piles.
94 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 

Hogback refers to a sharp-crested ridge formed by the 

outcropping edges of steeply inclined resistant rocks, 

and produced by differential erosion.

Infiltration is the flow of water into soil at land surface, as 

contrasted with percolation, which is movement of 

water through soil layers or other surficial material. 

Intermittent stream is a stream or reach of a stream that 

flows only certain times of the year when it receives 

water from springs or from some surface source such as 

melting snow.

Intrusive rocks are igneous rocks formed by emplacement 

of magma in pre-existing rocks.

Limestone is dense rock formed by chemical precipitation 

of calcium carbonate from solution in water.

Megagram is a unit of mass equivalent to 1,000 kilograms, 

one metric ton, or 1.102 short tons.

Metasedimentary rocks are sedimentary rocks that show 

evidence of having been subjected to metamorphism.

Milligrams per liter (mg/L) is a unit expressing the concentration of chemical constituents in solution as mass 

(milligrams) of solute per unit volume (liter) of water. 

Concentration of suspended sediment also is expressed 

in milligrams per liter and is based on the mass of sediment per liter of water-sediment mixture. 

Orographic refers to precipitation patterns that result when 

moist air encounters a topographic barrier and is forced 

to rise over it; for example, the increase in precipitation 

with elevation on the windward slopes of a mountain 

range and the rain-shadow of decreased precipitation on 

the leeward side of the range.

Particle size (grain size) is the diameter, in millimeters, of 

any given sediment particle. (See particle-size classification.) 

Particle-size classification used by the U.S. Geological 

Survey: 

Classification Particle-diameter range, in millimeters

Clay 0.00024-0.004

Silt .004-.062 

Sand .062-2.00 

Gravel 2.00-64.0 

Peak discharge (peak flow, flood peak) is the maximum 

instantaneous discharge during a specified time 

interval. The series of annual peak discharges at a 

gaging station is used to determine the recurrence 

interval (frequency) and exceedance probability of 

floods. 

Pediment refers to a broad, gently sloping, rock-floored 

erosional surface or plain of low relief, typically developed at the base of an abrupt, receding mountain front 

or plateau escarpment.

Perennial stream is a stream that flows continuously. 

Permeability is a measure of the relative ease with which a 

porous or fractured medium can transmit a liquid under 

a potential gradient (the capacity of a rock to transmit a 

fluid such as water or petroleum). 

pH is a measure of the acidity or alkalinity of water. It is 

defined as the negative logarithm of the hydrogen-ion 

concentration. This property is dimensionless and 

generally has a range from 0 to 14, with a pH of 7 representing neutral water. A pH of greater than 7 indicates 

the water is alkaline, whereas a pH value less than 7 

indicates an acidic water.

Placer deposit is a surficial mineral deposit, usually of a 

heavy mineral, formed by mechanical concentration of 

mineral particles from weathered debris.

Porosity is the property of a rock or soil that refers to the 

voids that the material contains. It may be expressed 

quantitatively as the ratio of the volume of voids to total 

volume of the material.

Porphyry refers to an igneous rock that contains conspicuous phenocrysts (relatively large crystals) in a finegrained groundmass.

Public-supply water use is water withdrawn by public and 

private water suppliers and delivered to users. Public 

suppliers provide water for a variety of uses, such as 

domestic, commercial, thermoelectric power, industrial, and public water use.

Recharge is the process by which water is absorbed and 

added to the saturated zone (aquifer), either directly 

into a body of rock or indirectly by way of an adjacent 

body of rock. Also, it is the quantity of water that is 

added to the saturated zone. 

Riparian vegetation refers to plants growing in near-stream 

locations where more moisture is available to plants 

than in adjacent upland areas, and where the dominant 

species of adjacent upland vegetation are absent or 

compose a minor part of the community.

Sandstone is the consolidated equivalent of sand. (See 

particle-size classification.)

Saturated zone is the subsurface zone in which all openings 

are full of water and are under hydrostatic pressure 

equal to or greater than atmospheric pressure. 

Sediment is unconsolidated solid material that originates 

mostly from disintegrated rocks and is transported by 

water or air. Also, it may include chemical and 

biochemical precipitates or decomposed organic material, such as humus. 

Shale is the consolidated equivalent of clay. (See particlesize classification.) 

Shear zone is a tabular zone of rock that has been crushed 

and brecciated by many parallel fractures due to shear 

strain. Shear zones often are mineralized by oreforming solutions.
GLOSSARY 95

Siltstone is the consolidated equivalent of silt. (See particlesize classification.) 

Silviculture (forestry) is the care and cultivation of forest 

trees.

Skarn refers to lime-bearing silicates derived from limestone and dolomite with the introduction of large 

amounts of Si, Al, Fe and Mg.

Stock is an igneous intrusion having less than 100 km2

 of 

surface exposure; resembles a batholith except in size.

Stratification refers to a statistical design for study or 

sampling in which a population is first divided into 

groups, or strata, having greater homogeneity than the 

whole population, according to relevant characteristics; 

then information is gathered about each group, often by 

selecting a sample from each.

Stratiform refers to geologic structure having the form of a 

layer, bed, or stratum; consisting of roughly parallel 

bands or sheets.

Streamflow is the discharge in a natural channel. Although 

the term “discharge” can be applied to a flow of a canal, 

the word “streamflow” is used only to describe the 

discharge in a stream. The term “streamflow” is more 

general than “runoff,” since streamflow may be applied 

to discharge whether or not it is affected by diversion or 

regulation. 

Superimposed refers to a stream or drainage system that has 

cut down from above by erosion, through the formations on which it developed, onto rocks of different 

structure lying beneath.

Surface water is an open body of water such as a stream or 

lake.

Terrace is a steplike landform above a stream and its floodplain, representing a former, abandoned floodplain of a 

stream. 

Unconsolidated refers to sediment grains that are loose, 

separate, or unattached to one another. 

Unsaturated zone is the zone between the land surface and 

the water table. It includes the capillary fringe. Generally, water in this zone is under less than atmospheric 

pressure, and some of the voids may contain air or other 

gases at atmospheric pressure. 

Volcaniclastic refers to a clastic rock containing volcanic 

material. 

Water table refers to the upper surface of the saturated zone 

where the water pressure is equal to atmospheric pressure.

Water year refers to the 12-month period from October 1 to 

September 30, and is designated by the calendar year in 

which it ends. Thus, the water year ending on 

September 30, 1995, is called water year 1995.

Yield is the constant or final pumping rate from a well in 

liters per minute at which a well-acceptance test is 

conducted.
96 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN 
T

ABLES97

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit AreRiver, Bighorn, and Powder River Basins

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali

Quaternary Sequence in 

table does not indicate position relative to other Quaternary entries

Alluvium and colluvium

“Unconsolidated clay, silt, sand, and gravel; includes terrace, flood-plain, and pediment deposits along major streams.”1, 2“Silt, sand, and gravel, unconsolidated; underlies flood plains and bordering terraces.”2“Clay, silt, sand, and gravel in flood plains, fans, terraces, and slopes.”3

Yield small to large supplies of water; large yields could be developed in some areas.1, 2 “The chemical quality 

alluvial deposits differs to place, depending on and amount of rechargebedrock underlying the Water in the flood-plain perennial streams is genin the upper reaches of but the quality deteriordownstream.” 1Water in alluvium near Mountains is better quawater in alluvium in cePowder River Basin.2

Quaternary Sequence in 

table does not indicate position relative to other Quaternary entries

Dune sand and loess

“Unconsolidated fine to very fine sand; present in eastern part of the Wind River Basin.”1

“Yields small supplies of water of suitable quality for stock or domestic use; it is an important source of water in areas underlain by the Cody Shale.”1Unknown in Bighorn and Powder River Basins.

Water is of good quality River Basin.1Unknown in Bighorn aRiver Basins.

Tertiary Oligocene White River 

Formation

“White to pale-pink blocky tuffaceous claystone and lenticular arkosic conglomerate.”3Present in the southeastern part of the Wind River Basin.1

“Highly permeable and productive waterbearing unit. Good intergranular permeability and porosity. Well yields generally range between 4 to 1,100 L/min, with maximum reported production 3,200 L/min. Saturated thickness ranges between 60 to 110 m.”4Present as isolated remnants in southwest Campbell County (Pumpkin Buttes), otherwise absent from Powder River Basin within the study unit.3 

Dissolved-solids concehistorical water samplethe Wind River Basin r207 to 397 mg/L. The calcium carbonate type

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone River Basin that lie within 

and Powder River Basins

(Table modified from Plafcan and others, 1995; Plafcan and Ogle, 1994; Plafcan and others, 1993; and Lowry and others, 1993)[m, meters; m/d, m per day; mg/L, milligrams per liter; μg/L, micrograms per liter; L/min, liters per minute; yields: small, less than 190 L/min; moderate, 190-1,100 L/m--, no data]
98 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Tertiary Eocene Tepee Trail Formation5 “Green and olive-drab hard and generally well bedded andesitic conglomerate, sandstone, and claystone.”3 “Yields minor amounts (less than 38 L/min) of water to springs and shallow wells along outcrop. Confining layer.”4 Limited water quality d

samples) showed that dsolids concentrations ra197 to 244 mg/L. 

Tertiary Eocene Aycross Formation5 “Brightly variegated bentonitic claystone and tuffaceous sandstone, grading laterally into greenish-gray sandstone and claystone.”3 “Would probably yield at least small, and possibly large, supplies from sandstone and conglomerate beds”1 “Confining layer.”4 Localized water yield primarily a function of sandstone content. Sandstone presence highly variable both vertically and laterally. Unknown. Tertiary Eocene Wagon Bed Formation “Green and gray tuffaceous claystone, sandstone, and conglomerate; some uranium- phosphate marlstone and 

variegated bentonitic claystone. Locally contains oil shale between Wind River and Bighorn Basins.”3

“Yields water locally to springs and shallow wells. Yields less than 38 L/min. Saturated zones include sandstone and conglomerate lenses... Not considered an aquifer.”3 

Dissolved-solids concehistorical water samplethe Wind River Basin r207 to 572 mg/L. The calcium-sodium carbon

Tertiary Eocene Wasatch 

Formation

Locally derived conglomerate around Wind River Basin margins. Lower part is Paleocene.3

Drab sandstone and drab to variegated claystone; numerous coal beds in lower part of Powder River Basin.3

Yields water from lenticular sandstone, and to a lesser extent from jointed coal and clinker beds. Yields can be expected to range from 10 to 190 L/min in north part of Powder River Basin becoming generally greater southward with 1,900 L/min or more possible in south part of basin.2

Unknown in Wind River Basin.

Dissolved-solids concehistorical water samplethe Powder River Basin from less than 200 to m8,000 mg/L, but commbetween 500 and 1,500 Sodium sulfate and sodbicarbonate were the dowater types.2

Unknown in Wind Rive

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
TABLES 99

Tertiary Eocene Wind River

Formation

“Variegated claystone and sandstone; lenticular conglomerate.”3

“Large supplies have been developed in the Riverton and Gas Hills areas and could be developed elsewhere, especially along the margin of the basin. Yields small supplies to many widely distributed stock and domestic wells”1, 6Major aquifer in Wind River Basin. “Yields water to wells and springs throughout basin.”4 Yields range between 4 to 

11,000 L/min. Locally contains artesian zones with sufficient head to produce 760 L/min. “Principal source of domestic and stock water on Wind River Reservation. Principal source of industrial water in southern part of basin”4

Dissolved-solids concehistorical water samplethe Wind River Basin r248 to 5,110 mg/L. Tha sodium-calcium sulfawater sample analyzed trace elements containeconcentration of 58 μg/above the Maximum CLevel of 50 μg/L set by Environmental Protecti

Tertiary Eocene Indian 

Meadows Formation

“Series of variegated claystone, argillaceous sandstone, massive limestone, and poorly sorted conglomerate.”4

“Confining layer.”4 Unknown. 

Tertiary Eocene Willwood 

Formation

Variegated, interbedded claystone and channel sandstone. Averages about 25 percent sandstone.

Might yield enough water from sandstones for domestic or stock use.

Dissolved-solids concehistorical water samplethe Bighorn Basin rangto 9,000 mg/L.

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
100 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Tertiary Paleocene Fort Union Formation Conglomerate, sandstone, shale, and carbonaceous shale in lower part of formation grading into very fine grained clastics in upper part, present at depth throughout most of the Wind River Basin.1 Northwest and central Wyoming—brown to gray sandstone, gray to black shale, and thin coal beds.3 “Conglomerate, sandstone, shale, siltstone, and carbonaceous shale in basal part of unit; grades upward to very fine-grained clastics.”4 Sandstones yield small supplies of water that is generally unsuitable for domestic use and may be marginal for stock in Wind River Basin.1 “Conglomerate and sandstone zones yield water to wells. Highly productive and permeable where fractured. Water is semi- confined to confined with sufficient head to produce 38 L/min... Basal part of unit is considered a regional confining unit. Upper part of unit contains complex series of permeable and confining layers.”4 Yields water from fine-grained sandstone, jointed coal, and clinker beds. Maximum yields are about 570 L/min in Powder River Basin.2 Might yield enough water from sandstones 

for domestic or stock use in Bighorn Basin.

Historic water quality dtwo samples taken in thRiver Basin had dissolvconcentrations of 994 a5,110 mg/L.Dissolved-solids concehistorical water samplethe Bighorn Basin rangto 4,890 mg/L. Samples taken in the PoBasin ranged from aboumore than 3,000 mg/L, commonly range betwe1,500 mg/L. Water wasodium bicarbonate, anextent sodium sulfate.2

Cretaceous Upper 

Cretaceous

Lance Formation

“North Wyoming—thickbedded buff sandstone and drab to green shale; thin conglomerate lenses.... Northeast Wyoming—brown and gray sandstone and shale; thin coal and carbonaceous shale beds.”3

“No known wells produce water solely from unit. Wells completed in Fort Union and Lance Formations. Unit is highly productive and permeable in Big Horn basin (yields range between 4 to 380 L/min)... Large development potential in Wind River Basin.”4Generally yields less than 76 L/min, but yields of several hundred liters per minute may be possible from the complete section of the formation in Powder River Basin.2

Dissolved-solids concehistorical water samplethe Bighorn Basin rangto 1,860 mg/L. Dissolved-solids concehistorical water samplethe Powder River Basin from about 200 to more 2,000 mg/L, but commbetween 500 and 1,500 dominant water type wprevalent.2Unknown in Wind Rive

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
TABLES 101

Cretaceous Upper 

Cretaceous

Fox Hills Sandstone

“Light-colored sandstone and gray sandy shale containing marine fossils.”3

Maximum yields in west part of Powder River Basin will probably be less than 380 L/min.2

Dissolved-solids concehistorical water samplethe eastern Powder Rivwere generally less than 1,000 mg/L, but generabetween 1,000 and 2,00west part. No dominanwas prevalent.2

Cretaceous Upper 

Cretaceous

Meeteetse Formation

“Chalky-white to gray sandstone, yellow, green, and dark-gray bentonitic claystone, white tuff, and thin coal beds.”3

Sandstones yield small supplies of water that is generally unsuitable for domestic use and may be marginal for stock in Wind River Basin.”“Regional confining layer.”4Sandstone might yield enough water for domestic or stock use in Bighorn Basin.

Dissolved-solids concehistorical water samplethe Bighorn Basin rangto 2,920 mg/L. Unknown in Wind Rive

Cretaceous Upper 

Cretaceous

Lewis Shale (Bear Paw Shale)

“Gray marine shale containing many gray and brown lenticular concretionrich sandstone beds.”3

Sandstones yield small supplies of water that is generally unsuitable for domestic use and may be marginal for stock in Wind River Basin.1Sandy zones may yield as much as 38 L/min, but most of the formation does not yield water in Powder River Basin.2

Unknown. 

Cretaceous Upper 

Cretaceous

Mesaverde Formation

“Light-colored massive to thin-bedded sandstone, gray sandy shale, and coal beds.”3

Permeable and productive water-bearing unit. Regional aquifer. Well yield data not available; however, artesian flows reported in numerous petroleum tests in central Wind River Basin.4

Yields of as much as 190 L/min are possible from sandstone beds, and as much as 760 L/min where fracturing has increased the permeability, generally near geologic structures.2

Dissolved-solids concehistorical water samplethe Wind River and Bigranged from 688 to 5,5The water type was varDissolved-solids can be range from about 300 t2,000 mg/L in the PowBasin. Most water will sulfate type.2

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
102 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Cretaceous Upper Cretaceous Cody Shale “Dull-gray shale, gray siltstone, and fine-grained gray sandstone.”3 Yields only meager supplies of poor quality water in Wind River Basin.1 “Regional confining layer.”4 Yields of as much as 76 L/min possible from sandstone beds, but other rocks in formation would yield little or not water in Powder River Basin.2 Thin sandstone might yield enough water locally for domestic or stock use in Bighorn Basin. Dissolved-solids conce

historical water samplethree basins ranged from 700 to over 8,000 mg/Lwas a sodium sulfate tyBighorn and Powder R

Cretaceous Upper Cretaceous Frontier Formation “Gray sandstone and sandy shale.”3 Yields small quantities of generally poor quality water although some supplies are usable for domestic purposes in Wind River Basin.”1 Upper two-thirds of unit is regional aquifer in Wind River Basin; lower one-third of unit is 

confining layer. Water is under confined conditions with sufficient head to produce flows of 38 to 95 L/min at selected petroleum tests. Yields 19 to 570 L/min to shallow stock and domestic wells”4Yields of as much as 190 L/min are available from sandstone beds in Powder River Basin.2Artesian conditions exist in the area along Kirby Creek in the Bighorn Basin.

Dissolved-solids concehistorical water samplethe Wind River and Bigranged from 280 to 9,9The water type was varDissolved-solids can be range from about 300 t3,000 mg/L in the PowBasin. Most water is sobicarbonate or sodium type.2

Cretaceous Upper 

Cretaceous

Mowry Shale “Silvery-gray hard siliceous 

shale containing abundant fish scales and bentonite beds.”3

Regional confining layer in Wind River Basin.4Some of these rocks may yield as much as 38 L/min, but most of the formation does not yield water in Powder River Basin..2Might yield very limited quantities of water to wells where fractured in Bighorn Basin.

Historic water quality done sample taken in the Basin had a dissolved-sconcentration of 648 mDissolved-solids concehistorical water samplethe Bighorn Basin rangto 1,150 mg/L.Unknown in Powder R

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
TABLES 103

Cretaceous Lower 

Cretaceous

Thermopolis Shale

“Black soft fissile shale; Muddy Sandstone Member at top.”3

Regional confining layer in Wind River Basin.4Ground-water possibilities not known in Powder River Basin, but probably poor.2Shales are relatively impermeable. Wells developed in Muddy Sandstone Member may yield enough water for domestic or stock use in Bighorn Basin.

Historic water quality dtwo samples taken in thRiver Basin had dissolvconcentrations of 223 a525 mg/L.Dissolved-solids concehistorical water samplethe Bighorn Basin rangto 1,100 mg/L.Unknown in Powder R

Cretaceous Lower 

Cretaceous

Cloverly Formation

North Wyoming—“rusty sandstone at top, underlain by brightly variegated bentonitic claystone; chert-pebble conglomerate locally at base. Northeast Wyoming—rusty to light-gray sandstone containing lenticular chertpebble conglomerate interbedded with variegated bentonitic claystone.”3

“Yield small to moderate supplies of water suitable for domestic use near outcrops”1Permeable and productive upper and basal sandstones in Wind River Basin. Water is under artesian conditions with sufficient head to produce flows of 4 to 95 L/min at selected petroleum tests. Yields water to stock wells along outcrops.4, 6Most yields range from 19 to 76 L/min in the Powder River Basin, but yields of 380 L/min or more are possible from complete section of rocks, and as much as several hundred liters per minute from zones of secondary permeability.2Sandstones might yield enough water for stock or domestic use in Bighorn Basin.

Dissolved-solids concehistorical water samplethree basins generally r300 to 3,000 mg/L.Sodium sulfate was the water type in the PowdBasin.2

Jurassic Morrison 

Formation

“North Wyoming—dully variegated claystone, nodular limestone, and gray silty sandstone. Northeast Wyoming—dully variegated siliceous claystone, nodular white limestone, and gray silty sandstone”3

Sandstones may yield enough water for domestic or stock use.

Dissolved-solids concehistorical water samplethe Wind River Basin r798 to 1,710 mg/L.Historic water quality done sample taken in the Basin had a dissolved-sconcentration of 673 mUnknown in Powder R

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
104 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Jurassic Sundance Formation “Greenish-gray glauconitic sandstone and shale, underlain by red and gray nonglauconitic sandstone and shale.”3 “Yield small to moderate supplies of water suitable for domestic use near outcrops”1 Regional aquifer in Wind River Basin. Large intergranular permeability in sandstone and chert lenses. Yields water to shallow stock and domestic wells along outcrops (4 to 95 L/min).4 Sandstone beds in Powder River Basin will probably yield no more than 76 L/min.2 Sandstones might yield enough water for stock or domestic use in Bighorn Basin. Dissolved-solids conce

historical water samplethe Bighorn Basin rangto 1,750 mg/L. Dissolved-solids can be range from about 500 t2,000 mg/L in the PowBasin. No water type i

Unknown in Wind Rive

Jurassic Gypsum Spring Formation “Interbedded red shale, dolomite, and gypsum. In north Wyoming wedges out south in T. 39 N.”3 No water wells known to tap this formation in the Wind River Basin, but it would probably yield only poor quality water.1 “Regional confining layer.”4

Solution zones in gypsum beds yield small amounts of water in Bighorn Basin.7Yields a few liters per minute locally for stock purposes from solution cavities in or near outcrop areas in the Powder River Basin.2

Historic water quality done sample taken in the Basin had a dissolved-sconcentration of 1,360 Dissolved-solids concehistorical water samplethe Bighorn Basin rangto 2,650 mg/L.Dissolved-solids are gegreater than 1,000 mg/L Powder River Basin, thwater type is calcium s

Jurassic(?)-Triassic(?)

Nugget Sandstone

“Gray to dull-red crossbedded quartz sandstone.”3 “Water-bearing potential not known in Wind River Basin, but probably would yield small 

supplies, and larger supplies might be developed in some areas”1Water-yielding characteristics unknown in Bighorn Basin.

Historic water quality done sample taken in the Basin had a dissolved-sconcentration of 1,470 Unknown in Bighorn B

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
TABLES 105

Triassic Chugwater 

Formation

“Red siltstone and shale. Alcova Limestone Member in upper middle part in north Wyoming. Thin gypsum partings near base in north and northeast Wyoming.”3

Yields small supplies of good quality water in and near outcrops in Wind river Basin.1Will probably yield as much as 76 L/min in Powder River Basin.2May yield sufficient quantities of water to wells for domestic and stock use in the Bighorn Basin.

Dissolved-solids concehistorical water samplethe Wind River and Bigranged from 251 to 2,9The water type was calcDissolved-solids generfrom about 500 to more 1,000 mg/L in the PowBasin, the dominant wacalcium sulfate.2

Triassic Dinwoody 

Formation

“Olive-drab hard dolomitic thin-bedded siltstone.”3 Yields small supplies of good quality water in and near outcrops in Wind River Basin.1

Water-yielding characteristics unknown in Bighorn Basin.

Yields small supplies oquality water in and nein Wind River Basin.1

Unknown in Bighorn B

Triassic and Permian

Goose Egg Formation

“Red sandstone and siltstone, white gypsum, halite, and purple to white dolomite and limestone.”3

Probably would yield only small supplies of mineralized water in Wind River Basin.1Yields little or no water in Powder River Basin.2Might yield small quantities of water to wells as a result of dissolution of gypsum in Bighorn Basin.

Dissolved-solids concehistorical water samplethe Bighorn Basin rangto 2,230 mg/L. Unknown in Wind RivePowder River Basins.

Permian Phosphoria 

Formation and related rocks

“Brown sandstone and dolomite, cherty phosphatic and glauconitic dolomite, phosphatic sandstone and dolomite, and greenish-gray to black shale.”3

“Complex series of permeable sandstones and impermeable limestone, dolomite and siltstone. Highly productive where fractured. Well yields range up to 3,800 L/min”4Yields as large as 3,800 L/min observed in Bighorn Basin.

Dissolved-solids concehistorical water samplethe Wind River and Bigranged from 215 to 3,6The water type was var

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
106 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Permian and Pennsylvanian Tensleep Sandstone “North Wyoming—white to gray sandstone containing thin limestone and dolomite beds. Permian fossils have been found in the topmost beds of the Tensleep at some localities in... Owl Creek Mountains.”3 Uppermost unit of the Tensleep aquifer system. Good intergranular permeability, excellent permeabilities where fractured. Saturated throughout Wind River Basin. Water is under confined conditions with sufficient head to produce flows of 4 to several hundred L/min from selected wells.4 Yields ranging from 76 L/min to as much as several hundred L/min are possible from these rocks, and where fracturing has increased the permeability, yields greater than 3,800 L/min may be possible in the Powder River Basin.2, 8 Flowing wells along the western flank of the Bighorn Mountains yield large dependable supplies of water. Dissolved-solids conce

historical water samplethe Wind River and Bigranged from 171 to 3,4The water type was calmagnesium carbonate iRiver Basin. “Dissolved solids commfrom 200 to 500 mg/L generally less than 1,00locally may be more th2,000 mg/L.”2 

Pennsylvanian and Mississippian

Amsden Formation

“North Wyoming—red and green shale and dolomite; at base is brown sandstone.”3

Part of Tensleep aquifer system. Sandstone member is permeable along joints and partings between bedding planes. Excellent permeabilities where fractured. Water is confined. Well yields range between 4 to several hundred L/min in Wind River Basin.4

Unknown. 

Mississippian Madison 

Limestone or Madison Group

Massive to thin bedded limestone, containing some thin beds of chert and red shales near the top.1

“Massive crystalline limestone and dolomite with siltstone and shale zones, cherty in places.”7“Dolomite and limestone, massive to thin bedded, cavernous in upper part”2

Part of Tensleep aquifer system. Poor permeabilities except where fractured. Some saturated caverns. Water-bearing throughout Wind River Basin. Water is confined. Well yields range between 4 to several hundred L/min.4Yields of more than 3,800 L/min are available where cavernous and fractured zones are present in Powder River Basin.2Unit in hydraulic connection with underlying Bighorn Dolomite forming the MadisonBighorn aquifer in Bighorn Basin.

Dissolved-solids concehistorical water samplethree basins ranged from 3,390 mg/L. Water typRiver Basin was sodium One sample in the WinBasin was a calcium-mcarbonate water type. Oin the Bighorn Basin wsulfate water type.

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
TABLES 107

Devonian Darby 

Formation

“Yellow and greenish-gray shale and dolomitic siltstone underlain by fetid brown dolomite and limestone.”3

“Part of Tensleep aquifer system. Generally considered a confining layer, but permeable along joints and fractures. Numerous joint controlled springs along Wind River Mountains.”4May hydrologically separate the overlying Madison Limestone from the underlying Bighorn Dolomite.

Unknown. 

Ordovician Bighorn 

Dolomite

“Gray massive cliff-forming siliceous dolomite and locally dolomitic limestone.”3

“Basal part of Tensleep aquifer system. Basal sandstones are permeable; also permeable along joints and fractures. Yields water to numerous springs along Wind River Mountains.”4Yields ranging from 76 L/min to several hundred liters per minute should be available from solution cavities and fractures in Powder River Basin.2In combination with the Darby Formation and the Madison Limestone, forms the Madison-Bighorn aquifer in the Bighorn Basin, which produces large and dependable supplies of potable water.7

Historic water quality dtwo samples taken in thRiver Basin had a dissoconcentration of 102 anDissolved-solids concehistorical water samplethe Bighorn Basin rangto 3,440 mg/L.Unknown in Powder R

Cambrian Gallatin 

Limestone

“Blue-gray and yellow mottled hard dense limestone.”3

“Confining layer. Permeable along joints and fractures. Yields small quantities (less than 19 L/min) to springs along the Wind River Mountains.”4Probably yields less than 38 L/min in Powder River Basin. 2, 9In combination with underlying Gros Ventre Formation, forms a confining layer for the Flathead Sandstone in Bighorn Basin.

Historic water quality done sample taken in the Basin had a dissolved-sconcentration of 296 mwater type was magnesbicarbonate.Unknown in Wind RivePowder River Basins.

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
108 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Cambrian Gros Ventre Formation “Limestone, shale, and calcareous shale, flat-pebble conglomerate at base.”4 “Green-gray thin-bedded limestone and limestone- pebble conglomerate.” “Ground-water possibilities not known, but rocks are potential sources of large supplies where fractured or cavernous.”1 “Confining layer.”4 Probably would yield less than 38 L/min in Powder River Basin.2, 10 Thin sandstone beds indicate potential for small yields in Bighorn Basin. Unknown. 

Cambrian Flathead Sandstone “Dull-red quartzitic sandstone.”3 Major aquifer in Wind River Basin. Permeable along partings between bedding planes, faults, fractures and joints. Small interstitial permeabilities. Water is semi- confined to confined. Yields 4 to 19 L/min to shallow stock and domestic wells. Excellent ground-water resource potential; however, relatively undeveloped because of 

availability of shallower ground-water sources.4Yields of 76 L/min are probably available in the Powder River Basin. 2, 11Yields over 7,600 L/min reported in Bighorn Basin.7

Dissolved-solids concehistorical water samplethe Wind River and Bigranged from 37 to 443 msamples from Hot Sprinin the Bighorn Basin wcalcium bicarbonate waUnknown in Powder R

1Whitcomb and Lowry, 1968.2Hodson and others, 1973.3Love and Christiansen, 1985.4Richter, Jr., 1981.5Part of the Absaroka Volcanic Supergroup.6Includes Morrison Formation.7Libra and others, 1981.8Includes Amsden, Hartville, and Minnelusa Formations.9Includes Whitewood Dolomite, Winnipeg, and Gros Ventre Formations.10Includes Whitewood Dolomite, Winnipeg, and Gallatin Formations.11Includes Deadwood Formation.

Table 16. Lithologic, water-yielding characteristics, and water quality of major geologic units in the Yellowstone NAWQA Study Unit Are

River, Bighorn, and Powder River Basins--Continued

System Series

Geologic 

unit Lithology Water-yielding characteristics Water quali
TABLES 109

Table 17. Shallow Hydrogeologic Units in the Powder River Basin, Montana and Wyoming

Hydrogeologic Unit

Range of 

Thickness

(m) General Description Water-Yielding Ch

Wasatch-Tongue River aquifer 0-1,190 Includes interfingering lenses of gravel, sand, silt, and 

clay of alluvium or terrace deposits (not shown on map) near the major streams and their tributaries; tuffaceous bentonitic claystone and siltstone which grade downward into lenticular fine-grained sandstone of the White River Formation; brownish-gray fine- to coarse-grained lenticular sandstone, interbedded with shale and coal, weathering to a buff color and the clinker deposits near coal outcrops of the Wasatch Formation; light-yellow to light-gray fine- to mediumgrained thick-bedded to locally massive cross-bedded and lenticular, calcareous sandstone and siltstone, weathering to a buff color, thick and laterally persistent coal beds, and clinker deposits near coal outcrops of the Tongue River Member of the Fort Union Formation; and may contain light-gray fine- to medium-grained sandstones and siltstones of the upper part of the Lebo Shale Member of the Fort Union Formation

Yields range from 19 to 380 L/mito 610 L/min for sandstone, coal, wells are shallow in the upper parsouthern part of the basin yields abe possible from sandstone or clinwell construction; well yields of 3measured. Wells completed in thmay flow as much as 38 L/min. Ueverywhere clinker deposits exist 

Lebo confining layer 0-920 Dark shale predominates with interbedded light-gray 

and brown to black carbonaceous shale, siltstone, and locally thin coal beds of the Lebo Shale Member of the Fort Union Formation. Altered and devitrified volcanic ash and brown ferruginous concretions are present in shale

Functions as a retarding layer wityield as much as 38 L/min to welsaturated thickness of lenticular cpenetrated

Tullock aquifer 0-600 Includes light-gray fine- to medium grained channel 

sandstone near the base of the Lebo Shale Member where present plus light-gray sandy or silty shale, locally resistant sandstone which grades downward into interbedded medium-gray to light-gray shale, light-gray fine-grained sandstone and siltstone, and thin coalbeds of the Tullock Member of the Fort Union Formation

Yields from fine-grained sandstonmay be as much as 150 L/min but more common. Where aquifer is flow less than 38 L/min

Table 17. Shallow Hydrogeologic Units in the Powder River Basin, Montana and Wyoming(Table modified from Lewis and Hotchkiss, 1981)[m, meters; L/min, liters per minute]
110 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Upper Hell Creek confining layer 0-610 Interbedded gray to brown siltstone and shale, locally lenticular fine- to medium-grained sandstone, and interbedded claystone, thin coal beds, silty sandstone, and bentonitic shale of the upper part of the Hell Creek (or Lance) Formation Limited water supply in study are

retarding unit. Locally, flowing wyield as much as 15 L/min Fox Hills-Lower Hell Creek aquifer 0-780 Interbedded carbonaceous shale, sandy shale, siltstone, and claystone with local deposits of gray to brown silty to clayey often crossbedded sandstone of the lower part of the Hell Creek (or Lance) Formation plus the marine gray to light-tan fine- to medium-grained sandstone of the Fox Hills Sandstone; locally contains thin beds of sandy shale Yields as much as 760 L/min to pwith yields generally less than 380 water for artesian wells; wells floalong major river valleys Table 17. Shallow Hydrogeologic Units in the Powder River Basin, Montana and Wyoming--Continued Hydrogeologic Unit Range of Thickness (m) General Description Water-Yielding Ch
TABLES 111

Table 18. Hydrogeology of southeastern Montana

Hydrogeologic Unit

Range of 

Thickness

(m) General Description Water-Yielding Ch

Alluvial aquifer 2-40 Includes lake-basin deposits, unconsolidated flood-plain 

deposits, and adjacent terrace deposits less than 15 m above current stream level—Largely unconsolidated sand, silt, and clay with local lenses of gravel. Coarse well-rounded gravel interbedded with finer material is common in basal alluvial deposits along the Yellowstone and Missouri Rivers, probably derived from reworked terrace gravels.

Coarse gravels along the Missouyield as much as 1,900 L/min to along smaller streams and adjacyields of 380 L/min might be poabout 57 L/min to stock and dom

Terrace gravel aquifer 2-30 Comprises terraces of Crane Creek, Cartwright, Flaxville, 

and Rimroad Gravels as described by Howard (1960)—Mostly gravel and sand with some silt and clay. Wellrounded 25- to 300-mm-diameter cobbles and boulders and sand-size particles of quartzite, chert, and igneous rocks are common. Deposits are largely distributed along upland areas and benches of the Yellowstone river. Many of the higher deposits are covered by loess.

Terraces are mostly isolated, tophaving limited saturation. Yield76 L/min to domestic and stock within larger deposits and along altitudes.

Wasatch-Tongue River aquifer 1-880 Consists mostly of Tongue River Member of Fort Union 

Formation but locally includes overlying basal rocks of Wasatch Formation—Light-gray to brownish-gray fine-to medium-grained thick bedded to massive and locally crossbedded lenticular calcareous sandstone and siltstone. Commonly weathers light yellow to buff. Contains lightbuff to dark-gray shaly siltstone and shale, brown to black carbonaceous shale, and coal beds as thick as 25 m.

Sandstone and coal beds are the in the study area; shales do not ymuch as 610 L/min to wells pensections. However, average yiedomestic wells are less than 76 fractured clinker beds may yield wells.

Lebo confining layer 0-180 Includes Lebo Shale Member of Fort Union Formation—

Predominantly dark shale interbedded with light-gray and brown to black carbonaceous shale, siltstone, and locally thin coal beds. Shales contain altered and devitrified volcanic ash and reddish-brown ferruginous concretions. White to light-gray argillaceous crossbedded and lenticular sandstones occur locally.

Relatively large percentages of this unit retard vertical movemelocal areas where saturated, medsandstones may yield as much a

Table 18. Hydrogeology of southeastern Montana(Table modified from Stoner and Lewis, 1980)[m, meters; mm, millimeters; L/min, liters per minute]
112 ENVIRONMENTAL SETTING OF THE YELLOWSTONE RIVER BASIN Tullock aquifer 0-180 Comprises entire thickness of Tullock Member of Fort Union Formation and locally includes upper 3 to 24 m of Hell Creek Formation—Interbedded medium-gray to light- gray shale, light-gray fine-grained sandstone and siltstone, and thin but persistent coal beds. A resistant sandstone commonly forms a rimrock at the top of the unit. Locally, sandstones and siltstones weather yellow to brown. Fine-grained sandstones and coa

yielding units. Well yields may but generally average about 57 Ltapping confined aquifers gener38 L/min. Lower Fort Union aquifer 0-170 Includes rocks in eastern part of study area equivalent to Lebo and Tullock Members of Fort Union Formation and may include uppermost part of Hell Creek Formation— Mostly interbedded light-gray to dark-gray shale, siltstone, and shaly sandstone. Upper part of unit is mostly shale; locally interbedded with lenticular siltstone and sandstone that commonly weather yellow to tan color. Lower part contains a few thin lignite coal beds. Overall appearance is darker than overlying and lighter than underlying units. Contact is conformable and generally transitional with underlying unit. Shaly sandstones and coal beds units. Typical well yields are 45 of low permeability, the entire amovement of water. Upper Hell Creek confining layer 0-180 Comprises upper part of Hell Creek Formation—Gray to 

yellowish-gray silty clayey carbonaceous and bentonitic shale and siltstone; locally contains yellowish-gray to tan fine- to medium-grained silty sandstone, thin coal beds, and brown ferruginous concretions.

Limited as a water supply in the wells yield about 19 L/min. Geunit.

Fox Hills-Lower Hell Creek aquifer 0-230 Includes lower part of Hell Creek Formation and Fox Hills 

(Lennep) Sandstone—Gray to yellowish-gray to tan fineto medium-grained sandstone; contains gray to olive-gray shale and shaly siltstone. Locally has massive or crossbedded sandstone layers and thin coal beds near top of unit. Lower contact considered to be the middle of the transitional siltstone located between sandstone above and shale of underlying unit. Contact is conformable with underlying unit.

Significant source of water in th76 L/min to flowing wells along where conditions permit. Yielddomestic and stock wells and 76industrial wells.

Bearpaw confining layer and older rocks, undifferentiated

0-340 Bearpaw confining layer consists of Bearpaw Shale and 

equivalent upper part of Pierre Shale; forms the base of the overlying shallow aquifer system—Bearpaw is gray to black marine shaly claystone and shale; thin beds of siltstone, silty sandstone, and bentonite occur locally.

Very low permeability; generallwells in the study area.

Table 18. Hydrogeology of southeastern Montana--Continued

Hydrogeologic Unit

Range of 

Thickness

(m) General Description Water-Yielding Ch
 113