Downstream Effects of

Downstream Effects of 

Dams on Alluvial Rivers

By GARNETT P. WILLIAMS and M. GORDON WOLMAN

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1286

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First printing 1984 

Second printing 1985

Library of Congress Cataloging in Publication Data

Williams, Garnett P.

Downstream effects of dams on alluvial rivers.

(Geological Survey Professional Paper; 1286)

Includes bibliographical references.

Supt. of Docs. No.: I 19.16:1286

1. River channels. 2. Rivers Regulation. 3. Dams.

I. Wolman, M. Gordon (Markley Gordon), 1924- II. Title. III. Title: Alluvial rivers. IV. Series.

TC175.W48 1983 551.48'2 82-600318

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

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

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

Scope of study ...................

Study sites and selection criteria .........

Acknowledgments ..................

Methods of analysis and data sources .........

Water discharge ..................

Sediment load ....................

Bed and bank materials ..............

Mean bed elevation .................

Measured cross sections ............

Published graphs of bed-elevation changes . 

Gage height-discharge records (rating tables)

Channel width ....................

Time origin of channel changes ..........

Vegetation ......................

Variability of natural channels

Downstream effects of dams .................

Water discharges .....................

Sediment loads ......................

Mean bed elevation ....................

General nature of changes in bed elevation . . . 

Degree of change attributable to dams ......

Degraded reach downstream from a dam .....

General function of degradation with time at a 

site .......................

Maximum degradation and associated time . . 

Standardized degradation-time plot ......

Length of degraded reach ...........

Zone of variable bed changes ...........

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Downstream effects of dams continued 

Mean bed elevation Continued

Longitudinal-profile changes ............. 26

Bed material and degradation ................ 29

Theoretical expectations ................. 29

Variations in bed-material sizes with time at a cross section ........................... 29

Variations in bed-material sizes with distance 

downstream ....................... 31

Channel width ......................... 31

General nature of width changes ............ 31

Distance affected ..................... 35

Factors affecting changes in channel width ...... 36

Alluvial-bank materials ............... 36

Bedrock-bank controls and downstream effects . . 37 

Water flow ...................... 37

Width-depth ratio .................. 39

Time trends of channel widening at a site ...... 39

Time trends of channel narrowing at a site ...... 42

Prediction of post-dam channel-width changes ..... 43

Role of a dam in effecting a change in channel width . 47 

Sediment volumes removed and channel equilibrium .... 49

Vegetation ........................... 50

Observed changes in vegetation ............. 50

Possible causes of vegetation changes ......... 52

Separating flow regulation from other factors affecting 

vegetation change ................... 54

Effects of vegetation growth .............. 55

Conclusions ........................... 56

References ........................... 61

Supplementary data (tables 13 and 14) ........... 65

ILLUSTRATIONS

Page

FIGURE 1. Map of conterminous United States showing location of major study sites ...................... 3

2-^4. Graphs showing:

2. Variation in annual suspended-sediment loads before and after closure of Hoover Dam, Colorado River, Arizona. 14

3. Suspended-sediment loads (concentrations) transported by various discharges at successive downstream stations

before and after closure of Canton Dam, North Canadian River, Oklahoma ............... 15

4. Post-dam/pre-dam ratio of annual suspended-sediment loads versus distance downstream from Gavins Point

Dam, Missouri River, South Dakota ...................................... 16

5-6. Photographs showing:

5. Time progression of bed degradation and channel armoring at the streamflow-gaging station downstream from

Jemez Canyon Dam, Jemez River, New Mexico ............................... 16

6. Degradation represented by successively lower water-intake pipes for the streamflow-gaging station 2.6 kilometers downstream from Fort Supply Dam, Wolf Creek, Oklahoma ..................... 17

7-16. Graphs showing:

7. Changes in mean bed elevation with time at streamflow-gaging stations 48 kilometers upstream and 1.3 kilometers downstream from Kanopolis Dam, Smoky Hill River, Kansas ..................... 17

8. Examples of irregular rates of bed degradation with time ........................... 18

9. Representative regression curves of bed degradation with time at selected sites .............. 20
IV CONTENTS

Page

FIGURE 10. Frequency distributions based on 111 measured cross sections on various rivers: (A) maximum expected 

degradation depth; (B) years needed to deepen to 95 percent of maximum depth; and (C) years needed 

to deepen to 50 percent of maximum depth .................................. 23

11. Standardized degradation-time dimensionless plot of degradation curves in figure 9 ............ 24

12. Longitudinal-profile changes downstream from four dams ........................... 28

13. Variation in bed-material size with time at a site, after dam closure .................... 30

14. Variation in bed-material diameter and bed degradation with distance downstream, at a given time after

dam closure .................................................... 32

15. Increase in bed-material size with bed degradation downstream from three dams ............. 33

16. Variation in median bed-material diameter with distance along the Colorado River downstream from Hoover

Dam, at successive times after dam closure .................................. 33

17-22. Photographs showing:

17. Jemez River downstream from Jemez Canyon Dam, New Mexico ...................... 34

18. Old streamflow-gaging site on Arkansas River 3 kilometers downstream from John Martin Dam, Colorado . 34

19. Wolf Creek about 2.6 kilometers downstream from Fort Supply Dam, Oklahoma ............. 35

20. North Canadian River about 0.8 kilometer downstream from Canton Dam, Oklahoma ........... 35

21. Sandy bank of Missouri River about 10 kilometers downstream from Garrison Dam, North Dakota ... 36

22. Stratified sand and silt bank, Missouri River downstream from Garrison Dam, North Dakota ...... 36

23-29. Graphs showing:

23. Changes in channel cross section of Sandstone Creek, Oklahoma ....................... 38

24. Examples of relative increase of channel width with time: (A) irregular rates; (B) regular rates with fitted

regression curves ................................................. 39

25. Frequency distributions of estimated eventual increases in channel width, based on 44 measured cross sections 

on various rivers: (A) final values of Wt/Wi, (B) years needed to widen to 95 percent of final W</Wi; (C) 

years needed to widen to 50 percent of final Wt/Wi ............................. 43

26. Dimensionless plot of relative increase in channel width with time, for 6 representative cross sections . 43

27. Examples of relative decrease in channel width with time: (A) irregular rates; (B) regular rates with fitted

regression curves ................................................. 44

28. Dimensionless plot of relative decrease in channel width with time, for 6 cross sections ......... 44

29. Computed versus measured values of post-dam average channel widths ................... 46

30. Photograph showing Canadian River about 3 kilometers downstream from Ute Dam, New Mexico ........ 47

31. Photograph showing Republican River downstream from Trenton Dam, Nebraska .................. 48

32. Graph showing variation in cumulative net sediment volumes of channel erosion with distance downstream from Denison

Dam on the Red River, Oklahoma-Texas ....................................... 50

33-35. Photographs showing:

33. Canadian River downstream from Sanford Dam, Texas ............................ 51

34. Washita River about 1.4 kilometers downstream from Foss Dam, Oklahoma ................ 53

35. Republican River downstream from Harlan County Dam, Nebraska, before and after dam closure ... 54 

36-49. Graphs showing changes in mean streambed elevation with time at streamflow-gaging station on:

36. Colorado River 6.4 kilometers downstream from Parker Dam, Arizona ................... 56

37. Jemez River 1.3 kilometers downstream from Jemez Canyon Dam, New Mexico, and at the control station

near Jemez 13 kilometers upstream from dam ................................ 57

38. Missouri River 13 kilometers downstream from Fort Peck Dam, Montana ................. 57

39. Missouri River 11 kilometers downstream from Fort Randall Dam, South Dakota ............. 57

40. Missouri River 8 kilometers downstream from Gavins Point Dam, South Dakota ............. 57

41. Smoky Hill River 1.3 kilometers downstream from Kanopolis Dam, Kansas, and at the control station at

Ellsworth 48 kilometers upstream from dam ................................. 58

42. Republican River 2.7 kilometers downstream from Milford Dam, Kansas, and at the control station at Clay

Center 49 kilometers upstream from dam ................................... 58

43. North Canadian River 4.8 kilometers downstream from Canton Dam, Oklahoma, and at the control station

near Selling 45 kilometers upstream from dam ................................ 58

44. Red River 4.5 kilometers downstream from Denison Dam, Oklahoma, and at the control station near Gainesville, Texas, 106 kilometers upstream from dam ............................... 58

45 Neches River 0.5 kilometer downstream from Town Bluff Dam, Texas, and at the control station on Village

Creek near Kountze in an adjacent drainage basin .............................. 59

46. Chattahoochee River 4 kilometers downstream from Buford Dam, Georgia, and at the control station on

the Chestatee River near Dahlonega 73 kilometers upstream from dam .................. 59

47. Rio Grande 1.3 kilometers downstream from Caballo Dam, New Mexico .................. 59

48. Marias River 3.2 kilometers downstream from Tiber Dam, Montana, and at the control station near Shelby

65 kilometers upstream from dam ....................................... 59

49. Frenchman Creek 0.3 kilometer downstream from Enders Dam, Nebraska ................. 59
CONTENTS V

TABLES

Page

TABLE 1. Selected examples of rates of change of channel width in alluvial reaches ........................ 6

2. Illustrative examples of long-term aggradation and flood deposition in alluvial reaches unaffected by manmade works . 6

3. Illustrative examples of long-term degradation and flood erosion in alluvial reaches unaffected by manmade works . 7

4. Water-discharge data for pre-dam and post-dam periods .................................. 10

5. Values associated with fitted degradation curves ...................................... 20

6. Maximum degradation downstream from various dams ................................... 22

7. Data on the degraded reach downstream from dams .................................... 25

8. Particle-size distributions of bed and bank material, Salt Fork, Arkansas River, downstream from Great Salt Plains

	Dam, Oklahoma ........................................................ 37

9. Flow data for Sandstone Creek near Cheyenne, Oklahoma, 1951-59 ........................... 38

10. Values associated with hyperbolic curves fitted to changes in channel width with time, at a cross section ..... 41

11. Data used to derive post-dam channel-width equation .................................... 47

12. Change in approximate percentages of riparian vegetation downstream from various dams .............. 52

13. Data on channel features, as measured from resurveyed cross sections ......................... 67

14. Changes in streambed elevations as estimated from streamflow-gaging-station rating tables .............. 81

SYMBOLS AND DEFINITIONS LIST

Ci Coefficient in empirical equations, with i = 1 to 4.

di Dominant size of bed material.

r Correlation coefficient.

t Time.

^max Time (years) needed to reach maximum degradation.

tp Time needed for degradation to reach a designated proportion of maximum degradation.

C{ Empirical coefficient in hyperbolic equation of channel change with time, with i I or 2.

D Bed degradation.

^max Maximum eventual bed degradation.

Qm Average daily discharge (arithmetic average of the annual mean daily flows for a post-dam period

	of years).

Qp Arithmetic average of the annual 1-day highest averaged flows for the pre-dam period of record.

W Channel (bankfull) width.

Wt Channel width at t years after dam closure.

Wi Channel width at time of dam closure:

W2 Channel width as of latest resurvey after dam closure.

DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL RIVERS

By GARNETT P. WILLIAMS and M. GORDON WOLMAN

ABSTRACT

This study describes changes in mean channel-bed elevation, channel width, bed-material sizes, vegetation, water discharges, and sediment loads downstream from 21 dams constructed on alluvial rivers. 

Most of the studied channels are in the semiarid western United 

States. Flood peaks generally were decreased by the dams, but in 

other respects the post-dam water-discharge characteristics varied 

from river to river. Sediment concentrations and suspended loads 

were decreased markedly for hundreds of kilometers downstream 

from dams; post-dam annual sediment loads on some rivers did not 

equal pre-dam loads anywhere downstream from a dam. Bed degradation varied from negligible to about 7.5 meters in the 287 cross sections studied. In general, most degradation occurred during the first 

decade or two after dam closure. Bed material initially coarsened 

as degradation proceeded, but this pattern may change during later 

years. Channel width can increase, decrease, or remain constant in 

the reach downstream from a dam. Despite major variation, changes 

at a cross section in streambed elevation and in channel width with 

time often can be described by simple hyperbolic equations. Equation 

coefficients need to be determined empirically. Riparian vegetation 

commonly increased in the reach downstream from the dams, probably because of the decrease in peak flows.

INTRODUCTION

Many alluvial channels are considered to be systems 

in equilibrium. This concept implies that the channel 

size, cross-sectional shape, and slope are adjusted to 

the quantities of sediment and water transported so 

that the streambed neither aggrades nor degrades. 

Similarly, the channel cross-sectional shape remains approximately constant. In this concept, both short-time 

changes (scour and fill) and long-term geologic or 

evolutionary changes (associated with climatic changes 

involving hundreds or thousands of years) are excluded. 

Neither the time scale nor magnitude of the changes 

involved in these concepts is precise. Nevertheless, the 

notion of adjustment and equilibrium implies that alluvial channels could be altered by significant manmade 

modifications, such as dams, in the regimen of water 

and sediment delivered.

This study deals with channel changes that have 

taken place downstream from 21 dams on alluvial rivers. Documentation of these changes can be useful in 

evaluating and (or) mitigating the expected effects of 

dams.

SCOPE OF STUDY

The primary emphasis of this study is on changes 

in bed elevation and width of river channels after alteration of the flow regimen by closure of dams. Information availability dictated the degree of study. Evidence 

of changes in bed material and in vegetation is presented where the data permit. Measured water discharges and sediment loads also are discussed because 

of their effect on all these features.

This study documents changes as they have occurred, 

particularly changes that have progressed for several 

decades. We have not been able to develop equations 

of sediment transport and erosion that might encompass 

the transient processes described, nor to produce a 

method of predicting the specific changes likely if a dam 

is built on a particular river. However, the data presented here should be useful for testing theoretical or 

empirical approaches. Brief discussion is devoted to the 

kinds of assumptions and constraints imposed on predictive models. Environmental impacts have received increasing attention during the past decade (see, for example, Turner 1971; Eraser, 1972; Gill, 1973; Sundborg, 

1977; American Society of Civil Engineers, 1978; and 

Ward and Stanford, 1979) but will not be discussed 

separately here.

STUDY SITES AND SELECTION CRITERIA

The preferred selection criteria for a damsite and 

downstream reach were:

1. An alluvial bed at the time the dam was built. Generally, this meant bed material in the silt-to-gravel 

range, as these sizes are more susceptible to erosion.

2. Monumented channel cross sections at various sites 

downstream from the dam, with repeat surveys 

(one of which was done at about the time of dam 

construction).

3. No significant dredging, channelization, or similar 

operations in the study reach.

4. No significant backwater effects from downstream

dams.

Data that met the above criteria were available for 

21 dams (fig. 1). Most of these are in the Plains States 

and semiarid West. Many other dams, too numerous
DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

to include in figure 1, also will be mentioned throughout 

this paper.

Although resurveyed cross sections were the preferred source of data for channel changes, gage height 

versus discharge relations at U.S. Geological Survey 

streamflow-gaging stations also were used to estimate 

bed-level changes, if the gaging station had: (1) An 

credible bed in the reach of the gage; (2) a location 

within about 10 km (kilometers) downstream from the 

dam; (3) gaging records beginning at the time of the 

dam closure (and preferably much earlier); and (4) a 

channel width that has not changed appreciably in the 

gaging-station reach, during the time period examined. 

Reaches downstream from 14 dams were found with 

a gaging station meeting these requirements. Eleven 

of these reaches have resurveyed cross sections and 

were among the 21 sites shown in figure 1.

Most of the analysis was based on information from 

sites that met the criteria noted above. However, 

where specific information was available on bed material, special channel characteristics, sediment loads, or 

vegetation, this information was used to illustrate specific changes and to enlarge upon the findings.

ACKNOWLEDGMENTS

We are grateful to Wayne Borough, John Turney, 

Kim Zahm, Ken McClung, Harry Hartwell, Cecil Courcier, Dave Shields, Brian Morrow, Jerry Buehre, Max 

Yates, Isaac Sheperdson (deceased), Joseph Caldwell 

(deceased), Albert Harrison, and Don Bondurant (retired) of the U.S. Army Corps of Engineers; Ernest 

Pemberton (retired), Willis Jones (retired), and James 

Blanton of the U.S. Bureau of Reclamation; Robert 

Wink of the U.S. Soil Conservation Service; and DeRoy 

Bergman, Lionel Mize, Harold Petsch, Bill Dein, Steve 

Blumer, Farrel Branson, Ivan Burmeister, Bob 

Liggett, John Borland, and John Joerns (retired) of the 

U.S. Geological Survey, for assistance in acquiring data 

and photographs and in making onsite observations. 

William Graf of Arizona State University, Ernest Pemberton, Wayne Dorough, and H. C. Riggs reviewed the 

manuscript and made many helpful and constructive 

suggestions. We also extend our warmest thanks to 

many others, too numerous to mention, who helped in 

various ways.

METHODS OF ANALYSIS AND DATA 

SOURCES

WATER DISCHARGE

Water-discharge data were available for all 21 sites 

from U.S. Geological Survey gaging-station records. 

These data were used to determine what effect the dam

had on the magnitude and frequency distribution of 

downstream flows. Comparison of pre-dam and postdam flow records of the nearest long-term gaging station downstream from the dam indicated the overall effect of the dam on downstream flow. However, any influence of the dam needs to be separated from other 

factors, such as regional climatic changes and upstream 

operations of man. Pre-dam and post-dam flow records 

were examined for the nearest gaging station both upstream and downstream from the dam. The "control" 

station upstream from the dam reflects to a significant 

degree the flows that would have occurred downstream 

from the dam if no dam had been built. A control station 

is most useful located as close as possible to the dam, 

as long as it is not within the backwater of the dam.

The flow record used for a damsite was the longest 

period common to both the downstream gaging station 

and the upstream control station. This common period 

sometimes was abbreviated to avoid the effects of a 

subsequently-built dam on the flow at one of the stations.

The flow characteristics examined in this paper include average daily flow (commonly called mean annual 

flow), average annual flood peak, and certain flow-duration features. The average daily discharge for a given 

year is computed by taking the average discharge during each day, adding these for 365 consecutive days, 

and dividing the total by 365. We averaged these annual 

figures for a number of years to get a representative 

average daily discharge for that period. Similarly, the 

instantaneous annual peak discharges were averaged 

for the period of interest. Flow-duration values used 

here are the discharges equaled or exceeded 5, 50, and 

95 percent of the time, where the duration curve is 

based on flow records for the appropriate period. These 

statistics represent only an approximate summary of 

flow characteristics and will not reveal changes in annual, seasonal, or daily mean flows. Daily variations, 

for example, can be large downstream from dams operated for power production.

SEDIMENT LOAD

Information about measured suspended-sediment 

loads before and after construction of dams is available 

for a few river reaches from U.S. Geological Survey 

and U.S. Army Corps of Engineers reports (some unpublished). Such data have been used in specific cases 

to show dam-related changes in suspended load.

BED AND BANK MATERIALS

Data on bed and bank materials were available for 

selected sites from research investigations or from preand post-engineering surveys for reservoir and dam
METHODS OF ANALYSIS AND DATA SOURCES

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DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

planning and design. These data have been supplemented by samples collected by the authors. In addition, the authors made pebble counts (Wolman, 1954) 

of coarse particles on the beds of several rivers 

downstream from dams. The results of these measurements are used to illustrate some aspects of channeland bed-material change.

MEAN BED ELEVATION

Mean bed elevation was determined from: (1) Measured cross sections; (2) published graphs of bed elevation at successive times after dam closure (Colorado 

River only); and (3) gage height-discharge relations at 

gaging stations.

MEASURED CROSS SECTIONS

The preferred method for determining mean bed elevation was based on plots of 248 resurveyed cross sections provided by the U.S. Army Corps of Engineers 

and U.S. Bureau of Reclamation. These 248 cross sections had been measured a total of 1,202 times. All measured cross sections were referenced to elevation above 

sea level (National Geodetic Vertical Datum of 1929). 

For each of the 1,202 cross-section surveys, we took 

from 15 to 30 elevation readings at equally-spaced intervals across the entire bed width, then averaged these 

readings for mean bed elevation.

Bars and unvegetated islands, indicated in field 

notes, aerial photographs, and published topographic 

maps, were included as part of the channel-bed data. 

In a few cases, the edge of a bar was high enough 

relative to the adjacent streambed that problems arose 

in defining the edge of the streambed or channel. The 

surveyors had the same difficulty.

The four chief sources of error or variability in determining mean bed elevation from plotted cross sections 

are: (1) Locations of placement of the stadia rod; (2) 

natural changes in the bed configuration with time; (3) 

recognition of the bed as opposed to the bank on the 

plotted cross section; and (4) operator error in choosing 

and averaging many bed elevations to get a mean value. 

Error due to location of the stadia rod can be assumed 

to be minor. Bed configurations do change with time, 

quite apart from scour and fill, because of passage of 

bedforms and redistribution of sediment. River surveys 

normally are conducted during low flow (wading conditions). Resurveys associated with the passage of a flood 

on the Colorado River near Lees Ferry, Arizona, 

showed about 2 m (meters) of change in mean bed level 

(Leopold and others, 1964, p. 228); low-flow resurveys 

of the present study undoubtedly involve changes considerably less than this. Exact error from changes in 

bed configuration with time is unknown. Recognition 

of the streambed and banks on plotted cross sections

was facilitated by the original notes of surveyors. 

Operator error was considered by comparing two 

operator's determination of the average of many elevations across the bed; differences of 0. to 0.4 m appeared, 

which is not a geomorphically significant error.

Because mean bed elevations naturally fluctuate with 

time at any alluvial cross section, fluctuations of less 

than about 0.1 or 0.2 m were considered insignificant 

in this study. Significance of a measured absolute 

change in bed elevation depends not only on measuring 

precision but also on the scatter in elevations, the rate 

of change of elevation with time, and the period of record. For example, for the magnitudes of changes occurring at one cross section downstream from Fort 

Peck Dam on the Missouri River, Wolman (1967, p. 

90) estimated that about 10 years of record would be 

needed to reliably show a degradation rate of 0.08 m/yr 

(meter per year), and 30 years would be needed to show 

a degradation rate of about 0.01 m/yr. These values 

will vary from site to site.

PUBLISHED GRAPHS OF BED-ELEVATION CHANGES

For an additional 39 resurveyed cross sections, 

downstream from Hoover, Davis, and Parker Dams on 

the Colorado River, U.S. Bureau of Reclamation reports for various years provide graphs of mean bed elevation versus time. We read bed elevations for selected 

times directly from the plotted curves, for all sections 

downstream from these dams. The authors of those reports derived the curves from measured cross sections 

by: (1) Planimetering the cross-sectional area below an 

arbitrarily chosen low bank-to-bank horizontal baseline, 

the elevation of which is constant with time for each 

site; (2) dividing this area by the baseline width (which 

stayed virtually constant with time), and (3) subtracting 

the mean depth thus obtained from the elevation of the 

baseline. In almost every case, no islands were present 

at the cross sections. The 39 cross sections in this category had been measured a total of 615 times. The total 

number of resurveyed cross sections for the study thus 

was 287, and these had been measured a total of 1,817 

times. On the average, then, each cross section in the 

study was measured about 6 times, at intervals ranging 

from about 1 to many years.

GAGE HEIGHT-DISCHARGE RECORDS (RATING TABLES)

Within the criteria listed earlier, the gage height corresponding to an arbitrarily chosen discharge is approximately proportional to the bed level. Lowering of such 

a gage height with time would indicate lowering of the 

streambed. For the reference discharge, a low flow is 

better than a high one, because the low-flow part of 

the gage height-discharge relation is more sensitive to 

changes in bed level and is better defined than the high-
METHODS OF ANALYSIS AND DATA SOURCES

flow part of the relation. (The elevation corresponding 

to zero discharge probably would be best, but it is not 

defined for many gaging stations.) Where possible, we 

used the discharge exceeded 95 percent of the time as 

the reference discharge. Where this discharge was not 

defined on a significant number of rating tables, the 

lowest discharge common to most of the tables was 

used.

Although this method can show general trends in bed 

elevation, it is not as accurate as measured cross sections. Water-surface elevations can be affected by 

changes in channel shape, channel roughness, and 

downstream features, even where width has remained 

approximately constant.

Where the rating-table method was used, a control 

station upstream from the dam, if available, also was 

examined. Control stations usually were located more 

than 10 km upstream from the dam in an attempt to 

avoid any effects of the reservoir.

CHANNEL WIDTH

Channel (banktop) width was measured directly from 

plotted cross sections. The survey notes in some cases 

were used to help define the banks. Defining the banks 

usually was not difficult.

Regulation of discharge by several dams reduced the 

channel-forming flows to such an extent that the postdam channel became narrower. The new banks, as well 

as the original banks, then appeared on a plotted cross 

section. In such instances, we measured the width between the newer banks, even though occasional flow 

releases could overtop those banks.

TIME ORIGIN OF CHANNEL CHANGES

As a preliminary step to constructing a dam across 

a channel, major or minor rearrangement of the stream 

and its channel usually is made. Thus, the normal movement of sediment and water are interfered with from 

the early stages of construction. Such interference can 

cause channel changes downstream. The extent of these 

changes will vary from one dam to another, according 

to the nature and rate of progress on the project. Several years usually are needed to complete construction 

and officially close a dam. Furthermore, storage in the 

reservoir generally begins before the dam is closed officially. The date of dam closure, therefore, may represent a rather belated time from which to date channel 

changes. A more logical date might be the date construction began. However, channel cross sections generally were not established at such an early stage. The 

available original cross-sectional measurements were 

made at times ranging from several years prior to the 

beginning of construction to a year or two after the 

dam was closed. The year of dam closure is used as

the reference date in this study because it is the only 

date commonly available to all sites. A cross-sectional 

measurement made no later than about 1 year after 

dam closure usually was accepted as representative of 

the channel at the time of dam closure.

VEGETATION

Analysis of vegetation changes in this study is limited 

to a gross quantitative approach, with little attention 

to individual plant types. Differences in vegetation 

cover for a number of study sites were determined in 

one or more of three ways: (1) Onsite mapping; (2) successive aerial photographs; and (3) successive ground 

photographs. Onsite, the simple method used consisted 

of comparing exposed areas of channel bars and islands 

clearly discernible on earlier aerial photographs with 

existing stands of vegetation in the same reaches. Mapping was confined to the channel itself and did not include the entire valley bottom.

VARIABILITY OF NATURAL CHANNELS

To evaluate the effect of manmade alterations on natural environment, the natural variability of an environment needs to be considered. A few observations of 

the characteristics and changes in alluvial rivers virtually unaffected by manmade structures are reviewed 

briefly here to provide a reference for subsequent 

analyses of apparent changes associated with dams.

Two kinds of variability are involved in any analysis 

of channel changes. First, at any time a channel's 

width, depth, and slope vary in space. For example, 

although the mean width of the Missouri River 

downstream from Garrison Dam in North Dakota in 

1957 was 415 m for a reach 87 km long, the standard 

deviation of 24 measurements was approximately 122 

m or 29 percent. The actual width ranged from a minimum of 255 m to a maximum of 845 m. This variability 

also shows that, in comparing present and past widths 

of the channels, a change needs to be demonstrable 

statistically and, thus, outside of the range of natural 

variability in any one set of measurements.

The second, more complex type of variability occurs 

with time at a given river cross section. Some selected, 

representative data from the literature on naturally-occurring changes in channel width and bed elevation are 

summarized in tables 1-3. These changes can be large. 

For example, within several weeks the Yellow River 

of China at any one spot may widen by as much as 

hundreds of meters (Chien, 1961). Another of the 

world's largest and most sediment-laden rivers, the 

Brahmaputra in India, also has extreme changes in 

width with time (Coleman, 1969). The rates of change 

range from a few meters to hundreds of meters per
DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS 

TABLE 1. Selected examples of rates of change of channel uridth in alluvial reaches

[ +, increase; -, decrease; USA, United States of America]

River, location

Brahmaputra River, Bangladesh

Brahmaputra River, Bangladesh

Brahmaputra River, Bangladesh

Brahmaputra River, Bangladesh

Brahmaputra River, Bangladesh

Brahmaputra River, Bangladesh

Brahmaputra River, Bangladesh

Brahmaputra River, Bangladesh

Katjuri River, India

Gila River, USA

Gila River, USA

Gila River, USA

Gila River, USA

Gila River, USA

Rio Salado, USA

Rio Salado, USA

Cimarron River, USA

Cimarron River, USA

Cimarron River, USA

Cimarron River, USA

Cimarron River, USA

Red River, USA

(average of 20 sites)

Red River, USA

(average of 20 sites)

Patuxent River, USA

Patuxent River, USA

Trinity River, USA

(many sites)

Approximate 

width of 

initial

channel 

(meters)

6,700

11,800

12,200

12,600

11,000

6,700

3,100

7,400

588

98

225

88

174

4.0

14.9

18

427

198

20

884

1,200

1,070

105

Flood changes Long

Change in width

Time tears ot

. . , .. observation (meters) (percent)

8

8

11

8

8

8

8

133

100

21

_ __ _ g

1

1

1

36

36

25

15

6

25

15

16

4

26 hours +6.2

About 200

days

0 to +45 0 to +43

[-term chanj

Rate of

(meters 

per

year)

+70

-65

0

+118

+98

+15

-42

+36

+16

-21

+12.5

-64

+85

-73

+4.3

+4.2

+16.4

-15.3

0

+34.6

-42.7

-25

+33

;es

change

(percent 

per

year)

+1.0

-.55

0

+.93

+.89

+.22

-1.4

+.49

-3.5

+12.8

-28

+97

-42

+108

+28

+91

-3.6

0

+173

-4.8

-2

+3

Reference

Coleman, 1969, p. 161.

Coleman, 1969, p. 161.

Coleman, 1969, p. 161.

Coleman, 1969, p. 161.

Coleman, 1969, p. 161.

Coleman, 1969, p. 161.

Coleman, 1969, p. 161.

Latif, 1969, p. 1689.

Inglis, 1949, p. 67.

Burkham, 1972, p. 5.

Burkham, 1972, p. 5.

Burkham, 1972, p. 5.

Burkham, 1972, p. 5.

Burkham, 1972, p. 5.

Bryan, 1927, p. 18.

Bryan, 1927, p. 18.

Schumm and Lichty, 1963, p.

Schumm and Lichty, 1963, p.

Schumm and Lichty, 1963, p.

Schumm and Lichty, 1963, p.

Schumm and Lichty, 1963, p.

Schumm and Lichty, 1963, p.

Schumm and Lichty, 1963, p.

Gupta and Fox, 1974, p. 503.

Gupta and Fox, 1974, p. 503.

Ritter, 1968, p. 17-52.

73-74.

73-74.

73-74.

73-74.

73-74.

86.

86.

TABLE 2. Illustrative examples of long-term aggradation and flood deposition in alluvial reaches unaffected by manmade works

[USA, United States of America; USSR, Union of Soviet Socialist Republics]

Flood deposition

River, location

Colorado River, USA

Yellow River, China

Alexandra-North Saskatchewan River, Canada

Kodori River, USSR

Last Day Gully, USA

Arroyo de Los Frijoles, USA

Nile River, Egypt

Mu Kwa River, Formosa

Brahmaputra River, Bangladesh

Rio Guacalate, Guatemala

James River tributaries, USA

Van Duzen River, USA

Little Larrabee Creek, USA

Trinity River tributaries, USA

Waiho River, New Zealand

Centre Creek, New Zealand

Time

6 months

2 hours

Several

hours

About 3

days

About 3

days

8 months

Depth

(meters)

_

0.6-8

1

0-1.5

.3-3

2.4

0-3.4

3-24

.44-. 55

Long-term

Years of

observation

31

358-2,400

32

11

6

1,900-2,800

3

aggradation

Rate 

(meters

per

year)

0.03

.03

.0007-. 003

.03

.006

.01

.00096-. 00 16

4

Reference

Cory, 1913, p. 1212.

Todd and Eliassen, 1940,

Smith, 1972, p. 182.

Mandych and Chalov, 1970,

Emmet t, 1974, p. 58.

Leopold and others, 1966,

Lyons, 1906, p. 315.

Lane, 1955, p. 745-747.

Coleman, 1969, p. 178.

Foley and others, 1978, p

Williams and Guy, 1973, p

Kelsey, 1977, p. 284-301.

Kelsey, 1977, p. 284-301.

Ritter, 1968, p. 53-54.

Gage, 1970, p. 621.

O'Loughlin, 1969, p. 697.

p. 446.

p. 35.

p. 219.

. 114.

. 42.

year; however, most of the changes are less than 1 percent of the channel width per year. The channel width 

of the Cimarron River in Kansas fluctuated significantly 

from 1874 to 1954 (Schumm and Lichty, 1963). Wolman 

and Gerson (1978) suggested that floods affect river

width more significantly in arid climates than in humid 

climates, but the magnitudes are not well-defined. A 

few instances are noted in table 1 for rivers comparable 

to those included in the present study. 

Natural bed aggradation measured during many
DOWNSTREAM EFFECTS OF DAMS 

TABLE 3. Illustrative examples of long-term degradation and flood erosion in alluvial reaches unaffected by mamnade works

[USA, United States of America; USSR, Union of Soviet Socialist Republics]

River, location

Castaic Creek, USA

Red Creek tributary, USA

2/ Beatton River, CanadaLena River, USSR-'

Klaralven River, Sweden

Trinity River and tributaries, USA

Wills Cove, USA

Yellow River, China

Pickens Creek, LSA

Centre Creek, New Zealand

Klaralven River, Sweden

-Estimated.

2/ 

 Classification uncertain.

Flood erosion

Depth Time , r .

(meters)

._

__

_

0-0.5

Several -^0-3

hours

About 12 5-9

hours

-''About 1 0-6

hour

8 months .2-. 5

About 1 4.7

month

Long-term

Years of

observation

1/100

about 815

250

20

about 7,000

__

degradation

Rate 

(meters

per

year)

0.01

.004

.011

.0005

.007

Reference

Lustig, 1965, p. 8.

LaMarche, 1966, p. 83.

Hickin and Hanson, 1975, p. 490.

Borsuk and Chalov, 1973, p. 461.

de Geer, 1910, p. 161.

Ritter, 1968, p. 54.

Williams and Guy, 1973, p. 35.

Todd and Eliassen, 1940, p. 376.

Troxell and Peterson, 1937, p. 93

O'Loughlin, 1969, p. 697.

de Geer, 1910, p. 174.

years can be very small (table 2). Examples are 0.0007 

to 0.0034 m/yr, or 1 m every 290 to 1,430 years 

(Alexandra-North Saskatchewan River, Canada) and 

about 0.001 m/yr, or 1 m every 1,000 years, for the 

Nile River near Aswan in Egypt. Values of about 0.03 

m/yr (1 m about every 30 years) have been given for 

the Colorado River in the United States, the Yellow 

River in China, and the Kodori River in the Soviet 

Union. The most rapid reported rate is about 4 m/yr 

for the Mu Kwa River in Formosa, where sediment 

from landslides during 3 years raised the streambed 

about 12 m.

In contrast to long-term average rates, bed aggradation during floods can be enormous. Some observed 

maximum depths of fill for a single flood are about 8 

m on the Brahmaputra River and 24 m in the Waiho 

River in New Zealand (table 2). Depths of 1 to 3 m 

are common for the cases reported in the literature.

Reported measurements of long-term natural degradation (table 3) range from about 0.0005 to 0.011 m/yr. 

For example, Borsuk and Chalov (1973) gave an averaged bed lowering of 0.0005 m/yr during 20 years for 

the Lena River, Soviet Union. LaMarche (1966) used 

vegetation to estimate an average of 0.004 m/yr during 

815 years for a small channel in Utah. The longest 

period examined seems to be 7,000 years by de Geer 

(1910), who counted varved clays and estimated an average bed degradation of 0.007 m/yr for the River 

Klaralven, Sweden. Hickin and Nanson (1975) reported 

an average degradation rate of 0.011 m/yr for the Beatton River, Canada, for 250 years.

During floods, streambeds in southern California in

a-matter of hours have eroded as much as 6 m (Troxell 

and Peterson, 1937), and the Yellow River in China 

has degraded by as much as 9 m (Todd and Eliassen, 

1940) (table 3). In some cases, the bed refills during 

the waning stages of the flood; in others, the bed refills 

during a number of years, and along some reaches the 

channel seems to be changed permanently.

Some data used in this study of channels downstream 

from dams may not demonstrate a cause and effect relation; instead, they may show a sequential or natural 

change. Cause and effect in certain cases needs to be 

inferred from the timing of the changes and from their 

nature and persistence; such proof can be demonstrated 

only occasionally. Commonly, the precise magnitude of 

the changes and the separation of manmade causes from 

those changes associated with climate and other natural 

phenomena may be difficult, as discussed below.

DOWNSTREAM EFFECTS OF DAMS 

WATER DISCHARGES

A number of papers in recent years (for example, 

Lauterbach and Leder, 1969; Moore, 1969; Huggins and 

Griek, 1974; DeCoursey, 1975; Petts and Lewin, 1979; 

and Schoof and others, 1980) have discussed the effects 

of dams on downstream flows. Because of the various 

purposes for which dams are built, there are large variations from one dam to another in the magnitude and 

duration of flow releases. At some dams (for example, 

Sanford Dam on the Canadian River, Texas, and Conchas Dam on the Canadian River in New Mexico), all
DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

or almost all the water is withheld from the 

downstream reach. Only drainage through the dam, 

tributary inflow, springs, ground water, and other 

downstream sources provide water downstream from 

the dam. At other dams, water is released only a few 

times per year. Discharge at hydropower dams may 

be stopped or curtailed for part of a day, and then a 

relatively large flow released during another part of 

the day (Fort Peck Dam on the Missouri River, Montana). At diversion dams, such as Milburn Dam on the 

Middle Loup River in Nebraska, large quantities of 

water may be diverted during the irrigation season, but 

all flows (and some sediment) may be passed directly 

through during the rest of the year. Even at dams built 

solely for irrigation, water may be released in a variety 

of patterns. At one extreme, virtually no water is ever 

released, and all irrigation diversions are made directly 

from the reservoir (Sanford Dam, Canadian River, 

Texas). Near the other extreme, practically no water 

is released during the winter storage period, but relatively large flows are released steadily during the irrigation season, with irrigation diversions made from various points downstream (Caballo Dam on the Rio 

Grande, New Mexico). Each dam, because of its purposes and the arrival of floods from upstream, has a 

unique history of daily, seasonal, and annual flow releases. Whatever the pattern of controlled releases, 

they are almost certain to be distributed differently 

from the natural flows.

The uniqueness of release policy at each dam precludes simple generalizations about the discharge distributions, except that flood peaks will be decreased (table 

4). For the 29 dams of table 4, average annual peak 

discharges were decreased to 3 to 91 percent of their 

pre-dam values (averaging 39 percent). The flow exceeded only 5 percent of the time was reduced in many 

(but not all) cases. High flows may be important, especially in controlling channel size and vegetation.

Average daily discharge in a reach may increase, remain the same, or decrease after a dam has been built 

(table 4). Low flows (equaled or exceeded 95 percent 

of the time) also were diminished in some instances and 

increased in others. Judging from the records at the 

control stations (table 4), some, and possibly all, of the 

changes in average daily flow (but not necessarily in 

other flow statistics) at a number stations in our sample 

would have occurred in the absence of regulation. 

Changes in climate, ground-water withdrawals, flow diversions, vegetation, or combinations of these factors 

could have been the causes.

SEDIMENT LOADS

In addition to changing the flow regimen, dams are 

effective sediment traps. The curtailment of sediment

supply, as with the change in water discharge, could 

have an important effect on the downstream channel. 

With some dams, such as those built mainly for hydropower generation, the sediment may be trapped as an 

incidental consequence of the dam's overall structure 

and operation. On other dams, sediment control may 

be a specific intent or purpose in building the dam. For 

example, Cochiti, Abiquiu, Jemez Canyon, and Galisteo 

Dams have been built on the Rio Grande and its major 

tributaries in an effort to reduce or eliminate aggradation on the Rio Grande.

A dam's role in trapping sediment can be shown by 

periodic reservoir surveys, by sediment-transport measurements, or by both. Sediment-transport measurements generally are given either as sediment concentrations (weight of sediment per unit volume of water-sediment mixture) or as annual sediment loads, in tons per 

year.

Hoover Dam on the Colorado River is a good example. Suspended loads in the Colorado River have been 

measured upstream and downstream from Hoover 

Dam. The upstream station is near Grand Canyon, 

Arizona, 430 km from the dam; the downstream station 

is near Topock, 180 km downstream from the dam. Two 

characteristics of the suspended load under natural conditions the large quantities and the very large annual 

variations are shown in figure 2. Before closure of 

Hoover Dam in 1936, annual loads at the two stations 

were similar. After closure, sediment inflow, represented by the data for the Grand Canyon station, continued to be large and variable. Downstream from the 

dam, at Topock, however, both the load and the annual 

variations were markedly decreased.

Data for several other dams also indicate a significant 

decrease in sediment load. For Glen Canyon Dam on 

the Colorado River (U.S. Bureau of Reclamation, 1976) 

the average annual pre- and post-dam suspended-sediment loads, as measured 150 km downstream at Grand 

Canyon, are as follows: Pre-dam (1926-62), 126 million 

megagrams; post-dam (1963-72), 17 million megagrams. 

This is a reduction of about 87 percent. On the Missouri 

River at Bismarck, North Dakota, 121 km downstream 

from Garrison Dam, sediment loads during 1949-52 averaged 48.6 million megagrams per year. The dam 

closed in 1953. During 1955, the sediment was 9.8 million megagrams, and during 1959, it was only 5.3 million 

megagrams. At Yankton, South Dakota, 7 km 

downstream from Gavins Point Dam, which began storing water in 1955, the Missouri River's pre-dam annual 

sediment load was about 121 million megagrams. The 

load then diminished to 8.1 million megagrams during 

1955 and was only 1.5 million megagrams during 1960.

Data for the above examples may not reflect accurately the actual trap efficiency, because the measuring
DOWNSTREAM EFFECTS OF DAMS 9

stations are a considerable distance downstream from 

the dam. The entrance of major tributaries, the erosion 

of sediment from the bed and banks immediately 

downstream from the dam, and various other factors 

(Howard and Dolan, 1981) can affect the apparent 

trends. Measurements made at or just downstream 

from the dam are much more suitable for an indication 

of trap efficiency. Such measurements show that the 

trap efficiency of large reservoirs commonly is greater 

than 99 percent. For example, during the first 19 years 

after closure of Canton Dam on the North Canadian 

River in Oklahoma, a total of 20.5 million megagrams 

of sediment arrived in the reservoir, and only 0.11 million megagrams went past the outlet works of the dam 

(U.S. Army Corps of Engineers, 1972, p. 6-8). The 

dam, therefore, trapped about 99.5 percent of the total 

sediment load. The trap efficiency of Denison Dam on 

the Red River, Oklahoma-Texas, during the first 12 

years after closure was 99.2 percent (U.S. Army Corps 

of Engineers, 1960, p. 11).

These examples illustrate the efficiency of dams that 

do not sluice appreciable volumes of sediment through 

the dam. Many diversion dams and some sediment-storage dams, however, are built and operated to permit 

sediment to be flushed out of the reservoir. For example, Milburn Dam on the Middle Loup River and other 

irrigation-type diversion dams such are those on the 

Rio Grande and Imperial Dam on the Colorado River 

are designed for flushing sediment either continuously 

or periodically through the dam to the downstream 

channel. Less commonly, a reservoir is emptied approximately once per year, such as at John Martin Dam 

on the Arkansas River in Colorado. The entire reservoir water storage at John Martin Dam typically has 

been released each spring during the irrigation season. 

The escaping water carves a channel in the stored sediment and transports sediment out with it. From 1943 

to 1972, the annual trap efficiencies at this dam varied 

randomly between 0 and 99 percent (U.S. Army Corps 

of Engineers, 1973). At John Martin Dam and at similar 

dams, annual trap efficiency (sediment storage) can 

vary with: (1) Volume of water stored during the winter 

and released (mainly a function of rainfall); (2) volume 

of sediment entering the reservoir since the previous 

year's release; (3) rate at which the reservoir release 

in made; (4) bottom topography of the pool (deep versus 

relatively shallow); (5) type and location of outlet gates; 

and (6) sizes of sediment particles (coarse versus very 

fine) entering the reservoir.

After dam closure, the downstream sediment loads 

at a particular site do not appear to recover from their 

greatly decreased values. Data on pre- and post-dam 

annual suspended loads were available for five stations 

downstream from Gavins Point Dam on the Missouri

River (U.S Army Corps of Engineers, unpublished 

data, various years). This dam does not sluice appreciable quantities of sediment. Further, several major reservoirs were built on the Missouri River upstream from 

Gavins Point Dam at about the same time that Gavins 

Point Dam was constructed. The post-dam annual loads 

for a given station downstream from Gavins Point Dam 

were relatively small and showed no significant change 

with time, for the 1 to 3 decades after dam closure 

for which data are available. Instead, the loads only 

fluctuate within the same relatively narrow range from 

year to year (as for the Colorado River downstream 

from Hoover Dam, mentioned above). Similarly, data 

from various sources show that sediment concentrations 

for a given discharge at four sites downstream from 

Canton Dam also have not changed significantly with 

time for as long as 3 decades (the period of record) 

after dam closure.

What river distance downstream from a dam is required for a river to recover to its normal pre-dam or 

upstream-from-the-dam sediment loads or concentrations? Sediment in the channel bed and banks and in 

tributary inflows are major factors in determining the 

length of channel needed. This distance for the North 

Canadian River downstream from Canton Dam is illustrated in figure 3. Upstream from the dam, at Seiling, 

Oklahoma, a given discharge transported about the 

same volume of sediment before and after the 1948 dam 

closure. Reduction in concentration 5 km downstream 

from the dam is dramatic. A significant post-dam decrease still is quite noticeable 140 km downstream from 

the dam. Even at Oklahoma City, 182 km downstream 

from the dam, sediment concentration for a given discharge is not as much as it was prior to dam construction. Finally at Wetumka, 499 km downstream from 

the dam, with a drainage area some 4,640 km2 (square 

kilometers) larger than that at the dam, sediment concentrations have recovered and may even be greater 

at high flows. Thus, the river required more than 182 

km, and possibly as much as about 500 km, of channel 

distance for bed and bank erosion, coupled with tributary inflows, to provide sediment concentrations equivalent to those transported in the same reach at a given 

water discharge prior to closure of Canton Dam.

Curves similar to those in figure 3 for the Red River 

downstream from Denison Dam (U.S. Army Corps of 

Engineers, 1960, plates 64 and 65) indicate the same 

order of magnitude of channel distance (or possibly even 

a longer required reach) for recovery of pre-dam sediment concentrations. At Arthur City, Texas, 150 km 

downstream from the dam, post-dam sediment concentrations for the 17 years after dam closure were only 

about 20 to 55 percent of the pre-dam concentrations 

for the same water discharge. At Index, Arkansas, 387
10 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 4. Water-discharge data

[km, kilometers; m3/s, cubic

Dam Year 

number River, dam, State of dam 

(fig. 1) closure

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

Colorado, Glen Canyon, 1963

Arizona

Colorado, Hoover, Arizona 1965

Colorado, Davis, Arizona 1950

Colorado, Parker, Arizona 1938

Jemez, Jemez Canyon, 1953

New Mexico

Arkansas, John Martin, 1942 

Colorado

Missouri, Fort Peck, Montana 1937

Missouri, Garrison, 1953

North Dakota

Missouri, Fort Randall, 1952

South Dakota

Missouri, Gavins Point, 1955

South Dakota

Medicine Creek, Medicine 1949

Creek, Nebraska

Middle Loup, Milburn, Nebraska 1955

Des Moines, Red Rock, Iowa 1969

Smoky Hill, Kanopolis, Kansas 1948

Republican, Milford, Kansas 1967

Wolf Creek, Fort Supply, 1942

Oklahoma

North Canadian, Canton, 1948

Oklahoma

River distance

 . . of station 

Downstream gaging station /

, , , I/ from dam and control station 

(km)

Colorado River at Lees Ferry, Arizona

C: No suitable station

Colorado River near Topock, Arizona

C: Colorado River near Grand Canyon, Arizona

Colorado River near Topock, Arizona

C: Colorado River below Hoover Dam, Arizona

Colorado River below Parker Dam, ArizonaCalifornia

C: Colorado River near Topock, Arizona

Jemez River below Jemez Canyon Dam,

New Mexico

C: Jemez River near Jemez. New Mexico

Arkansas River at Lamar, Colorado 

C: Arkansas River at LaJunta, Colorado

Missouri River near Wolf Point, Montana

C: No suitable station

Missouri River at Bismarck, North Dakota

C: No suitable station

Missouri River at Fort Randall, South Dakota

C: No suitable station

Missouri River at Yankton, South Dakota

C: Missouri River at Fort Randall,

South Dakota

Medicine Creek at Cambridge, Nebraska

Medicine Creek below H. Strunk Lake, Nebraska

C: No suitable station

Middle Loup River at Walworth, Nebraska

C: Middle Loup River at Dunning, Nebraska

Des Moines River near Tracy, Iowa

C: Des Moines River below Raccoon Pdver st

Des Moines, Iowa

Smoky Hill River near Langley, Kansas

C: Smoky Hill River at Ellsworth, Kansas

Republican River below Milford Dam, Kansas

C: Republican River at Clay Center, Kansas

Wolf Creek near Fort Supply, Oklahoma

C: No suitable station

North Canadian River at Canton, Oklahoma

C: North Canadian River at Woodward, Oklahoma

26

180

430

72

108

0-6.4

63

1.3

43

34 

70

100

120

0-11

8

110

10-15

0.8

19

31

19

94

1.3

48

2.7

49

2.6

.8

106

Period used 

(water yesrs)

P re-dam

1922-62

1923-34

1923-34

1935-49^'

1935-49-'

1936-37-'

1936-37-'

1937; 1944-52

1937-41; 195(£'

1914-41 

1914-41

1929-36

1929-52-'

1948-51-'

1948-54-'

1948-54-'

1938-48

-

1946-54

1946-54

1941-68

1941-68

1941-47

1941-47

1964-66

1964-66

1938-41

1939-47-'

1939-47-'

Post-dam

1963-78

1935-49

1935-49

1950-78

1950-78^'

1938-78

1938-78^-'

1953-78

1953-78

1942-55, 

1960-73 

1942-55, 

1960-73

1937-78

1953-78

1952-78

1955-78

1955-78^'

-

1951-78

1955-60

1955-60

1969-76

1969-76

1948-77

1948-77-'

1967-77

1967-77

1942-78

1948-78

1948-78
DOWNSTREAM EFFECTS OF DAMS 11

for pre-dam and post-dam periods

meters per second; C, control station]

Average 

discharge

Pre-dam

480

520

520

400

410

230

230

1.5

2.0

7.3

7.4

200

600

880

930

860

2.7

-

23

11.0

140

105

8.7

7.6

23

19.5

2.5

7.7

7.2

daily 

(m3/s)

Post-dam

320

400

500

340

360

340

380

1.5

1.9

4.8

6.6

280

660

680

740

670

-

1.9

22

11.5

200

155

9.9

8.9

24

23

1.7

4.7

5.0

Average annual peak 

3 discharge (m /s)

Pre-dam

2,200

2,200

2,300

640

650^

850

400

160

50

560

500

770

3,9003-/

6.3003-7

5.2003-'

5,1003-/

530

-

58

18

1,200

950

320

330

290

300

240

280

400

Post-dam

800

640

2,300

550

615*/

640

600

39

52

190

340

690

1,100

1,500

1,200

1,400

-

13.5

53

20

800

900

135

320

150

450

35

44

155

X " 5 g

Pre-dam

1,800

1,800

1,800

700

700

300

300

8.5

7.0

29

20

500

1,600

2,000

2,100

1,900

4.6

-

31

13.5

530

390

35

27

69

60

8.5

29

26

Flow equaled

ercent

Post-dam

560

700

1,800

560

590

630

620

6.2

6.7

16.0

20

630

1,100

1,400

1,400

1,300

-

8.2

30

14.5

560

560

54

37

90

79

6.0

26

19.5

or exceeded "x" percent 

(m3 /s)

x = 50 gercent

Pre-dam Post-dam

230 310

270 400

270 250

400 340

400 370

250 360

250 370

0.4 0.5

.9 .8

.2 .6

2.3 1.6

140 240

450 650

820 680

820 760

760 680

1.6

1.0

22 22

11.0 11.5

62 115

46 82

2.4 2.3

1.8 2.1

13.5 10.5

11.5 10.5

.5 .1

1.8 .2

1.2 1.1

of the time,

x = 95 gercent

Pre-dam Post-dam

100 31

120 145

105 120

145 140

155 140

125 140

135 155

0.006 0.0

.5 .4

.05 .07

.3 .4

70 40

140 250

195 155

250 220

220 145

0.8

.02

16.5 16.5

9.1 9.3

7.6 13.0

4.5 10.0

.5 .5

.4 .4

4.5 1.2

4.2 3.4

.006 .009

.0006 .03

.0006 .0
12 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 4. Water-discharge data for preRiver distance 

Dam Year 

number River, dam, State of dam Downstream gaging station o s a on 

, , . . . I/ from dam (fig. 1) closure and contro1 stat10"- 

(km;

18.

19.

20.

21.

~

~

--

Canadian, Eufaula, Oklahoma 1963 Canadian River near Whitefield, Oklahoma

C: Canadian River at Calvin, Oklahoma

Red, Denison, Texas-Oklahoma 1943 Red River at Denison Dam near Denison, Texas

C: Red River near Gainesville, Texas

Neches, Town Bluff, Texas 1951 Neches River at Evadale, Texas

C: Neches River near Rockland, Texas

Chattahoochee, Buford, 1956 Chattahoochee River near Buford, Georgia

Georgia C: Chestatee River near Dahlonega, Georgia

Rio Grande, Caballo, 1938 Rio Grande below Caballo Dam, New Mexico

New Mexico C: Not needed

Marias, Tiber, Montana 1955 Marias River near Chester, Montana

C: Marias River near Shelby, Montana

Canadian, Sanford, Texas 1964 Canadian River near Canadian, Texas 

C: Canadian River near Amarillo, Texas

Canadian, Conchas, New Mexico 1938 Canadian River below Conchas Dam,

New Mexico

C: Canadian River near Sanchez, New Mexico

Canadian, Ute, New Mexico 1963 Canadian River at Logan, New Mexico

C: Canadian River below Conchas Dam,

New Mexico

Republican, Trenton, Nebraska 1953 Republican River at Trenton, Nebraska

C: Republican River at Benkelman, Nebraska

Republican, Harlan County, 1952 Republican River near Hardy, Nebraska

Nebraska C: Republican River near Orleans, Nebraska

Washita, FOBS, Oklahoma 1961 Washita River near Clinton, Oklahoma

C: Washita River near Cheyenne, Oklahoma

13

108

0.5-4.0

106

93

63

4.0

73

1.3

3-8

65

120 

47

4.5-5.6

50

3.2

112

1.5

50

115

37

43

112

Period used 

(water years)

Pre-dam

1939-62-/

1939-62^

1937-42

1937-42

1922-50

1922-50

1942-55

1942-55

1946-47

1946-47

1939-63-/ 

1939-63-/

1937-38

1937-38

1943-62

1943-62

1948-52-/

1948-52

19 4 8-5 1-'

1948-5 1-''

1938-60

1938-60

Post-dam

1963-78

1963-78^

1943-78

1943-78^

1951-64

1951-64

1956-71

1956-71

1939-77

1956-78

1956-78

1964-78^ 

1964-7^

1943-72

1943-72

1963-72

1963-72

1953-78

1953-78

1952-77

1952 -7 1-1

1961-78

1961-78

 A long-term gaging station upstream from the dam.

2 /

 llain station is downstream from dam and control station is upstream from dam on same river.

 Flows affected by one or more upstream dams.

4/

 Highest mean daily flow used, rather than instantaneous peak.

 Only years available.

 Only data for water years 1961-78 available.

 Only data for water years 1921 and 1946 used (only data available).

of

 All post-dam flow is from seepage at dam and from springs and tributaries downstream from dam; no releases 

are made from the dam.

9/

 Water years 1936-39 (before dam closure) used.

km downstream from the dam, a given water discharge 

after dam construction transported about 50 percent of 

the volume of sediment it did before the dam. On the 

Red River, too, then, the deficit persists for hundreds

of kilometers. The actual length of reach required for 

complete recovery on the Red River cannot be determined from the above data. 

Five stations downstream from Gavins Point Dam
DOWNSTREAM EFFECTS OF DAMS 13

dam and post-dam periods Continued

Average 

discharge

Pre-dam

175

56

185

120

200

77

60

10.0

-

25

25

16.0

12.0

12.0

6.5

3.5^

l.J*/

6.0

3.4

32

16.5

4.1

1.2

daily 

(m3/s)

Post-dam

120

29

120

70

130

48 '

54

10.5

24

26

27

2.6

5.4

.8

4.3

1.2

.3

1.8

2.4

11.0

8.0

1.6

.4

Average annual peak 

discharge (m /s)

Pre-dam

3,600

2,300

3,000

2,400

1,100

550

660

220

-

115^

115^

1,100

1,100

1,000^

64<£'

550

145

290

105

530

380

290

290

Post-dam

740

1,300

950

1,400

800

360

270

260

77

90

560

370

640

100

420

66

32

24

60

185

125

69

66

Flow equaled or exceeded ' x" percent 

(m3 /s)

x_= 5 f

Pre-dam

760

270

720

540

700

290

140

23

-

79

76

66

48

35

22

14.0

1.7

16.0

7.7

135

52

15.5

4.0

lercent

Post-dam

480

120

400

280

500

195

150

24

63*/

74

105

7.6

24

1.1

16.0

8.6

.2

7.1

4.9

42

23

5.4

1.4

x = 50 percent

Pre-dam Post-dam

48 54

9.6 6.8

56 74

26 19.0

93 54

31 17.5

46 40

7.6 8.2

16.<£'

11.6 18.0

11.5 11.5

.9 .7

.7 .6

.5 .1

.6 1.0

.2 .07

.2 .1

4.0 0.08

2.8 2.3

14.5 4.8

9.3 5.4

.7 .7

.2 .1

of the time ,

x = 95 percent

Pre-dam Post-dam

3.1 1.

.3

7.1 3.

3.1 4.

8.8 5.

1.2

19.0 12.

2.8 3.

-

4.2 2.

4.2 4.

.01

.08

.02

.05

.006

.01

.001

.3

4.0 1

2.0

.01

.0003

,6

,1

,2

2

9

7

0

4

03^

8

0

003

05

009

02

02

05

.02

.03

.6

.5

.1

.0

provide an example of the degree of downstream recovery of suspended-sediment loads. Three dams Fort 

Randall (1952), Garrison (1953), and Gavins Point 

(1955) were closed on the Missouri River within 3 

years during the 1950's. Inspection of the yearly sediment data downstream from Gavins Point Dam shows 

that annual loads consistently decreased during this 

period (water years 1953-56), as expected. These years 

were excluded here in computing pre- and post-dam average loads. For the five downstream stations, the 

available water years of pre- and post-dam data, respectively, were: Yankton 1940-52, 1957-69; Omaha 

1940-52, 1957-73; St. Joseph, Kansas City, and Hermann 1949-52, 1957-76. These stations are 8 

(Yankton), 314 (Omaha), 584 (St. Joseph), 716 (Kansas 

City), and 1,147 (Hermann) km downstream from Gavins Point Dam. From the annual suspended-sediment 

loads, an average annual load was computed for the 

pre-dam period and again for the post-dam period. The 

ratio of these average loads as a function of distance 

downstream from the dam is shown in figure 4. At 

Yankton, just 8 km downstream from the dam, the average post-dam annual load was less than 1 percent of 

that for the pre-dam period. Even 1,147 km
14 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

500

or

<

LU

>-

or LU

Q. 

GO

or 

(D

o

2 

O

_i _j

^ 

2

<fe Q

O

_i

I 

2

LU

^

Q

Q

LU

Q

Z

LU 

Q_ 

C/5

400

300

200

100

I I

GRAND CANYON STATION, 

ABOUT 430 KILOMETERS 

UPSTREAM FROM HOOVER DAM

TOPOCK STATION, 

ABOUT 180 KILOMETERS 

DOWNSTREAM FROM 

HOOVER DAM

YEAR OF DAM CLOSURE

1925 1930 1935 

WATER YEAR

1940 1945

FIGURE 2. Variation in annual suspended-sediment loads before and 

after closure of Hoover Dam, Colorado River, Arizona, at a station 

upstream from the dam (Grand Canyon) and downstream from the 

dam (Topock).

downstream from the dam, the post-dam average annual load was only 30 percent of the pre-dam load. Data 

from the Mississippi River at St. Louis, downstream 

from the confluence of the Missouri and Mississippi Rivers, and about 1,300 km downstream from Gavins Point 

Dam, show that the mean annual suspended load decreased from about 320 million megagrams during 1949- 

52 to 109 million megagrams during 1957-80, after closure of the dams on the Missouri River. Changes 

elsewhere in the Mississippi River basin also may have 

contributed to the decrease in sediment load in the 

Mississippi. However, along the nearly 1,300 km of the 

Missouri River downstream from Gavins Point Dam, 

post-dam average suspended loads have not approached 

the much larger pre-dam average values.

Hammad's (1972, p. 601) data for the Nile River 

downstream from Aswan High Dam show that, even 

965 km downstream from the dam, annual sediment 

loads 2 years after dam closure were only about 20 percent of pre-dam values. The above examples indicate 

that, in some major rivers, sediment concentrations and

annual sediment loads may not achieve pre-dam values 

for hundreds or thousands of kilometers downstream, 

if at all. The cases documented here are large dams 

and reservoirs from specific geographic regions. As 

noted earlier, a variety of conditions will control the 

response on different rivers.

MEAN BED ELEVATION

The results of the bed-elevation analyses based on 

resurveyed cross sections are listed in table 13 at the 

end of this report. The data for changes in mean bed 

elevation determined from gaging-station rating tables 

are listed in table 14 also at the end of this report. 

From the latter rating tables, changes in mean bed elevation with time were plotted for each gage site (figs. 

36-49 at the end of this report). (Such graphic relations 

proceed in "stairsteps" because a constant bed elevation 

is assumed for the period during which a given rating 

table is in effect. The change to a new rating table 

brings what appears in figures 36-49 as a sudden switch 

to a new constant bed level. The actual change in bed 

level with time probably follows a smoother curve.) 

Similar plots of change in bed level with time were 

made for all resurveyed cross sections from the voluminous data in table 13, and representative examples 

are shown below.

GENERAL NATURE OF CHANGES 

IN BED ELEVATION

For all 21 channels (fig. 1) having resurveyed cross 

sections, a lowering of the mean bed level here called 

degradation occurred immediately downstream from 

the dam (figs. 5 and 6), unless constrained by very 

coarse material or bedrock. Such bed degradation 

downstream from dams is a well-known phenomenon 

on alluvial streams (Lane, 1934; Gottschalk, 1964, p. 

17-5). Analytical studies of open-channel bed degradation include those by Lane (1948), Mostafa (1957), Tinney (1962), Breusers (1967), Komura and Simons (1967), 

Aksoy (1970, 1971), Hales and others (1970), Komura 

(1971), Rzhanitzin and others (1971), de Vries (1973), 

Hwang (1975), and Strand (1977). Special flume studies 

of bed degradation have been conducted by Schoklitsch 

(1950), Harrison (1950), Newton (1951), Ahmad (1953), 

Willis (1965), Garde and Hasan (1967), Ashida and 

Michiue (1971), and others.

In some reaches, degradation can occur simply by 

the removal of bars in the absence of replenishment 

of sediment from upstream. This was observed on the 

Red River in the region about 10 to 15 km downstream 

from Denison Dam. Koch and others (1977) reported 

a similar removal of bars in the reach downstream from 

Yellowtail Dam on the Bighorn River, Montana.
DOWNSTREAM EFFECTS OF DAMS 

2.0

15

Q

LJJ to

o

cc.

1.0

0.5 -

0.0 

2.5

2.0

1.5

1.0

0.5

0.0

After Canton Dam

CONTROL STATION,

45 kilometers upstream from dam

5 kilometers downstream from dam

Before Canton Dam-

-After Canton Dam 

rrrf , i , ,

1.5

1.0

0.5

0.0 

1.0

0.5

0.0 

1.0

0.5

0.0

140 kilometers downstream from dam

After Canton Dam

I I I I I 1TT| I I T I I I I 11

182 kilometers downstream from dam

Before Canton Dam

After Canton Dam

 nj i i . i . ... |

499 kilometers downstream from dam

Before Canton Dam

"After Canton 

. . . ...I

Dam

10 100 1000 10 100 1000

DISCHARGE, IN CUBIC METERS PER SECOND

FIGURE 3. Suspended-sediment loads (concentrations) transported by various discharges at successive downstream stations before and after 

closure of Canton Dam, North Canadian River, Oklahoma. Control-station curve based on unpublished U.S. Geological Survey data; 

other curves redrawn from U.S. Army Corps of Engineers, 1958.

On four rivers Jemez River, Arkansas River, Wolf 

Creek, and the North Canadian River post-dam flow 

releases were so much less than pre-dam discharges 

that the channel became considerably narrower. In such 

instances, lowering of the mean bed elevation can result 

not only from bed erosion but also because the post-dam 

narrowed river occupies only the lowest part of the 

original channel.

Should a dam release little or no water, the bed 

downstream might not degrade. In fact, local aggradation sometimes occurs, because the controlled flows are 

not strong enough to remove deposits left by tributary 

flash-floods; by main-channel, sediment-removal works 

associated with canals; or by wind. Examples are found 

on the Rio Grande in New Mexico (Lawson, 1925;

Lagasse, 1980) and on the Peace River in Canada (Bray 

and Kellerhals, 1979). Downstream from Elephant 

Butte Dam on the Rio Grande the controlled releases 

are depleted systematically by irrigation intakes. In a 

reach beginning about 265 km downstream from the 

dam this decrease in flow strength, together with the 

deposits delivered by tributaries, brought the river bed 

in many places to an elevation higher than the adjoining 

farm area (Lawson, 1925).

DEGREE OF CHANGE ATTRIBUTABLE TO DAMS

The magnitude of the measured changes (as much 

as 7 m, as described below) greatly exceeds the expectable errors in measurement and analysis. Furthermore, 

occurring as they do during periods ranging from a few
16 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

~ 0.30

Q 0.25

0.20

o.,B

o

o.io -

°-05

0.00

.X'

1 1

Mississippi River 

downstream from 

confluence with IVIissouri River

1 1 1 1

200 400 600 800 1000 1200 

KILOMETERS DOWNSTREAM FROM DAM

1400

FIGURE 4. Post-dam/pre-dam ratio of annual suspended-sediment 

loads versus distance downstream from Gavins Point Dam, Missouri 

River, South Dakota.

FIGURE 5. Time progression of bed degradation and channel armoring at the streamflow-gaging station downstream from Jemez Canyon Dam, Jemez River, New Mexico. A, 1952; B, 1957; C and

years to 2 or 3 decades, the changes greatly exceed 

those that would be expected as part of a temporal fluctuation around a mean bed level and those generally 

observed to occur naturally (table 3). Several considerations support the view that the measured changes in 

alluvial channels downstream from the dams studied 

here are due to the dam and reservoir upstream:

1. As the longitudinal profiles discussed below show, 

degradation generally was greatest at or near the 

dams and usually decreased somewhat progressively downstream, though with local exceptions.

2. From the rating tables for the 14 streamflow-gaging 

stations downstream from dams (table 14 and figs. 

36-49), the relation of water-surface elevation to 

discharge for a reference low flow indicates that 

the channels generally were relatively stable prior 

to dam construction and began degrading just 

after the dams were built. This timing is illustrated by the bed changes, as assumed from the 

stage-discharge relation, for the Smoky Hill River 

near Langley, Kansas, about 1.3 km downstream 

from Kanopolis Dam (fig. 7).

3. Whereas the river bed downstream from the dam 

tended to erode, the elevation of the bed at eight

D, 1980. Dam was closed in 1953. Station is 1.3 kilometers 

downstream from the dam. White dashed line is at a constant elevation for reference.
DOWNSTREAM EFFECTS OF DAMS 17

FIGURE 6. Degradation represented by successively lower water-intake pipes for streamflow-gaging station 2.6 kilometers downstream 

from Fort Supply Dam, Wolf Creek, Oklahoma. Dam was closed 

in 1942; photograph was taken in 1951.

control stations upstream from dams for which 

data were available did not change significantly 

during the years after dam closure (fig. 7; table 

14).

4. For the channels having resurveyed cross sections, 

and for those with gaging-station records, extrapolation of the post-dam degradation rates back 

into the pre-dam years would place the pre-dam 

streambeds at unrealistically high elevations. 

Thus, the timing, magnitude, and spatial distribution 

of the measured changes in the alluvial channels studied 

here indicate that the dams and upstream reservoirs 

are responsible for the measured degradation.

DEGRADED REACH DOWNSTREAM FROM A DAM

Given the capacity of flow releases to entrain sediment from channel bed and banks, erosion of the bed 

and banks should continue downstream from a' dam 

until some factor or a combination of factors results 

in establishment of a new stable channel. These factors 

may include: (1) Local controls of bed elevation 

(emergence of bedrock; development of armor by winnowing of fines); (2) downstream base-level controls 

(ocean, lake, or larger river; manmade structure such 

as a dam; barrier of deposited sediment); (3) decrease 

in flow competence (flattening of slope by progressive 

degradation; expansion of channel width, resulting in 

decreased depth, redistributed flow velocities, or both); 

(4) infusion of enough sediment to restore the balance 

between arriving and departing sediment (upstream 

erosion; sluicing from the upstream dam; inflow from 

tributaries); and (5) growth of vegetation. Several of 

these changes or processes are considered or illustrated

£ 0.5

a z- -0.5 s l

Z I 

LU ^ -1.0

-1.5

i i i r

,DAM CLOSURE

UPSTREAM GAGE

DOWNSTREAM 

GAGE

1940 1945 1950 1955 1960 1965 1970 1975 1980

WATER YEAR

FIGURE 7. Changes in mean bed elevation with time at streamflowgaging stations 48 kilometers upstream and 1.3 kilometers 

downstream from Kanopolis Dam, Smoky Hill River, Kansas. Bed 

elevations assumed proportional to the gage height that corresponds to a constant low discharge of 0.42 cubic meter per second 

at the upstream gage and 0.51 cubic meter per second at the 

downstream gage.

separately here; any number of them may occur together along any given reach of river.

GENERAL FUNCTION OF DEGRADATION 

WITH TIME AT A SITE

Except for cross sections underlain only by sand beds 

of unlimited depth, the degradation rate at a section 

would be expected to decrease with time as the bed 

becomes armored, or until the channel slope in that 

reach becomes too flat for the bed material to be 

moved. Eventually, an equilibrium bed elevation should 

be reached, as postulated in many analytical studies 

cited earlier. A number of the sections for which data 

are given in table 13 show this trend. At many other 

cross sections, however, the rate of degradation with 

time varies considerably (table 13). For instance, one 

or more temporary periods of aggradation may be included within a long-term trend of degradation. Or, 

after some initial degradation, the bed level may become constant rather abruptly with time at a certain 

depth, probably an indication that bedrock was reached 

or that armor had developed. Other sites have an Sshaped curve, where initial degradation rates were 

slow, then increased with time for some years, and then 

reversed this trend to decrease in later years. (A possible cause of such a curve might be minimal releases 

the first few years after dam closure to fill the reservoir, and greater releases thereafter.) Some of these 

irregular degradation-time trends are shown in figure 

8. Besides variations in flow releases with time, departures from a regular degradation curve could be due 

to differences in bed material with depth, to changes 

in cross-sectional shape, and to development and death 

or eradication of vegetation.
18 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

Colorado River, Arizona 

13 kilometers downstream 

from Glen Canyon Dam Colorado River, Arizona 

16 kilometers downstream 

from Glen Canyon Dam

Q 

Qa 2

Jemez River, Mew Mexico 

2 4 kilometers downstream 

from Jemez Canyon Dam

Missouri River, Montana 

45 kilometers downstream 

from Fort Peck Dam

lissouri River, North Dakota 

17.5 kilometers downstream 

from Garrison Dam

Missouri River, Morth Dakota

28 kilometers downstream 

from Garrison Dam

Missouri River South Dakota 

1 6 kilometers downstream ~~ 

from Fort Randall Dam

Missouri River, South Dakota 

16.5 kilometers downstream 

from Gavins Point Dam

Morth Canadian River, Oklahoma 

58 kilometers downstream 

from Canton Dam

I I I 

Red River, Oklahoma-Texas 

3.2 kilometers downstream 

from Denison Dam

X. 1 . J - 1

Red River, Oklahoma-Texas 

27 kilometers downstream _ 

from Denison Dam

1 1 1

Chattahoochee River, Georgia 

5.8 kilometers downstream 

from Buford Dam

10 20 30 40 0 10 20 30

YEARS AFTER DAM CLOSURE

40 0 10 20 30 40

FIGURE 8. Examples of irregular rates of bed degradation with time. Data from table 13.

To search for a possible general function of bed degradation with time at a site, all 287 resurveyed cross 

sections were used except for those:

1. That did not show a general trend of bed lowering 

(84 sections, mostly in the zone of varied bed 

changes downstream from the degraded zone);

2. Of the remaining 203 sites that lacked enough data 

points to justify fitting a curve, our arbitrary requirement being at least three resurveys after the 

onset of degradation, not counting the original survey (49 sites); and

3. Survivors of the above two requirements that 

showed marked aberrations in general degradation, such as abrupt cessation of bed erosion or

even substantial aggradation after inital erosion

(40 sites, exemplified in figure 8). 

One hundred fourteen cross sections were left, after 

the above exclusions, for use in the degradation-time 

analysis. Plots of bed lowering with time (representative examples given below) for the 114 cross sections 

generally show that the rate of degradation is fastest 

immediately after erosion begins and gradually slows 

with time, becoming asymptotic toward some new stable bed elevation. Some of the plots have large scatter, 

scarcity of points, or an irregular trend; any of several 

types of functions could be fitted to such data with a 

large standard error. Empirically analyzing the trends 

for the more regular, better-defined curves, either of
DOWNSTREAM EFFECTS OF DAMS 19

two functions appeared to fit most cases: (1) 

Logarithmic, with degradation D (arithmetic scale) as 

a function of the logarithm of time, t; or (2) hyperbolic, 

with l/D as a function of 1/t, both on arithmetic scales.

Least-squares regressions were calculated applying 

each of these functions to each of the 114 cross sections. 

For the prediction of D (not l/D) at the observed times, 

the square of the correlation coefficient (r2) is as follows: for the logarithmic function, a range of 0.16 to 

1.00 with an average of 0.82; for the hyperbolic function, a range of 0.10 to 1.00 with an average of 0.81. 

The average of 0.82 corresponds to an r-value of about 

0.91, and the average of 0.81 corresponds to an r-value 

of about 0.90; they indicate a reasonably good fit.

The logarithmic relation had a greater r2 for 55 of 

the 114 cross sections; the hyperbolic relation had a 

greater r2 for 50 sections; and, at the 9 remaining cross 

sections, r2 was- the same for both equations. In most 

cases, there was little difference in the two correlation 

coefficients for a given cross section. However, at a 

few cross sections the hyperbolic equation predicted 

values of D that diverged greatly from the measured 

values; whereas, no such grossly disparate predictions 

resulted from the logarithmic relation.

Analytical considerations indicate that the bed erosion should decrease with time. The degradation-time 

plots indicate that this decrease or cessation of degradation tends to occur within decades or a few centuries. 

For the data of this study, the hyperbolic equation generally predicts this approximate time much more closely 

than the logarithmic equation, the latter in some instances predicting billions of years for degradation to 

cease.

The relative advantages of using each type of equation seem to be:

Logarithmic Equation:

1. Slightly but probably not too significantly greater 

correlation coefficient.

2. Reasonable predictions for a few cross sections at 

which the hyperbolic equation gives a very poor 

fit.

Hyperbolic Equation:

1. Better calculated-versus-measured agreement of the 

time within which approximate eventual maximum 

degradation occurs, and of the magnitude of this 

maximum limiting degradation.

2. The practical benefit of providing a reasonable value 

of maximum degradation for planning purposes.

3. Better consistency in the sign of the first coefficient 

(intercept) of the regression equation. (With degradation considered positive, only 3 of the 114 

cross sections have a negative coefficient using the 

hyperbolic equation, as opposed to 24 such cross

sections for the logarithmic equation. Reasons for 

such negative coefficients are mentioned below.) 

On the basis of the previous discussion, the hyperbolic equation seems more suitable as a model for bed 

degradation with time at the many sites analyzed here. 

This equation has the form:

D= t

C i+c2 t

(D

where

D is degradation, in meters, at t years after the start 

of bed erosion; and

GI and c2 are constants for a given cross section or 

graph.

Such a hyperbola is asymptotic to a line parallel to the 

x axis (time). Its equation (eq. 1) plots as a straight, 

line on arithmetic scales when written in the form:

(l/D) = (lit) (la)

where

c2 is the intercept; and

Ci is the slope of the best-fit straight line. 

For convenience, degradation is considered to be in a 

positive direction. Degradation can be considered negative simply by making the signs of c l and c2 negative. 

Regression coefficents for each cross section are given 

in table 5.

The reciprocal of c2 is the asymptote on a plot of 

D versus t; that is, l/c2 is the eventual limit of degradation. The reciprocal of GI on the same plot is the slope 

or tangent just after degradation has first begun; that 

is, I/GI is the initial degradation rate, in meters per 

year. Therefore, both c l and c2 have an important practical significance.

To fit a curve of this type, the time origin (t=Q years) 

needs to be taken as the year at which degradation 

began. Degradation at sites close to the dam can be 

taken as beginning at the time of dam closure. However, for some downstream sites, there is a response 

time or lag time between the date of dam closure and 

the start of degradation. In some instances the year 

in which degradation began was not determined accurately and had to be estimated from the plots of degradation versus time.

Twelve typical curves, using the hyperbolic function 

as the general model, are shown in figure 9. The data 

to which the least-squares regressions were applied are 

listed in table 13. These particular examples were chosen to reflect the range of r2 values of the 114 applicable 

cross sections.
20 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 5. Values associated with fitted degradation curves1

Distance Slope llaxof cross Correl- of imum Estimated Estimated

section Response ation Inter- best- expected time to time to 

11

from (years) icient ' c. straight dation 0.95 D 0.5 D 

dam r line D (years) (years) 

(kilometers) c (meters)

Colorado River, Arizona, Glen Canyon Dam

2.6 0 0.88 0.30 2.03 3.3 130 7 

4.3 0 .77 .24 .75 4.2 60 3 

6.4 0 .93 .52 1.78 1.9 65 3 

10.5 2.0 .99 .33 2.54 3.0 150 8 

19.5 2.0 1.00 .29 1.93 3.5 130 7

Colorado River, Arizona, Hoover Dam

2.3 .5 .69 .52 .57 1.9 20 1.1 

3.2 0 .87 .19 .17 5.3 17 .9

6.1 0 .56 .41 .18 2.4 8 .4 

7.1 0 .84 .27 .26 3.7 18 1.0

8.0 0 .77 .23 .14 4.4 12 .6 

9.7 0 .94 .18 .56 5.6 60 3 

11.0 0 .94 .48 .59 2.1 24 1.2 

12.5 0 .73 .18 .23 5.6 24 1.3 

13.5 0 .54 .32 .16 3.1 10 .5

15.5 0 .93 .18 .26 5.6 28 1.4 

16.5 .5 .79 .31 .20 3.2 12 .6 

18.0 .5 .92 .37 .25 2.7 12 .7 

19.5 .5 .74 .25 .23 4.0 18 .9 

21 .5 .84 .36 .34 2.8 18 .9

28 1.5 .98 .20 1.38 5.0 130 7 

36 1.0 .91 .22 1.97 4.6 170 9

42 .5 .84 .34 1.62 2.9 90 5 

51 2.0 .88 -.05 3.40 

57 1.3 .91 .10 2.30 10.0 440 22

63 2.0 .80 .18 1.31 5.6 140 7 

70 1.0 .94 .18 1.05 5.6 110 6

77 2.6 .93 .28 1.93 3.6 130 7 

87 3.5 .96 .35 1.90 2.9 100 5 

94 3.0 .84 .20 1.71 5.0 160 9 

104 3.0 .94 .23 1.65 4.4 140 7

117 4 .98 .031 2.00 32 1,200 65 

Colorado River, Arizona, Davis Dam

1.1 0 0.97 0.15 0.69 6.7 85 5 

8.8 .5 .95 .32 2.46 3.1 150 8

Colorado River, Arizona, Parker Dam

27 0 .97 .26 1.29 3.9 95 5 

39 2.0 .95 .19 1.05 5.3 100 6

46 1.0 .93 .17 1.29 5.9 140 8 

66 1.0 .77 .20 1.31 5.0 120 7 

80 1.75 .66 .45 1.11 2.2 45 2.5 

95 1.0 .63 .43 2.30 2.3 100 5

Arkansas River, Colorado, John Martin Dam

12.0 6 1.00 .57 13.29 1.8 440 24 

15.5 6 1.00 .66 10.02 1.5 290 15 

22 3.0 .77 .73 5.55 1.4 140 8 

26 7 .91 .94 6.10 1.1 120 6

9.2 0 .10 1.28 1.97 .78 30 1.5 

13.0 0 .64 .74 11.45 1.4 290 15 

16.5 0 .48 .58 6.33 1.7 210 11 

23 9 .83 .49 6.18 2.0 240 13

Missouri River, North Dakota, Garrison Dam

2.7 0 .99 .13 4.86 7.7 710 38 

6.4 0 1.00 .17 2.06 5.9 230 12

8.0 0 .90 .33 1.22 3.0 70 4 

10.5 0 .71 .46 .89 2.2 36 1.9 

12.0 2.0 .87 .39 3.09 2.6 150 8

15.0 2.0 .92 .60 4.99 1.7 160 8

32 0 .28 1.82 .77 .55 8 .4 

36 0 .82 .66 2.21 1.5 65 3 

38 0 .37 1.65 3.49 .61 40 2 

51 1.0 .37 .73 2.09 1.4 55 3

A small r2 (presumably indicative of a minimum correlation between variables for the type of function being 

used) can result not only from large scatter about the

TABLE 5. Values associated with fitted degradation 

curves1 Continued

Distance Slope Maxof cross Correl- of imum Estimated

from (years) icient-'' c 9 straight dation 0.95 Dmax 

dam r2 line D^ (years) 

(kilometers) ^ (meters)

Missouri River, South Dakota, Fort Randall Dam

3.1 0 0.77 0.74 4.25 1.4 HO 

4.2 1.0 .90 .70 3.23 1.4 90 

5.1 2.5 .77 1.26 3.61 .8 55 

6.6 1.0 .84 .40 4.59 2.5 220 

7 7 1.0 .81 .52 3.46 1.9 130 

11.0 0 .33 .52 .48 1.9 18 

12.5 2.0 .50 .96 1.52 1.0 30

Missouri River, South Dakota, Gavins Point Dam

23 0 .86 .33 2.37 3.0 140 

34 0 .99 .25 3.78 4.0 290 

4.3 4 .98 .28 3.71 3.6 250 

5.3 0 .99 .026 8.86 38 6,500 

(,'.8 0 .87 .44 10.59 2.3 460 

79 0 .96 .30 7.84 3.3 500 

8.4 0 .13 .88 1.04 1.1 22 

8.5 3.5 .98 .18 5.76 5.6 610 

» 95 o .94 .23 10.35 4.4 860 

llio 0 .75 .82 6.68 1.2 160 

12.5 0 .45 .92 6.72 1.1 140 

36 0 .48 .60 5.03 1.7 160 

44 0 1.00 1.51 12.53 .66 160

Middle Loup River, Nebraska, Milburn Dam

.2 0 .59 .48 .24 2.1 10 

1.6 6.3 .97 .64 2.76 1.6 80 

5.6 6.3 .98 .78 2.00 1.3 48

Smoky Hill River, Kansas, Kanopolis Dam

.8 0 .92 .61 1.68 1.6 50 

2.9 2 .97 .66 4.35 1.5 130

Wolf Creek, Oklahoma, Fort Supply Dam

0.3 2? 1.00 0.25 1.21 4.0 90 

1.0 2? .48 .46 .29 2.2 12 

1.3 2? .94 .33 1.92 3.0 110

1.6 6? .95 .31 3.70 3.2 230 

2.9 2? .96 .42 4.10 2.4 190 

3.9 2? .70 .51 1.68 2.0 65 

4.7 2? 1.00 .29 9.67 3.5 630

North Canadian River, Oklahoma, Canton Dam

1.8 0 .93 .35 .83 2.9 46 

3.1 0 .79 .63 1.00 1.6 30 

5.0 0 .40 .93 .80 1.1 16 

5.6 0 .54 .51 2.15 2.0 80 

7.4 0 .90 .33 2.43 3.0 140

12.0 0 .63 .25 6.01 4.0 460 

14.5 1.0 .85 1.05 3.08 .95 55 

35 0 .59 2.26 2.77 .44 24 

125 0 .99 1.34 1.99 .75 28

Red River, Oklahoma-Texas, Denison Dam

.6 0 .87 .64 .52 1.6 15 

2.1 0 .97 .36 1.15 2.8 60 

7.2 3 .97 .82 .92 1.2 22 

8.4 0 .84 .59 .87 1.7 28 

11.5 0 .98 .45 1.15 2.2 48 

15.0 0 .90 .31 1.20 3.2 75

Chattahoochee River, Georgia, Buford Dam

5 0 .31 .88 1.12 1.14 24 

1.9 0 .99 .10 4.32 10.0 820 

2.9 0 .95 -.02 7.96 

4.0 0 .88 -.11 11.49

 (1/D) = c. + c (1/t), where D = measured degradation, in me 

t years after start of degradation. 

 Years between dam closure and start of degradation. 

-^Listed r2 is for estimation of D rather than for 1/D.

Estimated

achieve

0.5 D max

(years)

6 

5 

2.9 

11 

7 

.9 

1.6

7 

15 

13 

340

24

26 

1.1 

32 

4.5 

8 

7 

8 

8

.5 

4 

2.6

2.8 

7

5 

.6 

6

12 

10 

3 

34

2.4 

1.6 

.9 

4

7

24 

3 

1.2

1.5

.8 

3 

1.1 

1.5 

2.6 

4

1.3

44

ters, at
DOWNSTREAM EFFECTS OF DAMS 21

i i i

v 

»------ -_

 Missouri River, South Dakota 

8.4 kilometers downstream 

from Gavins Point Dam

r 2 = 0.13 

I I I

^^ I I ' F 

^^-^ a*---._

16.5 kilometers downstream 

from Fort Peck Dam

r 2 = 0 48 

I I I

*_ . _L ' ' w 

Missouri River, North Dakota 

32 kilometers downstream 

from Garrison Dam

r2 = 0.28 

1 1 1

Colorado River, Arizona 

6.1 kilometers downstream

^» from Hoover Dam 

\ »

*--,«V % 

r 2 = 0.56 

I I I

: I I I

^- a.-^. _ _

Chattahoochee River, Georgia 

~~ 0.5 kilometer downstream ~ 

from Buford Dam

r2 =0.31 

1 1 1

*, 1 1 1

Vv... '

 - .?.. 

 "~~~-«---»- -- 

Colorado River, Arizona 

95 kilometers downstream 

from Parker Dam

I r2 T°-63 I

CO

cc 

LU 2

LJ U ' 

CC. 

LU

Q 

0 2

LU 

CO

4

0 <

2

4

6

C

\ ' ' '

North Canadian River, Oklahoma ~~ 

3.1 ki lometers downstream

from Canton Dam

I r2=l°' 79 I

\ ' II

Smoky Hill River, Kansas

0.8 kilometer downstream

from Kanopolis Dam

~ r2 =0,92 ~

I I I

) 10 20 30 4(

\ 

\

«.,

>

V

V

\.

\

1 1 1

** ""*" ^ ~ J

 _

Red River, Oklahoma-Texas

8.4 kilometers downstream

from Denison Dam

l r2 T 0' 84 l

l 1 I

Colorado River. Arizona

1.1 ki lometers downstream

from Davis Dam _

, r2 =0.97

"\.

\ ~~

__ ^

**t "*-*^*«'

I I *^~~ I

m<V. ' ' '

^--»- «_

North Canadian River, Oklahoma

 14.5 kilometers downstream _

from Canton Dam

r 2 = 0.85

I I I

fill

\

\

N

\

V

^>.^^

_ Wolf Creek, Oklahoma _

0,3 kilometer downstream

from Fort Supply Dam

r2 = 1 .00

I I I

10 20 30 40 0 

YEARS AFTER DAM CLOSURE

10 20 30 40

FIGURE 9. Representative regression curves (dashed lines) of bed degradation with time at selected sites. Data from table 13.

line, but from other factors, such as: (1) Small depths 

of degradation (flat slope of best-fit straight line); (2) 

small number of data points (especially during the first 

few years after bed erosion begins); (3) errors in estimating any response time; and (4) irregularities in the 

trend of the curve, such as a rather abrupt cessation 

of degradation or an S-shaped curve. These features 

also contribute to a negative value of the coefficient 

c2 (intercept). A negative c2 precludes estimation of the 

maximum limit of degradation and associated values.

The type equation used here could predict degradation with time at a cross section if the coefficients Ci 

and c2 could be predicted. These certainly are functions 

at least of bed material and water discharge. A third 

factor might be distance downstream from the dam. At

present (1982), the coefficients cannot be determined 

in advance. Therefore, prior to dam closure any estimates of bed erosion need to be based on some type 

of degradation analysis using measured bed-material 

sizes and expected water discharges (Strand, 1977; 

Priest and Shindala, 1969a). This approach requires: (1) 

Adequate sampling of the bed material with depth, distance across the section, and distance along the channel; 

and (2) accurate predictions of future flow releases. On 

the other hand, with a few years of measurements after 

the start of degradation, the model described above 

might be used with due caution. (References cited earlier include models of degradation based on transport 

equations, particle-size measurements, and the assumption of winnowing.)
22 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

MAXIMUM DEGRADATION AND ASSOCIATED TIME

At a given cross section within the degraded reach, 

the maximum observed depth of bed erosion (table 6) 

was negligible on some channels but was as much as 

7.5 m on others (Colorado River 12 km downstream 

from Hoover Dam, Arizona). (Maximum values for a 

cross section do not necessarily apply to an entire 

reach, as local features can affect the extent of degradation at any one site.) Had Da vis Dam not been built 

downstream, more degradation than 7.5 m probably 

would have occurred at the Colorado river section just 

mentioned. The data (table 13) show that the bed at 

this site was still degrading at a rapid rate 13 years 

after dam closure, when measurements were discontinued due to backwater from Davis Dam.

The maximum possible degradation in some cases can 

be limited or restricted by a fixed base level. For example, bedrock is encountered in places on the Smoky Hill 

River downstream from Kanopolis Dam (Kansas) and 

on the Republican River downstream from Harlan 

County Dam (Nebraska). On the Rio Grande and along 

reaches of the Colorado River, fans of very coarse debris are controlling. In contrast, certain wide, shallow 

cross sections in some reaches on the Missouri River 

downstream from Fort Randall Dam (South Dakota) 

may no longer degrade because flow velocities are too 

slow.

If one assumes that the bed does not contain a subsurface layer of erosion-resistant material and that the 

same discharges will continue, it is interesting to extrapolate the empirical hyperbolic equation for each of 

the 114 applicable cross sections discussed above into 

the future. With due regard both for the uncertainties

TABLE 6. Maximum degradation downstream from various dams

[Data from table 13, except last two entries which are from unpublished sources]

Rive

Colorado

Colorado

Colorado

Colorado

Jemez, Je

Arkansas

Missouri

Missouri

Missouri

Missouri

Medicine

Middle Lc

.r, dam, State

Glen Canyon, Arizona

Hoover, Arizona

Davis, Arizona

Parker, Arizona

mez Canyon, New Mexico

John Martin, Colorado

Fort Peck, Montana

Garrison, North Dakota

Fort Randall, South Dakota

Gavins Point, South Dakota

Creek, Medicine Creek, Nebraska

up, Milburn, Nebraska

Des Moines, Red Rock, Iowa

Smoky Hill, Kanopolis, Kansas

Republice n, Milford, Kansas

Wolf Creek, Fort Supply, Oklahoma

North Cat

Canadian

ladian, Canton, Oklahoma

Eufaula, Oklahoma

Red, Denison, Oklahoma-Texas

Neches, Town Bluff, Texas

Chattaho

South Car

>chee, Buford, Georgia

ladian, Conchas, New Mexico

Years 

since 

closure

9

13

26

27

12

30

36

23

23

19

3

16

9

23

7

27

28

6

16

14

15

7

Maximum lowering 

of bed elevation 

(meters)

7.3

7.5

5.8

4.6

2.8

.9

1.8

1.7

2.6

2.5

.6

2.4

1.9

1.5

.9

3.4

3.0

5.1

3.0

.9

2.6

3.0

Salt Fork, Arkansas, Great Salt

Plains Oklahoma 9 .6

of the assumptions and for the risks of extrapolation, 

we have nevertheless done this (table 5) to estimate: 

(1) Maximum eventual degradation Z)max ; (2) years 

needed to achieve 95 percent of the eventual maximum 

degradation (the function goes to infinity at maximum 

degradation); and (3) years needed for the bed to erode 

to 50 percent of its eventual maximum degradation. All 

estimates were computed from the regression coefficients G! and c2 and rounded off appropriately.

Anax values (l/c2) were estimated for all 114 cross 

sections. Three of these Dmax values obviously were unreasonable and were not considered further. A frequency distribution of DmSLX for the remaining 111 sections is shown in figure 10. The distribution is virtually 

the same if the 21 cross sections that narrowed considerably are excluded. Ordinarily degradation needs to 

be viewed in relation to the size of the channel rather 

than in absolute values. Thus, Z)max needs to be adjusted by a scaling factor, such as the channel width. 

Because widths were not available for 35 cross sections 

on the Colorado River (about 33 percent of the total), 

the frequency distribution was drawn without applying 

any scaling factor. The 111 cross sections used to compile figure 10A are downstream from the following 

dams (number of cross sections in parentheses): Glen 

Canyon (5), Hoover (27), Davis (2), Parker (6), John 

Martin (4), Fort Peck (4), Garrison (11), Fort Randall 

(8), Gavins Point (13), Milburn (3), Kanopolis (2), Fort 

Supply (8), Canton (9), Denison (7), and Buford (2). A 

variety of rivers and channel conditions is reflected in 

figure 10A

According to the data in figure 10A, the modal or 

average maximum expectable degradation for the cross 

sections represented on the graph is about 2 m. The 

range is from about 0.4 to 38 m; about 98 percent of 

the values are less than 10 m. Accuracy of these predictions is related to the fit from the data themselves, 

the number and duration of measurements, the assumed nature of the subsurface sediment, and the validity of the many other assumptions.

If the coefficients in equation 1 are known, then the 

time needed for the bed to degrade to any proportion 

of the maximum eventual degradation depth can be estimated quickly by the following method. The actual 

depth value need not be known. Let p = the decimal 

proportion of the maximum degradation depth, for example, 0.95 if the depth of interest is 0.95 Z)max . The 

time tp needed to reach any designated proportion of

'-'may IS

where cp=(^)-l. For example, the Colorado River 2.6 

km downstream from Glen Canyon Dam, at which
DOWNSTREAM EFFECTS OF DAMS 23

30

25

?0

15

10

5

0

0.1

D.

1 10 

av , IN METERS

100 1 10 

0.95t,

100 1000 

 , IN YEARS

10,000 0.1 10 100

0.50t mav , IN YEARS

FIGURE 10. Frequency distributions based on 111 measured cross sections on various rivers: A, maximum expected degradation 

B, years needed to deepen to 95 percent of maximum depth; and C, years needed to deepen to 50 percent of maximum depth.

1000

depth;

Cx = 2.03 and c2 = 0.30 (table 5), would be predicted to 

reach 0.95 Dmax in 19x2.03/0.30=129 years (or about 

130 years) after the start of degradation.

This shortcut method is preferable to equation 1 for 

estimating times needed to reach a given depth because 

rounding of D in equation 1 causes variations in the 

computed times, compared to the times calculated from 

the coefficients alone. Variations are insignificant for 

the steeper part of the degradation-time curve (early 

years, shallow depths) but can be as much as 60 percent 

where the curve flattens in later years.

The number of years predicted for the bed to achieve 

95 percent of its eventual total degradation (0.95 £max) 

for the 111 cross sections ranges from 7 to 6,500 years 

(fig. 105). About 91 percent of the values are between 

7 and 500 years. The modal time is about 140 years. 

Data for many of the cross sections (table 13) indicate 

that these individual estimates of 0.95 tmax are of the 

right order of magnitude. Each such computation, however, is based on the assumption that the remaining 

subsurface material does not differ substantially from 

the original channel bed sediment and that the same 

flow pattern will continue.

The adjustment period at a site, or predicted time 

required to reach the new stable depth (0.95 Dmax), 

does not seem to have any consistent relation to distance downstream from the dam, for the few reaches 

where this aspect could be assessed. Any relation probably is obscured by the irregular differences in degradation from one cross section to another along a river.

Most of the 111 cross sections eroded one-half of their 

predicted eventual maximum depths within the first 

few years after the start of degradation. The range of 

these predicted times (0.5 tmax) is from 0.4 to 340 years 

(fig. 10C); modal value is about 7 years. All distributions in figure 10 are skewed, with a preponderance 

of smaller values within the respective range.

Initial degradation rates (the reciprocal of the coefficient Ci) range from virtually negligible to as much as 

7.7 m/yr. Even downstream from the same dam, different sites show different initial degradation rates. No 

direct relation between initial degradation rate and predicted eventual maximum depth of degradation could 

be established.

STANDARDIZED DEGRADATION-TIME PLOT

The degradation-versus-time plots (fig. 9) can be 

standardized and made dimensionless by converting the 

D axis to D/0.95 Dmax and the t axis to £/0.95 tmax. 

(The extrapolated 95-percent value is taken as a reasonable approximation of the eventual maximum values, 

as the latter are unusable because tmax becomes infinite.) By substituting the type function (eq. la) for each 

of D and 0.95 £>max in the ratio D/0.95 £>max , the coefficients c l and c2 are eliminated, and -D/0.95 Dmax is proportional to £/0.95 tmax . This means that if the standardized, dimensionless plot is used, the site-specific coefficients Ci and c2 become irrelevant, and all the various 

D-versus-t curves collapse onto one general curve. The 

straight-line form of the equation for this generalized 

curve, in which the reciprocals of the two variables are 

used as in equation la, is

0.95 Dmax= Q .95+0 .05^

This general curve is shown in figure 11, with the 

data for the 12 representative cross sections of figure 

9. (Axes in fig. 11 correspond to the form of eq. 1 rather 

than eq. 2 for easier comparison to the plots of fig. 

9.) The scatter is greatest for sites having low correlations with the model curve on the unstandardized plots 

(fig. 9) and improves as the fit on the unstandardized 

plots improves.
24 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

U,U 1 

4

0.4 

0.6 

x 0-8

0}

E 

Q

a? 1.0 

o 

Q 

1,2

1,4 

1.6 

1 .8

(

I I I i I ! I I I I I I I I 

^ 8.4 kilometers downstream from Gavins Point Dam, South Dakota 

& 32 kilometers downstream from Garrison Dam, North Dakota 

X 0.5 kilometer dowmstr earn from Bufotd Dam, Georgia _ 

 16.5 kilometers downstream horn Fort P-ck Dam. Montana 

p ° 6.1 kilometers downstream from Hoover Dam Arizona 

JL ^7 95 kilometers downstream from Parker Dam Arizona 

4p A 3.1 kilometers downstiea from Canton Dam, Oklahoma _ 

\ <& 1 8.4 kilometers downstieat from DfiniMjn Dam, Oklahoma Texas 

y D 14.5 kilometers downstrean from Canton Dam, Oklahoma 

1^7 O 0.8 kilometer downstreat from Kanosjolis Dam Kansas 

 MJky ^ 1.1 kilometers downstreai froni DHVIS Dam Aii/rna 

r-ff /\ + 0.3 kilometer downstreat from Fort Supply Dam. Oklahoma 

|2\* 

^^ c D ° /CURVE OF 

~ ^5^> A DERIVED EQUATION ~~

v^^'^P " ^

n ^ J^P*^*^ x o

^ V X -Q

' * ° o 

 o

o

I I I I I I I I I I I I I I

0.0 0.2 04 0.6 0.8 1.0 1.2 1.4 1-6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

t/0 95 t max 

FIGURE 11. Standardized degradation-time dimensionless plot of degradation curves in figure 9.

The general curve (fig. 11) represents the bed degradation relative to maximum estimated degradation for 

the standardized period, for any eroding cross section 

that has the ideal degradation function of equation 1. 

According to equation 2, such degrading sections 

achieve 50 percent of their maximum eventual degradation after only the first 5 percent of their adjustment 

period. Similarly, they achieve 75 percent of their eventual total degradation after just 13 percent of the adiustment period. Thus, the vast majority of bed degradation occurs within the first 10 to 15 percent of the 

adjustment period, as the shape of the general curve 

shows. In other words, if normal releases are made 

when the dam is closed, the first years after dam closure tend to be the period of greatest bed degradation, 

and later years become relatively unimportant. Some 

river engineers previously have reported this in connection with specific dams (U.S. Army Corps of Engineers, 

1960, p. 13).

In the preceding paragraphs, we have described an 

empirical relation between degradation and time in a 

search for some potentially useful generalization. However, it is important to remind readers that the degradation at individual cross sections is variable, and that

40 sites with aberrant data and 84 sites that showed 

no regular trend (samples shown in fig. 8) were eliminated from the analysis. Assumptions about flow releases, particularly in the absence of high-flow releases, 

may well produce significant errors in estimating rates 

or depths of degradation, or rates of change of channel 

form. Nevertheless, for the class of channels included 

in this sample (predominantly sand, but including some 

coarser material, and with irregular depths to bedrock 

controls), the results may provide some boundaries on 

expected depths, rates, and times of degradation.

LENGTH OF DEGRADED REACH

The reach, downstream from each dam, in which all 

cross sections (except those of obvious bedrock control) 

showed significant degradation, was defined as the degraded zone. The length of this reach was taken as the 

distance from the dam to the farthest inclusive degrading cross section.

The length of degraded reach downstream from most 

dams increased with time, as expected (table 7). This 

downstream progression with time has long been recognized from onsite observations (Stanley, 1951, p. 944; 

Makkaveev, 1970, p. 109) and in theoretical studies
DOWNSTREAM EFFECTS OF DAMS 25

TABLE 7. Data on the degraded reach downstream from dams

[Dams not listed because of lack of data are Davis, Parker, Medicine Creek, Milburn, and 

Milford Dams]

Location . Location 

of front of Rate Distance Qf firgt 

Years after degraded reach of advance . rom am ° measured section 

dam closure downstream from (kilometers Slt, e ° ma^lmum downstream from 

dam per year) degradation dam 

(kilometers) (kilometers) (kilometers)

Colorado River, Arizona, Glen Canyon Dam

3 25 8 4 1 

7 4

9 -7 >25 16 

19 -^>25 16

Colorado River, Arizona, Hoover Dam

.5 21 42 8 2 

1 28 14 3 

2 50 22 3 

3 85 35 3

5 -^120 3 

7 - »120 15 

12 -^>120 13

Jemez River, New Mexico, Jemez Canyon Dam

6 -^2.1 .6 .6 

12 -^2.1 1.1 

22 -^2.1 1.0

Arkansas River, Colorado, John Martin Dam

9 26 3 22 4 

24 26 0 4 

30 26 0 22

Missouri River, Montana, Fort Peck Dam

13 -^>75 17 9 

18 i7 >75 17 

23 -^>75 23 

36 -7 >75 17 

Missouri River, North Dakota, Garrison Dam

1 12 12 28 3 

7 19 1.2 6 

11 18 -.25 6 

17 21 .5 6 

23 21 0 6

Missouri River, South Dakota, Fort Randall Dam

2 5 2.5 11 1.6 

5 13 2.3 1.6 

8 14 .3 11 

15 14 0 11 

23 15 .1 11

Missouri River, South Dakota, Gavins Point Dam

5 15 3 2 1.5 

10 14 -.2 4 

15 23 1.8 2 

19 23 0 2

Des Moines River, Iowa, Red Rock Dam

9 20 2.2 12 2.3 

Smoky Hill River, Kansas, Kanopolis Dam

3 5 1.7 .8 .8 

4 4-1.0 .8 

13 14 1.1 .8 

23 -- .8

Wolf Creek, Oklahoma, Fort Supply Dam

7 - '-7 .3 .3 

19 i/J >7 - .3 

27 ±/>7 - .3

(Mostafa, 1957; Albertson and Liu, 1957; Hales and 

others, 1970). Such lengthening occurred downstream 

from 9 of the 11 dams for which this feature could be 

determined (table 7). Lengths as of the latest resurvey 

ranged from 4 km on the Neches River downstream 

from Town Bluff Dam, Texas, to more than 120 km 

on the Colorado River downstream from Hoover Dam,

TABLE 7. Data, on the degraded reach downstream from 

da m s Continued

Location . Location

of front of Rate froTdaTto °f f±rSt 

Years after degraded reach of advance . . ° measured section 

. -. , .- /, -i site or maximum , ,- dam closure downstream from (kilometers . downstream from 

, degradation dam per year) ., ° , dam 

(kilometers) (kilometers) (kilometers)

North Canadian River, Oklahoma, Canton Dam

1 7 <1.8 1.8

2.8 7 "1.8 

3.4 7 ~1.8 

11 7 <1.8 

18 7 <1.8

Canadian River, Oklahoma, Eufaula Dam

6 16 2.7 .8 .8 

14 29 1.6 .8

Red River, Oklahoma, Denison Dam

3 7 2.3 15 .6 

6 -^>27 15 

16 -7 >27 1 

27 -7 >27 15

Neches River, Texas, Town Bluff Dam

9 .2 

14 4 .2

Chattahoochee Biver, Georgia, Buford Dam

7 712 .5 

9 912 

12 2 

15 10 .2 2

 Distance of farthest cross section that was established at time of dam 

closure.

California-Arizona. In most of these cases, there is no 

indication that the reach had stopped lengthening by 

the time of the most recent survey. This means that 

the zone of degradation can continue increasing in 

length for at least 30 years or more after dam closure, 

although it could stop sooner. The migration rate and 

the final length of the degradation zone should vary 

with flow releases, bed-material sizes, and topography. 

Consequently, growth rate and eventual length are 

likely to vary from one dam to another.

Migration of the front of the degraded zone means 

that at a downstream site a response time or lag time 

occurs before the bed reacts to the dam, if it is going 

to react. For some dams, this response time (and hence 

the migration rate of the edge of the degraded zone) 

could not be determined, because: (1) Cross sections 

were not established far enough downstream; (2) 

downstream measurements were not started until too 

many years after dam closure; or (3) a downstream base 

level interrupted or controlled the normal degradation 

process. A probable example of the latter is Wolf Creek 

downstream from Fort Supply Dam, Oklahoma. This 

stream joins the North Canadian River 6 km 

downstream from the dam. Successive profiles showed 

a hinge or base-level control at or near the confluence 

with the larger river. Similarly, the zero degradation 

point downstream from Town Bluff Dam on the Neches 

River, Texas, is sea level (Gulf of Mexico). Bedrock 

outcrops appear along the Republican River
26 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

downstream from Harlan County Dam, and there are 

cobble riffles that act as controls on the Red River 

downstream from Denison Dam.

Within a year or two after dam closure, the length 

of the degraded reach can range from little or nothing 

to as much as 50 km. After 2 or 3 decades, the length 

downstream from some dams remained as short as a 

few kilometers (and theoretically could be much less), 

but downstream from Hoover Dam it was more than 

120 km (table 7).

Hales and others (1970) proposed a method for predicting the temporary length of the degraded zone, 

based on 15 years or less of data for the Missouri River 

downstream from Garrison, Fort Randall, and Gavins 

Point Dams. This channel length in their treatment is 

a function of an average peak discharge, the time during which the degradation has been occurring, the dominant size of the bed material, and the area of the channel cross section. We have not tested their relation, 

mainly because of uncertainties in their definition of the 

peak discharge, uncertainties in the location of the cross 

section at which the area and particle sizes are to be 

measured, and their definition or way of determining 

the dominant grain size. Similarly, because of the few 

instances in which the degraded reach had stopped 

lengthening, a test of the method that Priest and Shindala (1969b) proposed for predicting that ultimate distance was not possible.

Migration rates of the leading edge ranged from very 

little to as much as 42 km/yr (kilometers per year) immediately after dam closure (table 7). Slower rates for 

subsequent periods ranged from virtually negligible to 

about 29 km/yr. Most of the travel rates range from 

about 0 to 2 km/yr. According to Makkaveev (1970, p. 

109), his countryman Fedorov determined migration 

rates of several kilometers per year on large lowland 

rivers, and several tens of kilometers per year on mountain rivers in the Soviet Union.

The rate of advance of the downstream edge of the 

degraded zone depends on the flow releases and bed 

materials; these vary widely from one stream to 

another. According to the data in table 7, the rate on 

any river is not constant; the front occasionally may 

appear to retreat for isolated periods, even though the 

long-term trend is downstream. Migration rates appear 

to be fastest during the years immediately after dam 

closure. The relatively slow rates of subsequent years 

might be expected in some cases due to a flattening 

of gradient (discussed below); however, variable flow 

releases also will affect the rate with time. Whether 

the rates eventually become constant or continue to get 

slower with time cannot be determined from available 

data.

ZONE OF VARIABLE BED CHANGES

Cross sections downstream from the degraded zone 

may aggrade, degrade, or stay at the same level (table 

13). There is some uncertainty as to whether bed-elevation changes in this downstream zone are due to the 

dam. Cross sections were not established prior to dam 

construction; therefore, the investigator does not have 

the benefit of this control. Marked trends, such as sudden and deep degradation typical of many cross sections 

near the dam and of the time when changes began, 

are not readily apparent on many measured sections. 

Most bed changes shown by the gaging-station data for 

control stations (table 14 and figs. 36-49) do not show 

trends. For these reasons, there is little basis for believing that the dam caused any observed changes in 

bed elevation beyond the degraded zone. Availability 

of ground and aerial photographs eliminates much of 

this uncertainty in regard to channel width and density 

of vegetation, but does not help to define bed elevations. We, therefore, have not evaluated observed fluctuations in bed level in the reach beyond the degraded 

zone.

It is possible that degradation results in aggradation 

at some point downstream. Borland and Miller (1960, 

p. 70) noted that after closure of Hoover Dam in 1935 

and Davis Dam about 1950 on the Colorado River, degradation downstream from the dams increased the aggradation in a reach farther downstream at Needles, 

California. Similarly, while only small changes in the 

overall longitudinal profile of the Rio Grande occurred 

after closure of Elephant Butte Dam and reservoir, J. 

F. Friedkin (International Boundary Commission, written commun., 1959) noted degradation of 1 to 2 m just 

downstream from the dam and deposition of about 1.5 

m at El Paso, Texas, about 225 km downstream. These 

data are suggestive but are too limited to support a 

generalization regarding downstream aggradation associated with upstream degradation. Our study provides no additional data.

A related intriguing possibility is that enlargements 

in channel width (discussed in detail below) could result 

in downstream aggradation. On the Missouri River 

downstream from Garrison, Fort Randall, and Gavins 

Point Dams, and on the Red River downstream from 

Denison Dam, significant increases in channel width at 

some cross sections are associated with bed aggradation 

near the approximate downstream edge of the degraded 

reach.

LONGITUDINAL-PROFILE CHANGES

To analyze changes in bed elevation with distance 

downstream (longitudinal profiles), we required at least 

four cross sections downstream from the dam and
DOWNSTREAM EFFECTS OF DAMS 27

enough post-dam resurveys, bed degradation, inclusive 

time, and total downstream distance to reveal trends 

and features. Of the 21 dams (fig. 1), these requirements eliminated Davis, Jemez Canyon, John Martin, 

Medicine Creek, Milburn, Milford, and Red Rock Dams, 

leaving 14 dams for this particular analysis.

Degradation models based on flume studies generally 

show maximum bed erosion at or near the dam, relative 

to the total reach undergoing bed changes (Ahmad, 

1953; Mostafa, 1957; Aksoy, 1970; Hwang, 1975). In a 

general way, our data support that finding. The cross 

section of greatest degradation at a given time was the 

closest section to the dam in five cases (Gavins Point, 

Kanopolis, Fort Supply, Canton, and Eufaula Dams). 

Downstream from seven other dams, the greatest degradation was some distance, generally 2 to 16 km, 

downstream from the dam, but still generally nearer 

the upstream than downstream end of the degrading 

reach. (Variations in the downstream location of 

maximum bed erosion were mentioned by Wolman, 

1967). For the two remaining dams, the location of 

maximum degradation was indeterminate.

Due to the spacing of the cross sections and the natural variations of bed and bank erodibility with distance 

downstream, the data do not reveal how close to the 

dam the maximum degradation will occur when bed material is homogeneous with depth and distance. Results 

of Ahmad's (1953) flume study indicate that the greatest 

degradation takes place closer to the upstream than to 

the downstream end of the degraded zone, but not right 

at the dam. Data in table 13 at least show more degradation closer to the upstream than downstream end of 

the degraded zone. Whether maximum degradation occurs immediately downstream from the dam needs to 

be determined by new measurements.

Flume studies also indicate progressively less degradation with distance downstream, at a given time. For 

our data, this occurs in some reaches, but others do 

not seem to have a well-defined trend of degradation 

with distance. Instead, downstream from some dams, 

varying depths of bed erosion seem to be distributed 

almost randomly. For example, the data for the Colorado River downstream from Hoover Dam (table 13) 

show considerable variability in degradation depths 

with distance downstream. Within the general degraded zone, some cross sections had only minor bed 

erosion, while others degraded many meters by the 

same year. Because flows were the same for all sections 

and channel width did not vary significantly, such degradation differences probably are due to differences 

in bed erodibility (Stanley, 1951, p. 945). 

~ Variations with time also occur. If degradation is a 

maximum at or near the dam, then the channel's 

downstream longitudinal profile should flatten with

time as degradation proceeds. This process has been 

observed in the laboratory, along with the expected decline in the rate of degradation. At a given cross section, the sediment-transport rate decreases progressively with time as the bed slope (and hence stream 

competence) decreases. Transport eventually should 

cease if the slope becomes sufficiently flat (Tinney, 

1962).

Where no bed controls exist, Ahmad's (1953) flume 

studies show that the point of maximum degradation 

migrates downstream with time. For most of our cross 

sections, maximum degradation either stayed at the 

same cross section with time (six dams) or varied from 

one cross section to another while showing a general 

preference for one site (seven dams), with one dam indeterminate. In several of the seven instances where 

the location varied with time, the first resurvey after 

dam closure showed maximum erosion at the cross section nearest the dam, but for later resurveys, the greatest bed degradation occurred at some fixed downstream 

cross section. In general, then, the site of greatest bed 

erosion tends to remain constant with time for the dams 

of this study, in which there is probably great variability of bed materials at or close to the surface.

In nature, the bed profile downstream from a dam 

is affected by differences in bed material with both 

depth and distance, the presence of local controls, the 

history of flow releases, tributary contributions of 

water and sediment, and other factors. The profile 

downstream from a dam varies irregularly with time, 

and a uniform flattening of slope is not common. In 

most cases, the rate and depth of degradation are greater closer to the dam, but, in other respects, each dam 

is unique in regard to profile adjustment. Four examples are shown in figure 12.

The Smoky Hill River downstream from Kanopolis 

Dam perhaps most closely approaches laboratory results and theoretical expectation, at least for the first 

10 km or so downstream from the dam. Beginning at 

or immediately downstream from the dam, degradation 

decreases progressively downstream (fig. 12).

The profile of the Colorado River downstream from 

Parker Dam is remarkably different in that only to a 

very slight extent is the expected flattening of the slope 

evident (fig. 12). Instead, degradation seems to be almost uniform throughout a reach at least 60 to 70 km 

long.

In contrast, the channel profile downstream from 

Fort Randall Dam on the Missouri River, though generally tending to flatten with time, has widely varying 

degrees of bed-level change with time from one cross 

section to another (fig. 12). Degradation, no change, 

and aggradation all have happened at different 

downstream locations.
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 23 years after dam closure

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 9 years after dam closure

 19 years after dam closure 

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KILOMETERS DOWNSTREAM FROM DAM

FIGURE 12. Longitudinal-profile changes downstream from four dams.

20 30

Finally, the Colorado River channel throughout about 

a 25 km-long reach downstream from Glen Canyon Dam 

has undergone both a decrease and increase in slope 

with time (fig. 12). By 3 years after dam closure, the 

expected trend in degradation and flattening of slope 

had developed. However, degradation then ceased near 

the dam and, despite local irregularities such as cobble 

riffles, increased in the downstream direction. By 9 

years after dam closure, this process had produced an 

average slope steeper than the slope at the time of dam 

closure. No change in the profile occurred during the

following 10 years. Coldwell (1948) shows other examples of variability in the development of post-dam longitudinal profiles.

The profile along the Green River downstream from 

Flaming Gorge Dam, Utah, is changing due to the development of rapids (Graf, 1980). The reduced (postdam) high flows are no longer able to move the coarse 

material. Some bed degradation near the rapids might 

accentuate the profile changes.

Detailed records from the Rio Grande (J. F. Friedkin, written commun., 1959) provide one of the best
BED MATERIAL AND DEGRADATION 29

illustrations of the variability of degradation and aggradation and their effect on the longitudinal profile. The 

reach for this example extends from Elephant Butte 

Dam, New Mexico, to and including a cross section 

downstream from El Paso, Texas; however, the upper 

reaches of the Rio Grande have similar problems 

(Lagasse, 1980). This complex case demonstrates both 

the effects of man (diversion dams) and the effects of 

sediment contributions from tributaries. From 1917 to 

1932, immediately downstream from Elephant Butte 

Dam, the streambed degraded to a depth of about 1.8 

m. Similarly, downstream from each of a number of 

diversion dams that control the channel elevation but 

provide little storage, degradation during 1917-32 

ranged from 0.3 to 2.0 m. Due to this degradation and 

the downstream controls, the slope flattened 

downstream from each diversion structure. For example, in a reach 17 km long downstream from Percha 

Dam (46 km downstream from Elephant Butte Dam), 

from 1917 to 1932, the slope decreased from 0.00080 

to 0.00065. Maximum depth of scour downstream from 

Percha Dam was about 2.0 m. In addition to the effects 

of these diversion dams, a number of steep arroyos, 

with intermittent large flows and large quantities of 

coarse material, periodically deliver that sediment to 

the Rio Grande. Because Elephant Butte Dam virtually 

eliminated downstream floods along the Rio Grande, 

the main channel can no longer transport the coarse 

material brought in by the arroyos. These sediment accumulations along some reaches block the channel and 

divert it completely. Along other reaches, such as those 

controlled by bank-protection works and jetties, such 

sediment accumulations provide a control by raising the 

elevation of the main stream at the confluence. This, 

in turn, induces deposition in the main channel for short 

distances upstream. The gradient of the Rio Grande 

is about 0.00028 to 0.00076, so deposition of coarse material can significantly flatten the local gradient.

A river's longitudinal profile and slope also can be 

affected by changes in river length or sinuosity. An 

increase in sinuosity (or in river length) has been noted 

in connection with local aggradation and vice versa 

(Hathaway, 1948; Ahmad, 1951; Frederiksen, Kamine 

and Associates, Inc., 1979).

BED MATERIAL AND DEGRADATION

THEORETICAL EXPECTATIONS

Few if any natural channels are underlain by perfectly uniform sediments. Because magnitude and frequency of high flows are significantly decreased by 

dams, and because released flows may not be able to 

transport sizes previously moved by higher flows, successive flows can winnow finer materials from the bed. 

Progressive winnowing concentrates the coarser fraction. As degradation proceeds, the average particle size 

on the bed increases, possibly eventually resulting in 

a surface covering or armor of coarse particles alone. 

This idealized theory has long been accepted in engineering planning.

Onsite and laboratory studies (Pemberton, 1976; Harrison, 1950; Little and Mayer, 1972) have demonstrated 

the importance of armoring in limiting degradation. In 

a general way, the number or extent of coarser particles should govern partly the depth of degradation in 

the cross section. Livesey (1965) has shown that as little 

as 10 percent coarse material in a standard sieved sample may be sufficient to provide the bed armor. (This 

underlines the importance of adequately sampling the 

surface and subsurface material for predictive purposes, 

before the dam is built. Representative sampling is difficult.) Livesey's observations show further that a postdam armored bed need not be covered entirely by 

coarse material, and that the percentage covered is 

about 50 percent. The estimated gravel cover for the 

bed of the Red River downstream from Denison Dam, 

as obtained by pebble counts throughout long reaches 

of the river, indicates that 30 to 50 percent cover limits 

or controls degradation.

Armor is a veneer underlain by normal or unwinnowed material. To date, onsite studies have not provided any proven examples of unravelling or unrolling 

of the veneer and reexposure of the subsurface sands. 

Assuming releases of large discharges from a dam, one 

would expect some unravelling of the surface. The extent should depend on the magnitude and duration of 

such excessive flows. Presumably restabilization and rearmoring of the bed should follow.

The progressive changes in particle size in the vertical should have their counterparts along the longitudinal profile, as degradation moves progressively 

downstream with time. Thus, armoring of the bed 

should appear first close to the dam, then disappear 

somewhere downstream.

VARIATIONS IN BED-MATERIAL SIZES 

WITH TIME AT A CROSS SECTION

An unpublished U.S. Army Corps of Engineers report gives median grain size (d50) at different years for 

two sites downstream from Gavins Point Dam on the 

Missouri River. Various U.S. Bureau of Reclamation 

reports, for example U.S. Bureau of Reclamation 

(1948), show size-frequency curves for the bed material 

at different locations downstream from Hoover, Davis, 

and Parker Dams on the Colorado River. The variation 

of d50 with time for these Missouri River and Colorado 

River sites is shown in figure 13.
30 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

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FIGURE 13. Variation in bed-material size with time at a site, after dam closure.

Initially, d50 increased with time following dam closure, at least at cross sections near the dam. The magnitude of this increase can be more than a factor of 

100 (and theoretically much more) in the value of d50 , 

depending on the sizes and number of coarse particles 

in the reach. Within about 1 to 10 years after the start 

of the coarsening, the particle sizes seemed to stabilize. 

From the graphs in figure 13, stabilization occurred relatively abruptly rather than gradually, but such an impression may be due to the sampfeg-intervals* In a 

few instances, the data suggest a subsequent reversal 

of the trend, that is, a decrease of d50 with time following the initial increase. Possible explanations, all 

speculative, are the arrival of finer material from upstream or from tributary inflow, the uncovering of finer

material at some depth below the original surface, lateral movement of the channel, and sampling inaccuracies. Such a decrease in d50 may or may not reach 

a new stable value, judging from figure 13. Thus the 

changes in median size of bed material at these sites, 

while initially tending to coarsen as expected, did not 

follow an ideal or common pattern thereafter, but varied in several ways during later periods.

Near the downstream end of the degraded zone, an 

increase of d50 with time may or may not occur. Where 

it does occur, it may lag behind the time of dam closure. 

Coarsening at these downstream sites seems to be less 

than that occurring near the dam. Whether this relatively limited coarsening is partly a function of distance 

from the dam, in addition to the distribution of particle
CHANNEL WIDTH 31

sizes in the subsurface material and other factors, cannot be determined from the available data.

Particle-size distributions given in U.S. Bureau of 

Reclamation publications for Colorado River reaches 

show that sorting as well as median size of streambed 

material downstream from dams varies with time. Some 

finer material is present in all samples, but later samples tend to have larger sizes (hence a wider range of 

sizes) than the earlier ones, as well as a greater percentage of coarse particles. Sorting, therefore, decreases 

with time. The presence of fine material in all samples 

may mean that such small grains really are on the bed 

surface, or it may result from the sampling technique, 

that is, the sample could include both surface and subsurface material.

VARIATIONS IN BED-MATERIAL SIZES 

WITH DISTANCE DOWNSTREAM

Some streams, such as the North Canadian River, 

have nearly constant sediment sizes for long distances 

downstream. Others, such as the Republican River 

downstream from Harlan County Dam in Nebraska and 

the Salt Fork of the Arkansas River downstream from 

Great Salt Plains Dam in Oklahoma, show great 

downstream variability in bed-material sizes. Such variability in these last two examples results in part from 

the sediment contributed by cliffs that abut the channel 

in places. Thus local geology can mask changes that 

might occur from dam construction.

Where bedrock controls are absent and the bed of 

the river has a mixture of grain sizes, the postulated 

succession of particle size with distance occurs. Kira 

(1972, fig. 11) showed a gradual decrease in the mean 

diameter of bed-surface particles with distance 

downstream from Huchu Dam on the Aya River, Japan, 

as of 5 years after dam closure. Downstream from 

Kanopolis Dam on the Smoky Hill River and Denison 

Dam on the Red River, pebble counts of the sediment 

on gravel bars exposed at low water were obtained in 

1960 throughout long reaches. Sieve analyses also were 

available for the bed material of the Colorado River 

downstream from Hoover Dam from U.S. Bureau of 

Reclamation sources. For these three rivers, the upper 

part of each of the three plots in figure 14 shows relatively coarse particles nearest the dam and a gradual 

grain-size decrease in the downstream direction. Bedsediment analyses (discussed below) made when Hoover 

Dam was closed show that the bed material at that 

time was much finer than it was 6Vfc years later (the 

year of the data plotted in fig. 14). The post-dam decrease of particle size with distance downstream, therefore, is reasonably attributable to the dam. 

Downstream from Kanopolis and Denison Dams, bedmaterial sizes were not measured at the time of dam

closure. Therefore, one cannot say with certainty 

whether the post-dam trend resulted from the dam or 

whether it occurred naturally. However, the similarity 

of the two grain-size versus distance curves to one 

another and to that for the Hoover Dam data, along 

with qualitative agreement with theoretical expectations, indicate that the decrease in grain size probably 

is due to the dams.

The lower plot for each dam in figure 14 shows variation in bed elevation with distance downstream, using 

the data of table 13 for the same year as the sedimentsize data. The relative changes in grain size, degradation, and distance downstream then can be compared. 

If one assumes that the sizes of pre-dam channel sediment downstream from these three dams did not vary 

significantly with distance within the reach examined, 

then the relation between bed-material changes, degradation, and distance downstream agrees with the 

theoretical model described above.

Reading the associated values of grain size and degradation at successive distances from the smoothed curves 

in figure 14, the curves in figure 15 were drawn to 

show the increase in bed-material grain size with degradation for each study reach. This shows more graphically the increase in bed-material sizes relative to the 

depth of bed degradation. The curves in figures 14 and 

15 might have been different in position on the graph 

if the data had been measured at some other time after 

dam closure; however, the trend would not be affected.

U.S. Bureau of Reclamation data permit an evaluation of how the grain size-distance relation varies with 

time for the Colorado River downstream from Hoover 

Dam (fig. 16). During the first year or so after dam 

closure, the reach that underwent changes (coarsening) 

in bed-sediment sizes was somewhat less than 10 km 

long. After 3 years, coarsening was quite noticeable at 

20 km but not at 70 km downstream from the dam; 

and by about 6 or 7 years after closure, coarsening was 

apparent 70 km, but not at 135 km, downstream from 

the dam. Coarsening did not seem to progress to the 

site 135 km downstream from the dam until about 13 

years after closure.

CHANNEL WIDTH

GENERAL NATURE OF WIDTH CHANGES

Channel widths downstream from the dams of this 

study narrowed, widened, or remained constant, depending on the site, in the years following dam closure 

(table 13). In general, the post-dam changes in channel 

width at a cross section as documented by measured 

cross sections, photographs, and maps, can be divided 

into five categories.
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CHANNEL WIDTH 33

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DISTANCE DOWNSTREAM FROM DAM, IN KILOMETERS 

FIGURE 16. Variation in median bed-material diameter with distance 

along the Colorado River downstream from Hoover Dam, at successive times after dam closure.

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in which the width at successive times is within about 

±4 percent (an arbitrary figure) of the width at the 

time of dam closure. In table 13, 231 cross sections 

downstream from 17 dams have meaningful width data. 

The width has remained virtually constant at 51 of 

these sections (about 22 percent of the total). (Such percentages are affected by the number of measuring sections downstream from the various dams and do not 

necessarily reflect relative frequency of the five 

categories of width change.)

Channel width in canyons, such as occur along some 

reaches downstream from Colorado River dams, obviously is constrained. Such sections were excluded from 

the total of 231 considered here. However, Howard and 

Dolan (1981) report that fine-grained terrace materials 

in depositional reaches on the Colorado River 

downstream from Glen Canyon Dam are being reworked by flow releases, resulting in slight channel 

widening.

A second category of channels widened, wnere widening arbitrarily is defined here as the most recently measured width being at least 5 percent greater than the 

width at the time of dam closure. About 46 percent 

(105 cross sections) are in this category. Although 

sometimes the channel has become about twice as wide 

during the post-dam period, most increases as of the 

latest resurvey wrere less than about 50 percent. Pronounced widening occurred at some cross sections 

downstream from Fort Peck, Gavins Point, Medicine 

Creek, Town Bluff, and Fort Randall Dams, but widths 

at other sites downstream from these dams did not 

change significantly. Also, changes in width were not 

consistent with distance downstream. Minor increases 

in width (less than about 15 percent) happened at a 

number of cross sections downstream from Milburn, 

Milford, Kanopolis, Red Rock, and Buford Dams. However, the magnitude varied considerably with distance 

along the river.

Category three consists of channels that have become 

narrower. Using the arbitrary 5 percent criterion, 59 

cross sections (about 26 percent) are in this group. 

About one-half of these are located downstream from 

Jemez Canyon, John Martin, Fort Supply, and Canton 

Dams (figs. 17-20). These channels are now only about 

17 to 50 percent of their pre-dam widths.

The fourth category includes channels that widened 

initially after dam closure, but later reversed this 

trend, and were most recently narrower than at the 

time of dam closure. Twelve cross sections (about 5 percent) are in this group. The North Canadian River 

downstream from Canton Dam, Oklahoma, has several 

such sections.

The fifth category, including only 4 of the 231 cross
34 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

FIGURE 17. Jemez River downstream from Jemez Dam, New 

Mexico, A, April 1936; B, spring 1951; C, June 1980. Dam was 

closed in 1953.

sections, shows an initial channel narrowing followed 

by widening. The channel as of the latest resurvey was 

wider than at dam closure.

Changes in width seem to have occurred at least from 

the time of dam closure; such changes tend to accompany changes in bed elevation. However, as with bed

FIGURE 18. Old streamflow-gaging site on Arkansas River 3 kilometers downstream from John Martin Dam, Colorado. A, March 1946; 

B, September 1959; C, July 1980. Dam was closed in 1943.

degradation, there can be a considerable lag time before 

effects become noticeable at some of the downstream 

sections. Examples occur along the Red River 

downstream from Denison Dam.
CHANNEL WIDTH 35

,-f

FIGURE 19. Wolf Creek about 2.6 kilometers downstream from Fort 

Supply Dam, Oklahoma. A, April 1940; B, September 1958; C, August 1972. Dam was closed in 1942.

* /

FIGURE 20. North Canadian River about 0.8 kilometer downstream 

from Canton Dam, Oklahoma. A, about 1938; B, July 1980. Dam 

was closed in 1948. Both scenes are looking downstream at the 

highway bridge.

DISTANCE AFFECTED

For those rivers having significant increases or decreases in width, the changes extend at least to the 

farthest measured cross section. (In most instances this 

was well beyond the zone of bed degradation.) Thus, 

the extent of a reach over which width has changed 

cannot be determined due to lack of data; however, it 

can be many tens of kilometers.

No downstreamward trend in the magnitude of 

change in width is discernible for most reaches. This 

is true whether one considers the degraded zone alone 

or the entire reach for which data are available. For 

example, width changes do not seem to be greater near
36 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

the dam. Rather, changes in most cases appear to vary 

randomly with distance or to remain about constant.

FACTORS AFFECTING CHANGES 

IN CHANNEL WIDTH

ALLUVIAL-BANK MATERIALS

Data on bank materials were not available for most 

of the cross sections and reaches described here. However, some analyses and onsite observations illustrate 

the variety of bank-material factors that affect channel 

widening.

At different locations along the Missouri River 

downstream from Garrison Dam, two distinctive types 

of channel bank occur. In one location (fig. 21) about 

10 km downstream from the dam, the entire bank is 

composed of almost uniform sand (median diameter 0.17 

mm (millimeter), standard deviation 0.027 mm). Erosion of these sugar sands, as they are called, appears 

to be a function of the shear of the flow against the 

surface of the bank. The rate of erosion is likely to 

be proportionate to the discharge and time the bank 

is subjected to the flow. Moderate fluctuations in flow, 

without major high flows, result in erosion of the sand 

deposit at the base of the bank, forming a narrow 

beach. With minor changes in water stage, this beach 

provides some protection against further erosion of the 

bank. The river bank in figure 21 eroded at a rate of 

about 3.6 m/yr between 1946 and 1957. (This approximate rate is mentioned only in a general sense and is 

not meant to show any effect of Garrison Dam, which 

was closed in 1953.)

In contrast to the uniform sands in the bank in figure 

21, the banks at other cross sections consist of layers 

of sand interbedded with finer-grained strata (fig. 22). 

The sand, about 1 m thick, is overlain by stratified silts 

(median diameter 0.009 mm, 99 percent finer than 0.074 

mm) about 5 m thick. Low water on the outside of the 

bend impinges directly upon the sand, which is eroded 

readily by the continuing flow, even at low stages. The 

bank collapses by undercutting, with large blocks dropping vertically into the flow. Such silt-clay blocks retard 

bank erosion for a time, but eventually disintegrate and 

then are transported by the continuing flow. The bank 

in figure 22 eroded at a rate of 73.2 m/yr during 1946- 

57. Similar banks composed entirely of sand erode even 

more rapidly. This is a very rapid rate of erosion, but 

even at other cross sections downstream from Garrison 

and other Missouri River dams, the erosion rates generally exceed 20 or 30 m/yr (table 13; Rahn, 1977).

All manner of permutations and combinations of bank 

materials and stratigraphy occurs on the Plains rivers, 

which dominate the sample of rivers studied here. On 

the Missouri River in the 100 km reach downstream

FIGURE 21. Sandy bank of Missouri River about 10 kilometers 

downstream from Garrison Dam, North Dakota.

 ** '

FIGURE 22. Stratified sand and silt bank, Missouri River 

downstream from Garrison Dam, North Dakota.

from Garrison Dam, the percentage of silt (particles less 

than 0.074 mm) in the banks ranges from 3 to 100 percent. The bank commonly has thin strata containing 

large percentages of silt and some clay; however, on 

the average, silt and smaller sizes constitute no more 

than 33 percent. In this reach, the average of samples 

of bed material contained less than 2 to 10 percent silt
CHANNEL WIDTH 37

size or finer. In contrast, a representative sample of 

bank material from 1 m above low water on the Smoky 

Hill River downstream from Kanopolis Dam had 75 percent of the particles finer than 0.074 mm.

Samples from the bed and banks of the Salt Fork 

of the Arkansas River illustrate both the stratigraphy 

of the flood plain or channel banks and the contrasting 

character of bed and bank materials. As the data in 

table 8 show, 54 percent of the bed material is larger 

than 0.5 mm (coarse sand). With increasing distance 

above the bed, the proportion of silt-clay in the channel 

perimeter increases; that is, the percentage of coarse 

and medium sand decreases. Only in the upper 0.5 m 

of the flood plain is the percentage of silt and clay appreciable, a fact clearly evident in the stratigraphy of 

the bank as seen at the site. For the 2 m-high bank 

as a whole, 75 percent of the vertical section is composed of sand coarser than 0.125 mm. The remaining 

25 percent is very fine sand or smaller. Considering 

the entire bank as a whole, the percentage of silt and 

clay (weighted according to the proportion of the vertical section described by the sample) is about 12 percent.

For the few rivers where bank materials were 

examined in detail, no general and simple correlation 

could be made between erosion rates and the percentage of sands or silt and clay in the banks, except for 

isolated examples along the Missouri River and for 

straight reaches several kilometers downstream from 

Kanopolis Dam on the Smoky Hill River, where erosion 

of the silty banks appeared minimal. (Bank erosion on 

the Smoky Hill River, however, was significant at 

bends or where the thalweg of the channel meandered.) 

Although cohesive banks retard erosion, tests of stability criteria based on a weighted silt-clay content in bed 

and banks, using the method proposed by Schumm 

(1960, fig. 10, p. 23), indicate that measured channel

sections known to be either aggrading, widening, stable, or unstable are not distinguishable on the basis 

of the width-depth ratio and weighted mean percentage 

of silt and clay. The difficulty appears to be that weighting of the particle size of the sediments by the channel 

width significantly distorts a controlling relationship between actual differences in bed and bank sediments. 

Generally, a cohesive bank will limit both channel width 

and the tendency to bank erosion or lateral migration; 

however, many other factors occurring simultaneously 

appear to dominate in the control of bank erosion.

BEDROCK-BANK CONTROLS AND DOWNSTREAM EFFECTS

Several cross sections on the Missouri River 

downstream from Garrison Dam indicate that channel 

shifting and bank erosion may increase downstream 

from cross sections at which bank erosion is controlled 

or retarded. Bedrock on one or both sides of the valley 

constricts the valley and channel in places. Lateral erosion at such constrictions usually is minimal, but in the 

expanding valley width downstream from such controls, 

erosion of one or both banks is relatively much greater. 

Further work is needed to determine whether the lateral erosion downstream from the constricted sections 

is greater than it would be without the constrictions.

WATER FLOW

In a detailed analysis, Chien (1961, p. 751) showed 

that the shifting of a river's course varies directly with 

the rate of rise and fall of flood flows, bed shear stress, 

relative width of water surface at peak floods and at 

bankfull stage, width-depth ratio at bankfull stage, and 

varies inversely with particle size. Chien also noted 

(1961, p. 744) that channel shifting is related to the

TABLE 8. Particle-size distributions of bed and bank material. Salt Fork, Arkansas River, downstream from Great Salt Plains Dam,

Oklahoma

[Total height of flood plain above water surface is 1.K3 meters]

River bank and 

bed features 

(Distance below

flood plain, in 

meters)

Top surface of clay band in

silt

0.30-0.43

0.49-0.67

1.47-1.22

1.49-1.77

Channel bedBar at water level

Percent finer than indicated size 

(millimeters)

0.002 0.004 0.008 0.016 0.031 0.062

42 50 59 69 79 87

9 9 11 16 24 44

__ ___ __ __ ___ A

1

o

 

0.125

93

68

36

6

4

0

0

0.25

99

90

96

77

25

10

7

0.50

100

99

100

100

84

46

81

1.0 2.0

100

98 99

88 99

98 100

4.0

100

100

 Sand sample beneath shallow water in braided channel.
DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

downstream spacing of control points. These relationships probably are not precise correlatives of bank erosion, but channel shifting is related closely to bank erosion. It has not been possible to obtain sufficiently complete data with which to verify equations provided by 

Chien. However, many observed phenomena in the alluvial channels described here qualitatively support his 

conclusions.

Observations on the Missouri River downstream from 

Fort Peck Dam in Montana indicate that bank erosion 

increases markedly with discharges equal to or greater 

than about 500 m3/s (cubic meters per second). This 

is equivalent to flows that occurred equal to or less 

than about 12 percent of the time prior to closure of 

the dam (U.S. Army Corps of Engineers, 1952, p. 37). 

The increase in erosion accompanying increases in flow, 

particularly at or near the bankfull stage, also is 

documented elsewhere (Chien, 1961, p. 741; Leopold 

and others, 1964, p. 88).

Net erosion in terms of enlargements in cross-sectional area (width and depth increases) can occur even 

with decreases in certain flow statistics. A decrease in 

mean daily discharges and in peak discharges during 

the years immediately after dam closure on the Red 

River downstream from Denison Dam nevertheless was 

accompanied by about a 25 percent increase in the average bankfull cross-sectional area of the downstream 

channel. Reductions in those same flows in the North 

Platte River downstream from Guernsey Dam in 

Wyoming were accompanied by a doubling of the average cross-sectional area throughout a 5 km-long reach 

downstream from the dam. Thus in these cases the 

mean daily flow and the annual peaks do not reflect 

adequately the erosive flows.

Along other channels, decreases in flows have been 

accompanied by decreases in (cross-sectional area and 

in width. Flow reductions due largely to various dams 

probably have caused the observed decrease in width 

of the Platte River in much of Nebraska to as little 

as 10 to 20 percent of its 1865 width (Williams, 1978). 

Where the decrease in flow has been significant, as in 

the lower Rio Grande, J. F. Friedkin (International 

Boundary Commission, written commun., 1959) has 

shown that the channel almost may disappear as vegetation, windblown sand, and sediment deposited by low 

flows clog the channel. Comparable changes on the 

Canadian River downstream from Sanford Dam, Texas, 

are described later in this report.

Sandstone Creek near Cheyenne, Oklahoma, provides 

one of the better-documented examples of cross-section 

decreases and channel narrowing due to dams, in this 

instance, a combination of dams (Bergman and Sullivan, 

1963). Sandstone Creek has a drainage area of 277 km2. 

Land-treatment measures were begun during the

1940's; by 1952, 24 floodwater-retarding structures and 

17 gully plugs had been built in the watershed. Further 

construction continued during the 1950's and 1960's. 

During the 1950's the hydrologic regimen of the stream 

was altered significantly (table 9). From 1951 to 1959, 

mean daily flow tended to increase as the number of 

days of zero flow decreased from almost two-thirds of 

the year to zero. In addition, a significant increase in 

the number of peak flows occurred during 1953-56, 

suggesting a brief period of increased rainfall. In 1954, 

the channel cross section still retained the box-like form 

characteristic of an arroyo (fig. 23). By 1961, however, 

a much narrower channel (about one-third the former 

width), stabilized by vegetation (grass, shrubs, and 

some trees) had formed within the original cross section. A new flood plain had been created, virtually as 

an inset fill. The effect of the new channel cross section 

and vegetation is illustrated by the decreases in crosssectional area and flow at successive stages (fig. 23).

The metamorphosis of Sandstone Creek seems to follow a pattern typical of a number of other dammed 

streams (see Frickel, 1972, p. 29; Gregory and Park, 

1974; Petts, 1977). Once the larger flows are eliminated, 

the flows occupy a somewhat narrower channel. Vegetation commonly tends to become established on the 

lesser-used part of the old streambed. This plant 

growth probably traps sediment during any inundaTABLE 9. Flow data for Satidstone Creek near Cheyenne, Oklahotna, 1951-59

[m3/s = cubic meters per second]

Water 

year

Mean

daily

flow

Days of 

zero 

flow

Number of 

peaks greater 

than 14 m /s

1952

1953

1954

1955

1956

1957

1958

1959

0.028

.020

.36

.14

.075 

.25 

.11 

.45

222

184

114

39

87

62

7

0

PERC ENT_DECRE ASE

AREA FLOW 

- 4 32

15

12

40

23

FIGURE 23. Changes in channel cross section of Sandstone Creek, 

Oklahoma, at the streamflow-gaging station, 1954-61, caused by 

many upstream flood-detention dams (modified from Bergman and 

Sullivan, 1963).
CHANNEL WIDTH 39

tions. The vegetated zone thereby aggrades (fig. 23) 

and becomes a new flood plain. The old flood plain becomes inactive (a terrace), rarely or never flooded. In 

this manner, the stream channelizes itself, commonly 

in more stable banks from the binding properties of 

the vegetation.

Although it is intuitively obvious that the magnitude 

and frequency of flows must affect bank erosion, a precise characterization of such flows for purposes of a general equation has not yet been obtained. Some preliminary efforts to develop a general equation are described 

later in this discussion.

WIDTH-DEPTH RATIO

Analysis of a number of cross sections indicated that 

wide, shallow channels tend to increase in width at a 

somewhat greater rate than relatively narrow, deep 

sections. A large initial width-to-depth ratio indicates 

that bank material in such sections may be more erodible, and that these sections are likely to predominate 

in braided reaches. Because such a process cannot continue forever, channels may narrow by taking a new 

course or by developing several distributary sections.

TIME TRENDS OF CHANNEL 

WIDENING AT A SITE

A dimensionless relative change in width can be defined as Wt/Wi, where Wl is the bankfull channel width 

at the time of dam closure at the cross section of interest and Wt is the bankfull channel width t years later 

at the same section. A plot of this ratio with time was 

made for each cross section downstream from the 17 

dams for which data (table 13) were available. On these 

plots, nearly 50 percent of the 105 cross sections that 

became wider have either too many aberrations, no 

noticeable pattern, or insufficient data to warrant an 

attempt to fit a line to the points (see fig. 24A for some 

typical examples of such cross sections).

The trend of relative increase in width with time for 

the remaining 54 cross sections can be described by 

a simple hyperbolic equation of the same type used for 

bed degradation. As applied to relative channel-width 

changes, this equation has the straight-line form

(Wt/Wt) = c3 + c4 (I/O (3) 

where

c3 is the intercept; and

c4 is the slope of the fitted straight line on a plot of 

Wt/Wt versus lit.

(A) Irregular rates of increase (B) "Regular" rates of increase

1.5

^ 1.0

I 

O

I 

O

0,5 

2.5

2.0 

1.5

1.0 

1.5

1.0 

0.5

rr_: r

Missouri River. 

North Dakota

61 kilometers

downstream from

Garrison Dam

I I

\ \

Missouri River, 

South Dakota,,

1.6 kilometers 

downstream from 

Fort Randall Dam 

_J_____I

I I

Missouri River.

South Dakota

- - 

^

19 ki lometers

'downstream from 

Fort Randall Dam

\ I 

Red River, 

Oklahoma-Texas

80 ki lometers

downstream from

Denison Dam

Red I River, I 

Oklahoma-Texas

150 ki lometers ~^* 

downstream from 

Denison Dam 

1 1

Missouri River, 

South Dakota

11 kilometers 

downstream from 

Fort Randall Dam 

I I

10 20 30 0 10 20 30

1,5

1.0 

2.5

2.0

1.5

1.0 

2.5

2,0

1.5

I I I 

Medicine Creek, Nebraska 

0.8 kilometer downstream 

_ from Medicine Creek Dam

r2 =0,33

1.0

I I I

Missouri River, South Dakota

26 kilometers downstream

/x from Gavins Point Dam 

- / 

1 r2 =0.81

Missouri River, North Dakota

'47 kilometers downstream 

r from Garrison Dam

r2 =0.91

I I I

Missouri River, South Dakota

14.5 kilometers downstream

from Gavins Point Dam

^^^-~- * > r2 =0.79

 -*'"}_____I_____I

\ I I 

Missouri River, South Dakota 

43 kilometers downstream 

from Fort Randall Dam

r 2 =0.90

I I I

Missouri River. South Dakota

48 kilometers downstream

from Gavins Point Dam

r2 =0.99

0 10 20 30 40 0 10 20 30 40

YEARS AFTER DAM CLOSURE

FIGURE 24. Examples of relative increase of channel width with time: A, irregular rates; B, regular rates with fitted regression curves

(dashed lines). Data from table 13.
40 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

The coefficients are positive where width has increased, 

as in the present discussion, and negative where width 

has decreased. The reciprocal of c4 is the initial rate 

of change, in relative-change (Wt/Wi) units per year. 

The reciprocal of c3 ordinarily would be the asymptote 

or eventual new value of Wt/Wi. However, with the 

present application, the value of W/Wi at t = 0 is 1.0 

rather than 0. To adjust the data to an origin of 0, 

1.0 first needs to be subtracted from each Wt/Wi before 

performing the regression. Consequently, the 

asymptote or final predicted Wt/Wi is (l/c3) + 1.0. Similarly, the value of Wt/Wi at any time, t, is

+ 1.0

<cs+i)

Values associated with these channel-width regression curves, such as coefficients c3 and c4, estimated 

final equilibrium values of Wt/Wi, time needed to complete the estimated total change (here given as 95 percent of the estimated change), time needed to attain 

50 percent of the estimated total change, and the square 

of the correlation coefficient (r2) are given in table 10.

As discussed in connection with bed degradation, several features that do not directly affect the goodness 

of fit influence the value of r2 . For example, the value 

of r2 is somewhat sensitive to the location of the origin; 

that is, to the specification of the response time where 

such a lag period occurs. Hence, measurements in the 

first few years after (and before) dam closure are very 

important in defining the curve.

The asymptote of the curve, or extrapolated eventual 

value of Wt/Wi, needs to be treated with caution. 

Where the basic data fit the curve, the extrapolated 

final value is valid. However, in several instances, the 

data show enough departures from a smooth curve that 

illogical values of the asymptote obtain. These few cases 

are noted in table 10.

Based on the coefficients and the observed fit of the 

curve to the data points, the equations for 10 of the 

54 sections are questionable. The remaining 44 cross 

sections, listed by dam and number of sections, were 

downstream from Fort Peck (3), Garrison (11), Fort 

Randall (3), Gavins Point (20), Medicine Creek (1), 

Kanopolis (3), and Denison Dams (3).

The regression features for these widening cross sections show a wide range in initial rate of increase of 

channel width, predicted (or observed) final relative increase, and time required for the new width to develop. 

Some representative trends are shown in figure 24B. 

The initial rate of increase (reciprocal of the coefficient 

c4) for the 44 cross sections ranges from 0.0032 to 4.0 

relative-change units per year.

The predicted final values ofWt/W1 (called (Wt/Wi)max

in table 10) as extrapolated from the regression curves 

(again keeping in mind the risks of extrapolation) range 

from very slight (1.05) to about 2.8. (The latter number 

would indicate that the final width would be 2.8 times 

the width at the time of dam closure.) The frequency 

distribution of these 44 values (fig. 25A) shows most 

of them closer to the smaller end of the range, with 

the mode at about 1.12.

The estimated time needed for completion of 95 percent of the eventual change in width (see bed-degradation section for computation details) ranges from about 

2 to nearly 1,900 years (table 10). The modal value of 

the 44 estimates is about 35 years (fig. 25B). Assuming 

no radical changes in the flow regime, most sections 

are predicted to need from about 1 decade to 600 years 

to complete their widening. Within this range, the 

longer durations (as much as hundreds of years) of 

course are mathematical results. We have no evidence 

that channel widening continues for such durations, and 

there is considerable evidence of discontinuity and 

change.

As with bed degradation, much of the estimated 

widening occurs relatively quickly. One-half the total 

estimated overall increase in channel width can occur 

in as little as 1 or 2 months (table 10). For the 44 cross 

sections, the maximum estimate of the time needed for 

a section to complete 50 percent of its widening was 

100 years. The distribution within this range (fig. 25C) 

has its mode at about IVz to 2 years. At most cross 

sections, 50 percent of the total eventual increase in 

width probably occurs within 2 or 3 decades after dam 

closure, according to these data.

The above estimates of magnitudes of eventual widening and of adjustment time also apply to many cross 

sections for which the data were not fitted by a regression curve, judging from plots of Wt/Wi versus time 

(fig. 24).

Curves of relative increase in width with time (fig. 

24B) can all be combined onto one general, dimensionless curve (fig. 26) similar to the one for bed degradation. The ordinate in this case is the ratio of observed 

relative change in width (Wt/Wi) at a given time to the 

extrapolated maximum expectable relative change, the 

latter being approximated by 0.95 (Wt/Wi)max . The 

abscissa on the plot is the proportion of total adjustment time that has elapsed, £70.95 tmax. Here the denominator (0.95 tmax) is 19 c4/c3 (as explained earlier). 

The dimensionless equation, referred to as the derived 

equation in figure 26, is identical to equation 2, for degradation, with the new dependent variable inserted.

To the extent that the data fit the standardized 

curve, the same tendencies that described bed degradation with time also apply to the rate of channel widening. One-half the total change occurs during the first
CHANNEL WIDTH

TABLE 10. Values associated with hyperbolic curves fitted to changes in channel width with time, at a cross section'

[km, kilometer; yr. year; r2 , square of correlation coefficient; c3 , coefficient (intercept) of fitted straight line on plot of W,/Wt versus 1/t; c4 , coefficient 

(slope) of fitted straight line on plot of Wi/Wt versus 1/t; Wt , channel width at t years after dam closure; W,, channel width at time of dam closure]

41

Distance

of cross

section

downstream

from dam

(km)

Response

time

(yr)

 , /TT x 4/ 5/ Time to reach

^ C 3« c. 4

Jemez River, New Mexico,

1.0

1.3

1.6

1.8

2.4

3.1

15.5

22

33

36

0

0

0

0

0

0

0?

0?

0?

0?

0.37 -1.041

.79 -.943

.77 -.565

.83 -.941

.52 .010

.64 -1.279

Arkansas River

.99 -.926

.99 -.902

.79 -1.15

.91 -1.62

-2.66

-2.32

-8.51

-5.87

-19.2

-1.77

, Colorado,

-10.74

-11.35

-13.83

-12.78

Missouri River, Montana

9.2

16.5

75

12.0

15.0

17.5

21

32

38

44

47

54

58

78

87

7.7

43

58

4.3

5.3

6.8

11.0

12.5

14.5

16.5

22

26

27

28

30

32

34

48

52

61

64

69

72

82

85

93

.8

13.0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

5

0

0

0

0

0

0

0

0?

0?

5?

4.5?

3?

4?

5?

4?

4?

0

0

.79 4.89

.62 1.51

.90 3.12

Missouri River

0.65 11.48

.81 2.16

.48 .901

.61 19.4

.94 9.01

.36 4.24

.98 .764

.91 .780

.57 1.05

.83 5.67

.68 4 . 95

.56 2.46

Missouri River,

.84 5.62

.90 1.88

.59 16.2

Missouri River,

.58 4.19

1.00 11.5

.27 5.03

.95 4.37

.95 2.18

0.79 1.79

.94 2.24

.97 8.77

.81 .552

.84 3.17

1.00 2.03

1.00 3.81

.95 3.54

.90 1.88

.99 2.69

.98 .990

1.00 9.80

.95 2.90

.39 15.5

.97 2.37

1.00 4.68

.95 1.75

.86 .290

Medicine Creek,

.33 4.78

.35 .937

26.36

31.39

308.8

P 0.9 wmax

Jemez Canyon Dam

0.039

.22

John Martin Dam

_

.13

.38

, Fort Peck Dam

1.20

1.66

1.32

5(W /W )^

max

(yr)

49

26

230

150

100

395

1,880

Time to reach

0.5(Wt /W1 )-/

max

(yr)

2.6

1.4

12

8

5

21

100

, North Dakota, Garrison Dam

5.62

10.51

.306

31.64

3.52

2.16

10.33

2.09

.350

8.73

6.36

.249

1.09

1.46

2.11

1.05

1.11

1.24

(2.31)

2.28

1.95

1.18

1.20

1.41

9

90

6

30

7

10

(260)

50

6

29

24

2

0.5

5

.3

1.6

.4

.5

(14)

3

.3

1.5

1.3

.1

South Dakota, Fort Randall Dam

42.1

7.01

42.2

1.18

1.53

1.06

140

70

50

7

4

3

South Dakota, Gavins Point Dam

3.04

196.1

3.03

9.71

17.00

41.0

4.50

210.5

9.71

7.59

13.6

1.80

14.0

63.6

6.28

23.0

22.9

23.5

21.6

47.6

144.3

5.91

4.95

Nebraska,

13.5

52.4

1.24

1.09

1.20

1.23

1.46

1.56

1.45

1.11

2.81

1.32

1.49

1.26

1.28

1.53

1.37

(2.01)

1.10

1.35

1.06

(1.42)

1.21

1.57

(4.45)

Medicine Creek Dam

1.21

(2.07)

14

320

11

42

150

440

38

460

330

45

130

9

75

640

44

(440)

44

150

26

(380)

590

65

(320)

54

(1,060)

.7

17

.6

2

8

23

2

24

18

2

7

.5

4

34

2

(23)

2

8

1.4

(20)

31

3

(17)

3

(55)
42 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 10. Values associated with hyperbolic curves fitted to changes in channel width with time, at a cross section' Continued

Distance 

of cross 

section 

downstream 

from dam 

(km)

Response 

time

(yr)

2 , / \ 4/ 51 Time to reach

r c c, I 77- I 0.95(W /W.. ) 3 4 \ W, / t 1 \ I/ max

max , . (yr)

Time to reach

0.5(Wt /W1 )-/ 

max 

(yr)

Smoky Hill River, Kansas, Kanopolis Dam

6.8

8.7

13.0

25

73

2.9

3.9

4.7

6.6

0

0

0

0

0

0?

0?

0?

0?

0.64

.80

1.00

1.00

.75

.98

.34

.90

.99

1.89

.866

6.75

6.03

5.49

Wolf Creek,

-1.22

-1.24

-.912

-.901

North Canadian

1.8

5.6

14.5

114

125

134

4

2.5

1.0

2.8

2.8

0

.81

.71

.99

.75

.72

.30

-1.13

-1.93

-1.01

-3.00

-3.64

-5.99

-338.9

45.1

4.42

79.9

7.26

Oklahoma, Fort

-1.47

-1.28

-8.35

-5.47

River , Oklahoma

-4.82

-2.80

-7.48

-1.37

-2.80

-1.33

_

(2.15)

1.15

1.17

1.18

Supply Dam

.18

.19

(0)

(0)

, Canton Dam

.12

.48

(.01)

.67

.73

.83

_

(990)

12

250

25

23

20

80

28

(140)

9

15

4

_

(50)

.7

13

1.3

1.2

1.0

4

1.5

(7)

.5

.8

.2

Red River, Oklahoma-Texas, Denison Dam

.6

18.5

27

34

48

90

132

0

0

0

0?

0?

0?

0?

.63

1.00

.83

0.82

.97

.60

.93

1.74

8.33

2.28

0.416

.725

4.52

1.52

34.0

125.1

11.1

39.0

3.42

40.5

63.5

(1.57)

1.12

1.44

(3.40)

(2.38)

1.22

(1.66)

(370)

285

90

(1,780)

(90)

170

(790)

(20)

15

5

(95)

(5)

9

(42)

2/ Lj 22. Listed r is for W /W , not the reciprocal.

 All values of W /W.. were adjusted to an origin of 0 by subtracting 1.0 prior to the regression.

 The predicted final values of W /W (called W /W in table) are computed as (1/c ) + 1.0. 

c- / max

 Values in parentheses seem unreasonable. Leaders mean that a value cannot or was not 

listed due to curve-fitting difficulties.

5 percent of the adjustment period. Three-fourths of 

the total increase takes place within the first 13 percent 

of the adjustment period. Channel changes are most 

pronounced in the early years after the onset of widening.

TIME TRENDS OF CHANNEL 

NARROWING AT A SITE

Fifty-nine cross sections became narrower, and 39 of 

these have a sufficiently irregular trend of relative 

width with time that no smooth curve can be fitted 

to the points; some representative examples of such 

cases are shown in figure 27A. At some cross sections, 

the new width already was established by the time of

the first resurvey and changed little thereafter. Other 

sites show fluctuations in width with time.

At the remaining 20 of the 59 narrowed cross sections, the data of table 13 again indicate the hyperbolic 

curve of the type used earlier in this report (eq. 3). 

The regression statistics (table 10) indicate that only 

11 of these cross sections are suitable for estimating 

final channel widths and adjustment periods. The 11 

cross sections are downstream from Jemez Canyon, 

John Martin, Fort Supply, and Canton Dams. Six typi0al regression curves are shown in figure 275.

Initial rates of narrowing ranged from 0.05 to 0.78 

relative-change units per year. The extrapolated final 

values of Wt/Wl (table 10) ranged from 0.83 to 0.04 for 

the 11 curve-fitted cases that could be assessed reliably.
CHANNEL WIDTH 43

3 1 10

ESTIMATED FINAL Wt /W., 0.95t

100 1000 

. IN YEARS

10,000 0.1

0.50t

10 

.. IN YEARS

100

FIGURE 25. Frequency distributions of estimated eventual increases in channel width, based on 44 measured cross sections on various 

rivers: A, final values of Wt/Wi, B, years needed to widen to 95 percent of final Wt/Wi\ C, years needed to widen to 50 percent 

of final Wt/Wi.

i O . 

generally was only about 25 to 50 percent of the initial 

width, except for a few of the cross sections 

downstream from Canton Dam.) Theoretically, the relative decrease in width for a channel can range from 

almost 1.00 to 0, depending on flow regulation.

The estimated time needed for the channel to reach 

its new, narrow width varies from 4 to 230 years for 

the 11 cross sections (table 10). Most estimates from 

the fitted curves are about a few decades or less. Onehalf the total adjustment can occur virtually immediately or within as much as about a decade. Less 

than 1 or 2 years is typical for the available data.

The dimensionless standardized curve of the type applied above to bed degradation and channel widening 

is shown in figure 28. The derived equation is that of 

equation 2 with the appropriate dependent variable 

(proportional relative decrease in width).

PREDICTION OF POST-DAM 

CHANNEL-WIDTH CHANGES

Channel width depends primarily on water discharges 

and the boundary sediment. A multitude of regime- and 

hydraulic-geometry equations relate width to discharge. 

Unfortunately, most of those that are not site-specific 

require a resistance coefficient, a characteristic or dominant discharge, or both. Bed-material sizes change with 

time during the armoring process downstream from 

many dams (fig. 13), so even in the rare case where 

the size distribution had been measured adequately, it 

would be hard to build this changing particle-size variable into a resistance coefficient to predict eventual 

channel width. Similarly, identification of the most diagnostic or dominant discharge to use in an equation for

1 .4

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 _

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0,000 »

*7 ' *

cy

/ V 0.8 kilometer downstream from

&/ Medicine Creek Dam Nebraska

/ ^7 "/ & 14.5 kilometers downstream from

/ Gavins Point Dam, South Dakota

I D 26 kilometers downstream from ~

C^> Gavins Point Dam, South Dakota

L O 43 kilometers downstream from

D Fort Randall Dam, South Dakota

T~ A 47 ki lometers downstream from 

_ II Garrison Dam, North Dakota

P U 48 kilometers downstream from

^ I Gavins Point Dam, South Dakota

0,0* III

00 0.2 0.4 0.6 0,8 1.0

t/0,95t max

FIGURE 26. Dimensionless plot of relative increase in channel width 

with time, for the 6 representative cross sections of figure 24B. 

Data from table 13.

(At sections not describable by a hyperbolic curve 

downstream from these same four dams, the new width
44 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

(A) Irregular rates of decrease (B) "Regular" rates of decrease

1.5

1.0

55 0.5 

I)

I

Q 0.0 

E 1.5

at

I 

O

1.0

0.5

0.0

I I I 

Wolf Creek, Oklahoma 

0,3 kilometer downstream

T from Fort Supply Dam

^ I ' I

I I I

^_II__^..._

North Canadian River, 

 Oklahoma 3,1 kilometers 

downstream from 

Canton Dam 

I I I

I I I

Red River, Oklahoma-Texas 

122 kilometers downstream 

from Denison Dam ~~

I I I

I I I 

Arkansas River, Colorado 

19 kilometers downstream

^^ x^from John Martin

^^x. Dam

I I I

North Canadian River, 

Oklahoma

I I I

North Canadian River 

Oklahoma 134 kilometers

 *V » 

 

_ downstream from _ 

Canton Darn

r 2 =0.30 

I I I

I I I 

Arkansas River, Colorado 

36 kilometers downstream

"\ "~ 

x

_ ^"~~~*--J. _

from John Martin Dam

r2 =0.91 

I I I

I I I

Wolf Creek, Oklahoma 

3.9 kilometers downstream

1 from Fort Supply Dam

_\ r2 = 0.34 _ 

\ 

"^ -.._!_ __

1 1 ' 1

1 1 1

Jemez River, New Mexico 

1.0 kilometer downstream

from Jemez Canyon Dam

\

-\ r 2 = 0 37

f

?r -r i

12.0 kilometers 

downstream from 

Canton Dam

0.0

I I I

North Canadian River,

Oklahoma 7.4 kilometers

downstream from

Canton Dam

2

T 

l 

\ \

1 1 1

Wolf Creek, Oklahoma 

9 kilometers downstream

from Fort Supply Dam

^ r 2 =0-98 

1 1 1

-

North Canadian River. 

Oklahoma 1.8 kilometers

downstream from 

Canton Dam

\ r 2 =0.81

10 20 30 40 0 10 20 30 40 0 10 20 30 40 0 10 20 30 40

YEARS AFTER DAM CLOSURE

FIGURE 27. Examples of relative decrease in channel width with time: A, irregular rates; B, regular rates with fitted regression curves

(dashed lines). Data from table 13.

 I I I I I I I I

 134 KILOMETERS DOWNSTREAM FROM CANTON DAM. OKLAHOMA

O 36 KILOMETERS DOWNSTREAM FROM JOHN MARTIN DAM, COLORADO

 2.9 KILOMETERS DOWNSTREAM FROM FORT SUPPLY DAM, OKLAHOMA 

3.9 KILOMETERS DOWNSTREAM FROM FORT SUPPLY DAM, OKLAHOMA 

1.0 KILOMETER DOWNSTREAM FROM JEMEZ CANYON DAM, NEW MEXICO 

1.8 KILOMETERS DOWNSTREAM FROM CANTON DAM, OKLAHOMA

CURVE OF 

DERIVED EQUATION

0 0.2 0.4 0.6 0.8 1.0 12 1.4 1.6 1.8 2.0 2,2 2.4 2-6 2.8

FIGURE 28. Dimensionless plot of relative decrease in channel width with time, for the 6 cross sections of figure 275. One point (for 

the section 134 kilometers downstream from Canton Dam) plots off the graph. Data from table 13.
CHANNEL WIDTH 45

channel form remains an unsolved problem. An empirical effort was made (no acceptable theory being available) to determine those measures of discharge best related to width and to changes in width. Due to lack 

of data on bank cohesiveness, the search involved only 

water discharge.

We used stepforward multiple regressions to test 

possible correlations between channel width and magnitudes, frequencies, and characteristics of discharge, 

namely: (1) Mean daily discharge; (2) average annual 

instantaneous peak flow; (3) single highest and lowest 

instantaneous annual peak flow; (4) highest average 

daily flow for consecutive periods of 1, 3, 7, 15, 30, 

60, 90, 120, and 183 days for each year; (5) flow equaled 

or exceeded 1, 5, 10, 25, and 50 percent of the time; 

and (6) variability of flows within periods ranging from 

1 to many years. Many different possible expressions 

for flow variability (item 6) involving the ratio of a high 

flow to a standardized flow were tested. Variations 

within a day, however, could not be considered, and 

such variations could be important on channels where 

flows are regulated by dams for hydroelectric power. 

Seasonal sequences were not explored. All of the above 

discharges, including ratios thereof, were examined for 

the pre- and post-dam periods, separately. The many 

discharge statistics, plus the log of each, amounted to 

115 independent variables.

For each river, the average width for all cross sections as a group was taken as the representative channel width for the particular year. These reaches in general have little significant tributary inflow throughout 

their lengths. Average width was calculated for the 

year of dam closure (first surveys of cross sections), 

yielding W\, and for the year of the latest resurvey, 

W2 . The relative change in width is then W2/W\. Nine 

cross sections had special local topographic features and 

were not included in the calculation of the average 

change in width for the entire reach downstream from 

a dam. These nine sections are downstream from a'total 

of 6 dams and probably do not affect significantly the 

regression results described here.

Along some reaches, sparseness of cross sections is 

a drawback of this sampling approach to generalizing 

the change in width of a reach. Locations of cross sections is another possible disadvantage, in regard to: (1) 

Position around or near meander bends versus straight 

reaches, and (2) spacing with river distance 

downstream. Usually, the sections are close together 

immediately downstream from the dam and become 

farther apart with distance downstream.

Ideally, the length of river reach within which the 

change applies needs to be standardized for the entire 

group of rivers. The first standardized length of reach 

we considered was 47 channel widths (from the most

recent resurvey), this being the longest distance common to the 15 reaches for which enough data were available. Second, we tried defining the standard reach as 

the zone of bed degradation, again from the most recent 

resurvey. A third reach used the entire distance covered by the measured cross sections was not standardized for the group. Best correlations came from this 

last approach, probably because the greater number of 

cross sections provided a better representation.

A general estimate of W2 downstream from the 15 

dams is given by

0.5Q,n (4)

where

W2 is the average bankfull width at the time of

the latest resurvey, in meters; 

Qm is the arithmetic average of the annual mean

daily flows during the post-dam period from dam

closure to the latest resurvey, in cubic meters

per second; and 

Qp is the arithmetic average of annual 1-day highest

average flows for the pre-dam period of record,

in cubic meters per second.

Thus, both pre- and post-dam flows are represented, 

though by different flow statistics. As with many empirical expressions, the relation is not correct dimensionally. The r2 for the regression equation is 0.99, and 

the average absolute error in the predicted W2 is ± 

19 percent. Computed versus observed values of W2 

are compared in figure 29.

The ranges of values used in determining equation 

4 are 30 < W2 < 939 m, 22 < Qp < 5,000 nr'Vs; and 

1.6 < Qm ^ 830 m3/s (table 11). Average daily discharges differ slightly from those of table 4 because 

only flow data up through the latest channel resurvey 

were used for equation 4. Also, filling of the reservoirs 

for Fort Peck, Garrison, Fort Randall and Gavins Point 

Dams was not completed until about 1964, so mean 

daily discharges were computed beginning with 1965 

for these dams. The period of reservoir filling for the 

other dams was assumed to be negligible. This empirical equation applies only to the ranges of data included 

in the analysis. For example, the equation may not be 

valid for dams which release little or no flow. We have 

no explanation for why the post-dam mean daily flow 

and pre-dam average annual 1-day high flow turned out 

to be the significant variables.

Two sites with the required flow data were found 

to test equation 4. The tests are only approximate because the measurement or estimate of post-dam width 

is not made for a long reach of the river. The Canadian 

River at 3 km downstream from Ute Dam (closed in 

1963) is shown in figure 30. Three measurements of
46 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

.. 1000

I

3 900 

i! 800

T. 700 

O

60°

<S

IS

Q

500

400

300

200

100

Line of perfect agreement _

0 100 200 300 400 500 600 700 800 900 1000

MEASURED POST-DAM AVERAGE CHANNEL WIDTH.

IN METERS 

EXPLANATION 

NUMBER DAM AND STATE

1 Jemez Canyon, New Mexico

2 John Martin, Colorado

3 Fort Peck, Montana

4 Garrison, North Dakota

5 Fort Randall, South Dakota

6 Gavins Point. South Dakota

7 Medicine Creek, Nebraska

8 Red River, Iowa

9 Kanopolis, Kansas

10 Fort Supply, Oklahoma

11 Canton, Oklahoma

12 Eufaula, Oklahoma

13 Denison, Oklahoma-Texas

14 Town Bluff, Texas

15 Buford, Georgia

FIGURE 29. Computed (by the equation W2 =13+0.5Qm +0.1 Qp) versus measured values of post-dam average channel widths.

channel width were made in 1981 at different sections 

along a nearly uniform Q.3-km reach (fig. 30). This reach 

is typical in regard to present channel width, according 

to the local U.S. Geological Survey engineers. The measured widths ranged from 43 to 46 m, averaging 44 

m. According to equation 4, the width should be 55 

m (Qm = 1.2 m3/s, Qp = 410 m3/s). This estimate is 

therefore slightly (25 percent) too large.

The second test area is the Republican River 

downstream from Trenton Dam, Nebraska (fig. 31). 

The flow records provide Qm = 1.8 m3/s and Qp = 112 

m3/s, for which equation 4 yields a width of 25 m. Judging from figure 31, the present width is an estimated 

30 percent less than 25 m.

None of the 115 variables correlated particularly well 

with the relative change in channel width (W^W^, although some approximate correlations will be mentioned below. With respect to W2/Wi, the 15 damsites

can be divided into two distinct groups. The first includes the 11 dams downstream from which the channel 

either has undergone a slight widening, on the average, 

or has not changed appreciably. All hydropower dams, 

and some others, are in this group. The channels in 

the second group (downstream from Jemez Canyon, 

John Martin, Fort Supply, and Canton Dams) have narrowed considerably (as described earlier). The distinctive feature of the post-dam flow regime for the latter 

group seems to be that these channels convey little or 

no flow during a large part of the year. In contrast, 

the channels in which the width has remained constant 

or has enlarged are rarely dry and generally convey 

substantial (though not overbank) flows. Osterkamp 

and Hedman (1981) studied regulated Kansas streams 

on which the flow releases, though sustained, were not 

large and erosive. The channels tended to be narrower 

downstream from the dams than upstream. A general 

knowledge of a proposed dam's release policy, therefore, might indicate whether significant channel widening or narrowing is likely to occur. Further study needs 

to be given to this possibility.

Some of the narrowed channels may have conveyed 

little water during much of the year even during the 

pre-dam era; however, periodic floods then probably 

kept the channels wider. With the virtual elimination, 

or marked curtailment, of such high flows (table 4), lowflow periods appear to have assumed much greater importance. Such prevailing low flows form their own new 

(narrower) channel. Those high post-dam flows that are 

released may not be sufficient to maintain the former 

channel, especially since such flows generally are lower 

than pre-dam high flows (table 4). Vegetation has a better chance to become established on the lesser-used 

part of the streambed, and the course of events described above in connection with Sandstone Creek 

(table 9; fig. 23) can occur. Northrop (1965) reported 

similar processes on the Republican River in Nebraska, 

although flows there have been greater.

W2/Wi did show an approximate correlation with flow 

durations of low flows and also of certain high flows, 

namely: (1) The percent of time which a low flow equal 

to about 0.06 Qm was equaled or exceeded; (2) the percent of time a high flow equal to 8 Qm was equaled 

or exceeded; and (3) the percent of time a high flow 

equal to 0.1 times an estimated bankfull discharge was 

equaled or exceeded. In all three cases, correlation was 

improved by adding the average bankfull width-depth 

ratio as of the year of dam closure as a second independent variable. From these tests, it seems quite possible 

that flow durations help determine relative channel 

changes (W2/Wi); however, the general cause-and-effect 

relation remains unsolved. Part of the difficulty lies in 

the fact that the mechanisms are erosional in some
CHANNEL WIDTH 47

TABLE 11. Data used to derive post-dam channel-width equation

[F, flood control; I, irrigation and water conservation; L, low-flow augmentation; M, municipal and industrial supply; N, navigation; 

P, hydropower; R, re-regulation of flow; S, sediment control; m, meters; m:Vs, cubic meters per second]

Main 

Year 

Dam purP°se

no. I/ Dam of 

closure , dam

5.

6.

7.

8.

9.

10.

11.

13.

14.

16.

17.

18.

19.

20.

21.

Jemez Canyon

John Martin

Fort Peck

Garrison

Fort Randall

Gavins Point

Medicine Creek

Red Rock

Kanopolis

Fort Supply

Canton

Eufaula

Denison

Town Bluff

Buford

 In figure 1

1953

1943

1937

1953

1952

1955

1949

1969

1948

1942

1948

1963

1943

1951

1956

and

S,F

I.F

N,P,

N,P,

N,P,

N,P,

I

F,L,

L,F

F,M

F,M

P,S,

P,F

R,I,

P,F

table 4.

F

F

F

F

I

F

M

Water years Latest 

included in average 

analysis width

Pre-dam

1937;

1944-52

1914-41

1929-36

1929-52

1948-51

1948-54

1938-48

1941-68

1941-47

1938-41

1939-47

1939-62

1937-42

1922-50

1942-55

Post-dam

1954-75

1943-72

1965-73

1965-76

1965-75

1965-74

1951-78

1970-78

1949-71

1943-69

1949-71

1965-77

1944-69

1952-65

1957-71

W2 

(m)

46

50

299

703

820

939

47

167

40

31

30

357

373

126

73

Relative 

change 

in width

Vwi

0.

0.

1.

1.

1.

1.

1.

1.

1.

0.

0.

0.

1.

1.

1.

22

31

16

08

12

18

18

03

03

15

47

97

10

19

04

Post-dam 

average 

daily 

discharge

Qm 

(m3 /s)

1.64

3.37

332

795

779

830

1.88

170

10.1

1.93

5.32

135

120

119

56.4

Pre-dam 

average 

1-day high 

flows

QP 

(m3 /s)

22.2

283

746

3,420

4,460

4,990

170

1,160

228

119

219

2,920

2,760

1,110

566

FIGURE 30. Canadian River about 3 kilometers downstream from 

Ute Dam, New Mexico. A, August 1954; B, April 1980. Dam was 

closed in 1963.

channels (those that have widened) but not in others 

(those that have narrowed).

ROLE OF A DAM IN EFFECTING 

A CHANGE IN CHANNEL WIDTH

Through control of water and sediment flow, the 

change in hydrologic regime associated with reservoir 

releases could result in an increase, decrease, or no 

change in downstream channel width. Channel widening 

conceivably might result from : (1) A decreased sediment load in the flow, enhancing the capacity of the 

flow to entrain sediment from the bed and banks; (2) 

a decrease in the volume of sediment brought to, and 

deposited on or near, the banks, due to the reduced 

sediment transport and decreased high flows (net removal of material); (3) diurnal flow fluctuations (power 

or other controlled releases) causing consistent bank 

wetting and promoting greater bank erodibility; (4) bed 

degradation, where it occurs, resulting in flows impinging at a lower level on the banks, undermining vegetation and the higher section of the banks; and (**) rapid 

changes in flow releases (common with power dams) 

causing the river position to wander indiscriminately 

from one side of the channel to the other, encouraging
48 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

FIGURE 31. Republican River downstream from Trenton Dam, Nebraska. Dam was closed in 1953. Looking downstream from bridge 

at Trenton (4 kilometers downstream from the dam), about 1949 

(A) and July 1980 (B). Looking downstream from bridge at Culbertson (19 kilometers downstream from dam), July 1932 (C) and 

July 1980 (D).

periodic erosion of first one bank and then the other 

without compensatory deposition. Whether the specific 

increases in width reported in this study are due to 

the dams cannot be determined because of lack of conclusive data, especially pre-dam cross-section measurements.

The dam's role on the four channels that have become 

narrower is clearer. Photographs of the Jemez River 

(fig. 17) show that little if any significant narrowing 

of the channel occurred from 1936 to 1951, while a very 

marked reduction in width took place sometime between 1951 and 1980 (dam closure was 1953). Measured 

cross sections (table 13) indicate a relatively wide channel around the time of dam closure and a striking decrease in width in the years immediately thereafter. 

No major reduction in water discharge occurred at the 

control station upstream from the dam during the postdam period (table 4), and 1978 aerial photographs show 

that the channel upstream from the reservoir is still

relatively wide (about as wide as the pre-dam channel 

downstream from the dam). This upstream-downstream 

aerial-photograph comparison of reaches which are 

geographically near one another rules out any climatic 

effects or other factors that might be noticeable on 

other mountain streams in the western United States. 

Finally, the data of table 13 indicate that the overall 

bed degradation and channel narrowing on the Jemez 

River downstream from Jemez Canyon Dam have not 

been affected significantly by any changes farther 

downstream, such as on the Rio Grande. The channel 

narrowing since 1951 downstream from Jemez Canyon 

Dam, therefore, must be due to the dam.

Wolf Creek downstream from Fort Supply Dam (fig. 

19) in 1969 was only about 15 percent as wide as it 

was when the dam was closed in 1942. Aerial photographs taken in 1973 show a relatively wide channel 

upstream from the reservoir, compared to the narrow 

channel downstream from the dam. The similarity of 

the present upstream reach to the pre-dam channel upstream and downstream from the dam, coupled with 

the decreases in width shown by onsite measurements 

(table 13) and photographs (fig. 19) indicate that the 

radical post-dam decrease in width downstream from
SEDIMENT VOLUMES REMOVED AND CHANNEL EQUILIBRIUM 49

the dam very probably is due to the altered flow regime 

controlled by the dam.

Measured cross sections (table 13) and photographs 

(fig. 18) of reaches downstream from John Martin Dam 

on the Arkansas River show a pronounced decrease in 

width (an average of nearly 70 percent for the W2/Wi 

values) after the 1943 dam closure. Such a radical 

change has not occurred upstream. Cableway discharge 

measurements at Las Animas, about 25 km upstream 

from the dam, show no change in the channel width 

during 1946-57, the period for which usable data are 

available. At Nepesta, about 100 km upstream from the 

dam, a similar analysis of cableway measurements for 

1943-65 shows only about a 5 percent decrease in channel width. Two ground photographs of the latter site, 

taken in 1938 and 1963, also indicate no significant 

change in width. Aerial photographs taken in 1950 and 

1970 seem to show a slight channel narrowing and an 

increase in vegetation for many tens of kilometers upstream from the reservoir during that period. The vegetation change had been occurring since at least 1936 

(Bittinger and Stringham, 1963). Due to man's extensive effect on the hydrology of the Arkansas River, 

some channel narrowing and vegetation growth probably would have occurred even without the dam. The 

differences upstream and downstream from the dam are 

large enough, however, that most of the channel narrowing downstream from the dam probably has resulted 

from the dam.

Bankfull width at the streamflow-gaging station 4.8 

km downstream from the site of Canton Dam was about 

60 m in 1938, according to the station description of 

that year. In 1947, the first cross-section surveys 

downstream from Canton Dam (closed in 1948) showed 

channel widths of 65 m 3.1 km downstream from the 

dam and 47 m 5.0 km downstream from the dam. These 

figures indicate some, but not a major, decrease in 

channel width in the reach 5 km downstream from the 

dam during the 9 years before construction of the dam. 

According to the 1976 resurvey, the channel by then 

was 74 percent and 37 percent of its 1947 width at the 

same two cross sections. No control station at which 

water discharges and channel changes are unaffected 

by flow regulation is available for the North Canadian 

River at Canton Dam. However, the fairly stable predam width compared to the decrease in post-dam width 

indicates that much of the decrease in channel width 

is due to Canton Dam.

SEDIMENT VOLUMES REMOVED 

AND CHANNEL EQUILIBRIUM

Year-to-year estimates of volumes of sediment removed from the entire channel boundary within a finite

reach can be determined from end-area measurements 

of cross sections. Such estimates, made by the Corps 

of Engineeers and Bureau of Reclamation, show many 

of the same features as bed degradation. For example, 

U.S. Bureau of Reclamation (1976) computations of this 

type for separate reaches on the Colorado River 

downstream from Davis Dam show that the largest volumes of sediment removal per year take place soon 

after dam closure. As years go by, the estimated volumes removed tend to approach zero net change. These 

tendencies agree with observed degradation and channel-width changes with time, described by the hyperbolic curve discussed above. Large differences, however, can be found from one year to the next, and in 

some years net deposition takes place. Net erosion in 

one reach can occur during the same year as net deposition in an adjoining reach.

Similar data obtained by the U.S. Army Corps of 

Engineers for the Red River show how the volume of 

sediment removed varies with distance downstream. 

Successive times after dam closure also can be compared. A plot of cumulative volumes of sediment removed from the channel boundary as a function of distance downstream is shown in figure 32. A steep line 

on the plot indicates a large increase in the volume removed from one cross section to the next, or, in other 

words, a large volume of erosion has occurred throughout a unit downstream distance during the inclusive 

period represented by the plotted line. The steepness 

of the curve is proportional to the erosion rate for the 

unit reach. A horizontal line indicates that the cumulative volume removed, as of the survey year, no longer 

changes with distance downstream. In the latter case 

neither net erosion nor deposition occurs with distance, 

presumably an indication of a stable channel unaffected 

by the dam.

Both curves in figure 32 show maximum channel erosion in the reaches closest to the dam, with the volume 

of erosion (slope of line) decreasing with distance 

downstream. In 1948, 6 years after closure, the reach 

of appreciable sediment removal extended downstream 

about 55 km. By 1958, the steep curve extended to 

about 90 km; even 160 km downstream, it had not become horizontal. From 1942 to 1948, the first 6 years 

after dam closure, the average rate of sediment removal 

from the first 25 km downstream from the dam was 

about 863,000 m3/yr. By 1958, this rate had decreased 

to about 620,000 m3/yr.

The downstream patterns of degradation and channel 

widening discussed earlier show that the relative volumes of erosion of bed and banks along a given river 

are variable. The contribution from the bed appears to 

be greater closer to the dam; therefore, the longer the 

eroded reach (or the farther the subreach of interest
50 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

50

40

D o

LU S3 30

co 20

10

r i i T i i

1942-48

0 20 40 60 80 100 120 140 160 180 

DISTANCE DOWNSTREAM FROM DAM. IN KILOMETERS

FIGURE 32. Variation in cumulative net sediment volumes of channel 

erosion with distance downstream from Denison Dam on the Red 

River, Oklahoma-Texas.

from the dam), the greater the relative contribution 

from the banks.

Variations from one river to another also are considerable, as has been shown by Petts (1979) for British 

rivers. The U.S. Army Corps of Engineers (1952, p. 

37) examined measured cross sections and estimated 

that, of the channel erosion downstream from Fort Peck 

Dam, about 60 to 70 percent of the sediment removed 

came from the banks and 30 to 40 percent from the 

bed. Estimates for the Red and North Canadian Rivers 

would make this percentage about 80 to 95 percent from 

the banks. In comparison, in certain reaches 

downstream from several dams on the Colorado River, 

the width is constrained, directing most of the erosion 

to the bed alone.

The persistence of disequilibrium or the reestablishment of equilibrium in the relation of sediment inflow 

and outflow in reaches downstream from dams probably 

varies considerably from river to river. Clear water released from the dam may receive a new supply of sediment mainly from the channel bed, the channel banks, 

or from tributary inflows. Unless tributary inflows supply a relatively large proportion of the sediment, the 

regulated river has difficulty in regaining its former 

sediment load from the bed and banks alone.

VEGETATION

OBSERVED CHANGES IN VEGETATION

Vegetation cover in and along channels downstream 

from the dams of this study either remained about the 

same or (most commonly) increased, following dam closure. A decrease in vegetation after a dam was built 

was reported by other investigators in only one case, 

cited below.

Noticeable, and in some cases very extensive, encroachment of vegetation onto former streambeds is apparent downstream from dams on the Jemez River (fig. 

17), Arkansas River (fig. 18), Wolf Creek (fig. 19), 

North Canadian River (fig. 20), Canadian River (fig. 

30), Republican River (fig. 31), and others shown 

below. Considerable vegetation has grown on the Platte 

River downstream from Kingsley Dam in Nebraska 

(Williams, 1978).

A striking increase in vegetation has occurred on the 

Canadian River downstream from Sanford Dam, Texas 

(fig. 33), where virtually no releases of any magnitude 

have been made since dam closure in 1964. Due to the 

scarcity of major tributaries, the effect still is very pronounced 120 km downstream from the dam and probably much farther. Vegetation cover increased in direct 

proportion to the reduction in channel width.

Studying the flood plain rather than the channel, 

Johnson and others (1976) reported a post-dam decrease 

in overall extent of forest cover and in certain kinds 

of trees downstream from Garrison Dam on the Missouri River. Green ash (Fraxinus pennsylvanicd), however, increased.

Vegetation changes in selected reaches downstream 

from 10 dams were mapped in the present study. Vegetated zones were marked on aerial photographs taken 

about the time of dam closure. About 7 to 13 years 

after the date of the aerial photograph, the same areas 

were visited, and vegetated areas again were mapped 

on the same aerial photographs. Of the 10 reaches 

examined, vegetation had covered as much as 90 percent of the channel bottomland in some cases (table 12). 

Seven of the 10 areas showed an increased growth of 

more than 50 percent.

The alternative presence of willow (Salix sp.) or 

saltcedar (tamarisk sp.) for the sites in table 12 appears 

to be dictated at least in part by water quality. For 

example, saltcedar seems to thrive in the saline water 

of the Salt Fork of the Arkansas River in Oklahoma, 

while willow covers large areas on the Republican River 

in Nebraska. Differences are less apparent between the 

Arkansas, Canadian, and Republican Rivers.

Distribution of vegetation in and along channel areas 

appears in at least three common patterns. In the first 

pattern, the increase in vegetation occurs in a strip 

along each bank. Turner and Karpiscak (1980) beautifully document such increases in riparian vegetation on 

the Colorado River between Glen Canyon Dam and 

Lake Mead, Arizona. Of their many sets of photographs, even those that were taken 0 to 13 years prior
VEGETATION 51

FIGURE 33. Canadian River downstream from Sanford Dam, Texas. 

Dam was closed in 1964. Looking downstream from about 400 

meters downstream from damsite, October 1960 (A, prior to dam)

and April 1980 (B). White arrow points to 1980 channel, about 5 

meters wide. Looking upstream from railroad bridge near Canadian, about 120 kilometers downstream from dam, October 1960 

(C, before Sanford Dam) and March 1980 (D). (Photograph credits: 

A and C, U.S. Bureau of Reclamation; B, U.S. Soil Conservation 

Service; D, U.S. Geological Survey.)

to closure of Glen Canyon Dam (1963), compared to recent (1972-76) photographs almost all show a definite 

increase in vegetation. The authors note (p. 19) that 

"in the short period of 13 years the zone of post-dam 

fluvial deposits has been transformed from a barren 

skirt on both sides of the river to a dynamic double 

strip of vegetation." The Des Moines River downstream 

from Red Rock Dam, Iowa, also exemplifies this kind 

of distribution. Overbank areas that formerly had relatively frequent flooding now have significantly more 

trees. Most of the trees sprouted naturally, the remaining few having been planted by residents who found 

the land along the riverside much more habitable after 

dam closure upstream.

In the second pattern, vegetation encroachment occurs within and adjacent to the former channel, leaving 

a much narrower single channel to carry the decreased 

post-dam flows. The succession of changes that a reach 

undergoes in this transformation is illustrated by the 

Washita River 1.4 km downstream from Foss Dam, Oklahoma (fig. 34). (Other smaller dams, farther upstream, also have affected this and other streams in 

this part of Oklahoma, as discussed below.)

The Republican River downstream from Harlan 

County Dam in Nebraska exemplifies a third characteristic pattern. This is shown on aerial photographs 

taken in 1949 and in 1956 (fig. 35). The dam was closed 

in 1952. The discharge on the day of the 1949 photograph was 29 m3/s, whereas the regulated flow on the 

day of the 1956 photograph was only 1.8 m3/s. The vegetation changes are quite evident, nevertheless. In 1949 

the channel shown in the photograph had the typical 

island and bar topography of a braided channel, with 

exposed expanses of clean white sand. In contrast, in 

1956 the channel consisted of thin threads of open water 

in channels converging and diverging around "dark" islands fixed by vegetation. The vegetation consists of 

a dense growth of willows that form a virtually impenetrable jungle (see also fig. 31, Republican River 

downstream from Trenton Dam, Nebraska).
52 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS 

TABLE 12. Change in approximate percentages of riparian vegetation downstream from various dams

River, dam, location

Arkansas, John Martin, above

Lamar, Colorado

Republican, Trenton, below 

Trenton, Nebraska

Republican, Harlan County, 

near Franklin, Nebraska

Republican, Harlan County,

Superior, Nebraska

Red, Denison, Denison, Texas

Salt Fork, Arkansas, Great Salt

Plains, Jet, Oklahoma

North Canadian, Canton,

Oklahoma

Wolf Creek, Fort Supply, Fort 

Supply, Oklahoma

Year 

of 

dam 

closure

1942

1953

1952

1952

1943

1941

1948

1942

Post dam 

time 

period 

analyzed

1947-60

1952-60 

1960-80

1949-56 

1956-80

1950-56

1948-55

1941-54

1960

1960

1951-59 

1959-72

Length 

of 

reach 

(kilometers)

40

6 

6

32 

32

26

35

31

16

16

5 

5

Average 

percent change 

in area 

covered by 

vegetation

90

50-60 

85-95^

60-80 

85 -95-^

65

6

33

60

30-50

(local)

0 

80-90^

Type of 

vegetation

Saltcedar

Willow 

Willow

Willow 

Willow

Willow

Saltcedar

Saltcedar

Willow

Grass, shrub.

willow

 Estimated for short reach from ground photographs.

POSSIBLE CAUSES OF VEGETATION CHANGES

The roots of a plant are vital to its survival; therefore, the scouring effect of high flows can be devastating to vegetation. (The root depth and strength, the 

age and size of the plant and its trunk flexibility all 

affect a plant's ability to withstand the scouring action 

of floods.) Even when a plant is not uprooted completely by a flood, germination and seedling survival 

generally depend on species flood tolerance. This in 

turn is a function of flood magnitude, frequency, and 

duration (Turner, 1974; Teskey and Hinckley, 1977). A 

reduction in such flood characteristics, therefore, often 

enhances vegetation survival and growth.

If one deals only with the flood plain as opposed to 

the channel and banks, the effect of floods is less clear. 

Some trees, for example, may grow better under 

periodic flooding, especially where vigorous scouring is 

less active or less effective than gentler inundation. 

Johnson and others (1976) attributed a post-dam decrease in cottonwood (Populus deltoides Marsh), box 

elder (Acer negundo L.), and American elm (Ulmus 

americana L.) on the flood plain of the Missouri River 

downstream from Garrison Dam in part to the reduction 

of floods that formerly brought more nutrients and produced a higher water table.

An increase in low flows has been thought to increase 

riparian plant growth. Such augmentation would raise 

the water table and increase the soil moisture, thus 

effecting an increase in vegetation. Some of the dams 

listed in table 12, such as Wolf Creek downstream from 

Fort Supply Dam, seem to support this thesis, insofar 

as an increase of both vegetation and low flows has 

occurred. However, a number of other dammed rivers, 

such as the Jemez and part of the Republican, have 

considerably reduced low flows, and yet vegetation also 

increased downstream from the dams on these rivers 

(figs. 17, 31, and 35). Thus, while increased low flow 

can encourage the spread of riparian vegetation along 

rivers, it does not appear to be a requirement, provided 

moisture is available.

Ground-water withdrawals downstream from some 

dams have increased in recent years. Such withdrawals 

theoretically should lower the water table and decrease 

soil moisture, tending to inhibit many plant species. The 

importance of ground-water withdrawals in regard to 

post-dam vegetation changes could not be determined 

for the rivers studied here.

Climatic changes could bring new conditions of temperature, humidity, and rainfall. The reaction of vegetation type and density to such changes may not be 

readily apparent. A period of less annual rainfall, for
FIGURE 34. Washita River about 1.4 kilometers downstream from Foss Dam, Oklahoma. A, February 1958; B, May 1962; C, March 1967;

D, February 1970. Dam was closed in 1961.

example, could mean fewer flood peaks and an 

attendant establishment of vegetation, or it could mean 

less moisture in the ground and less vegetative growth. 

(Flood intensity and spatial distribution, which in turn 

depend on the intensity and distribution of precipitation, may be as important for plant survival as flood 

frequency. Total annual rainfall might not show changes 

in any of these factors.) In any event, changes in plant 

species might accompany climatic changes.

Channel shape also could be a factor in vegetation 

changes. Little change can be expected on a narrow, 

deep channel. In comparison, a wide, shallow channel 

offers a better opportunity for vegetation to become 

established.

Rate of channel meandering has not been treated 

separately in this paper. However, if sinuosity is affected by a dam (as mentioned briefly above), then rate 

of channel meandering also would change. Gill (1973) 

explains that the nature of the flood-plain plant 

community is very closely related to lateral migration 

of the channel. Johnson and others (1976) attributed 

a lack of young stands of cottonwood (Populus sp.) 

along the Missouri River downstream from Garrison 

Dam to a lesser rate of meandering after dam construction.

The U.S. Fish and Wildlife Service studied the seed 

germination and seedling establishment of willows and 

cottonwoods along the Platte River in Nebraska,
54 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

FIGURE 35. Republican River downstream from Harlan County Dam, Nebraska, before and after dam closure. Reach shown is near 

Bloomington, Nebraska. Flow was 29 cubic meters per second at time of 1949 photograph and only 1.8 cubic meters per second at 

time of 1956 photograph, but the increase in vegetation (dark areas) and the change in channel pattern are nonetheless apparent.

downstream from Kingsley Dam (U.S. Fish and 

Wildlife Service, 1981). Favorable features were a significant soil-moisture content, bed material in the fine 

sand-to-sand size class, at least 1 to 2 weeks of ground 

exposure during the time when viable seed is available 

(mid-May to August), and moderate water depth during 

flooding. New seedlings survived only on bare 

sandbars, and not in upland shrub and woodland areas. 

Many factors, most of which could be affected or controlled by flow regulation, affect plant growth.

SEPARATING FLOW REGULATION FROM OTHER 

FACTORS AFFECTING VEGETATION CHANGE

A downstream increase in vegetation following dam 

construction does not necessarily mean the dam caused 

the change. As noted above, a number of factors, not 

all of which are dam-related, can affect vegetation. In 

addition, a particular plant may spread rapidly and even 

achieve dominance in a given region. Saltcedar growth,

for example, is highly suspect as an indicator of the 

effects of flow regulation. This plant has been spreading 

at a rapid rate along innumerable valleys in the southwestern United States since its introduction late in the 

18th century (Everitt, 1980). Although many of the valleys into which it has spread have been subjected to 

flow regulation, reaches or sections of many others have 

not. Larner and others (1974) concluded that, in view 

of the regional spread of saltcedar in west-central Texas 

since about the 1920's, the observed accelerated increase in saltcedar downstream from various dams in 

that area meant that flow regulation by dams contributed to, but was not solely responsible for, the increase in riparian vegetation. On the Arkansas River, 

infestation by saltcedar is, if anything, more extensive 

on flood plain and channel bottom upstream from John 

Martin Dam, including several hundred kilometers 

beyond the backwater reach, than it is downstream 

from the dam (Bittinger and Stringham, 1963). Flow
VEGETATION 55

regulation at John Martin Dam does not appear to be 

a sufficient explanation of the spread of saltcedar along 

this reach of the Arkansas. Similar growth has been 

observed on the Pecos River both upstream and 

downstream from Red Bluff Reservoir near the New 

Mexico-Texas State line (memorandum and photographs of Trigg Twichell, U.S. Geological Survey, December 21, 1961). In low areas along a 25-km reach 

of the Gila River valley in Arizona, saltcedar has become a dominant species of vegetation from 1944 to 

1964. Turner (1974, p. 10) notes that changes in vegetation since 1914 have not coincided with channel changes. 

Moreover, while natural changes in flow regime have 

reduced winter flood frequency, and although increased 

summer low flow would enhance saltcedar growth, a 

decrease in cottonwoods does not correspond with 

changes in hydrologic regime (Turner, 1974 p. 13). 

Turner (1974, p. 18) concludes that, for the reach 

studied on the Gila River, neither disruption of the 

channel nor changes in flow regime account for the ascension of saltcedar to dominance over the indigenous 

vegetation. Rather, saltcedar competed succesfully with 

native plants and appears to be able to sustain its position indefinitely (Turner, 1974, p. 19).

Climatic variability can complicate any attempt to determine the extent of channel changes and of increased 

vegetation growth attributable to dams. For example, 

a number of major dams were built in Oklahoma in the 

1950's and 1960's. At the same time, hundreds of 

smaller flood-detention reservoirs were installed on 

tributaries. In the Washita River basin, 476 such reservoirs were completed from 1952 to 1972 (Carr and 

Bergman, 1976). Average annual rainfall in west-central 

Oklahoma during 1961-71 was about 12 percent less 

than during 1938-60. This reduced rainfall alone could 

have resulted in decreased streamflows and in observed 

changes in channels. In fact, streamflows during 1961- 

70 were decreased by as much as 60 percent, compared 

to the earlier period. Both dam construction and less 

rainfall probably were responsible for this reduction; 

although, given the very large changes in flow regime 

associated with the dams, their effect may well have 

been more significant than the change in rainfall.

Even discounting possible effects of rainfall variability, the extensive simultaneous construction of small 

flood-detention dams and of dams on major rivers in 

parts of Oklahoma means that observed channel 

changes on the bigger rivers in those areas cannot be 

assumed to be entirely due to just the one dam immediately upstream. Thus, for example, some of the 

drastic changes on the Washita River downstream from 

Foss Dam (fig. 34) might have occurred even without 

Foss Dam because of the many upstream detention 

dams.

Because vegetation can increase regardless of 

changes in post-dam low flows, an increase in vegetation cannot be attributed necessarily to low-flow augmentation from reservoir regulation.

Regulation of high flows (magnitude, frequency, and 

duration) seems to be the only dam-related factor that 

is reasonably certain to encourage an increase in vegetation. Even with this feature, evidence on the extent 

to which the dam is accountable commonly may be absent. Little information exists on the response of riparian vegetation to changes in climate and hydrology unaffected by man. Vegetation changes comparable to 

those observed downstream from dams have occurred 

in the past in the absence of dams, though not as abundantly. Examples are on the Gila River (Turner, 1974; 

Burkham, 1972) and on the Cimarron River (Schumm 

and Lichty, 1963). Thus, although an increase in riparian vegetation due to flow regulation might logically 

be expected, the degree of the change ascribable to the 

dam cannot always be fixed from available data. Regulation of high flows in some cases could be virtually 

the sole cause of the change, while in other cases, it 

could be only a contributory part of the cause. In general, however, information from this and other studies 

indicates that the reduction of high flows by dams, if 

not controlling, often contributes significantly to the 

downstream growth of riparian vegetation, especially 

in cases where the channel has become narrower (figs. 

17-20, 30^31, 33-35).

EFFECTS OF VEGETATION GROWTH

Channel vegetation blocks part of the channel, resulting in reduced channel conveyance, faster flow velocities in the channel thalweg or both. Conveyance is decreased both by physical reduction of flow area by the 

vegetation and by impeding the sediment transport process and inducing bed aggradation. On the Republican 

River in Nebraska, vegetation decreased the channel 

capacity by 50 to 60 percent in some reaches (Northrop, 

1965). Such reduced conveyance leads to more frequent 

and longer-lasting overbank flooding. Faster velocities 

in the channel thalweg have been observed in some 

California streams, resulting in chutes where riffles 

used to be (John Hayes, California Fish and Game Commission, oralcommun., 1980).

Vegetation also enhances greater bed stability. Not 

only does vegetation impede the flow, but the roots 

help bind the sediment. Sediment within vegetated 

areas can be extremely difficult to erode.

A potential effect of vegetation, though not 

documented specifically in this study, is greater bank 

stability, due to the binding and protective effects of 

the vegetation. Such bank stability would be enhanced 

by decreases in damaging flood flows.
56 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

Another potential major effect of significant new vegetation growth is an increase in water losses by evapotranspiration. No comparative studies of water losses 

from sand channels before and after vegetation growth 

have been made. The only work done seems to have 

dealt with flood plain rather than channel vegetation. 

Similarly, it is still unclear whether more water is lost 

from a plain water surface than from one with a plant 

cover. On flood plains, comparisons of evapotranspiration before and after phreatophyte removal, as well as 

studies using evapotranspirometers, indicate possibly 

significant increase in consumptive use of water by 

phreatophytes compared to volumes for sand and bare 

soil (Van Hylckama, 1970; Culler and others, 1982; Leppanen, 1981). Several studies (Meyboom, 1964; Bowie 

and Kam, 1968; Ingebo, 1971) suggest that increased 

vegetation depletes streamflow, but the variety of conditions under which this occurs is not yet established. 

The elevation of the water table also has an effect. 

Evaporation is decreased significantly if the water table 

is lowered 0.6 m (Hellwig, 1973, p. 106).

CONCLUSIONS

The large data set compiled and examined in this report includes 287 measured cross sections downstream 

from 21 dams. Each cross section was resurveyed 

periodically, under the auspices of the U.S. Army 

Corps of Engineers or the U.S. Bureau of Reclamation, 

since about the time of dam closure. We have analyzed 

1,817 such cross-section surveys (table 13). For each 

resurvey, we determined the mean bed elevation and 

measured the bankfull channel width. In addition, gage 

height-water discharge relations at 14 streamflow-gaging stations (table 14, figs. 36-49) were inspected. 

Thirdly, numerous supplementary observations and 

measurements (such as time-sequential photographs, 

grain-size measurements, and vegetation mapping) 

downstream from other damsites have been included 

in the study.

Data published here and in many other reports show 

that the construction of dams on alluvial channels, by 

altering the flow and sediment regimen, is likely to result in a number of hydrologic and morphologic changes 

downstream. For example, average annual peak discharges for the rivers of this study were reduced by 

from 3 to 91 percent of their pre-dam values by the 

dams. Mean daily flows and average annual low flows 

were decreased in some instances and increased in 

others.

On most of the alluvial rivers surveyed, the channel 

bed degraded in the reach immediately downstream 

from the dam. Channel width in some cases showed 

no appreciable change, but in others, increases of as

0.5

0.0

CO

cc

-0.5

<

Q

LLJ 

03

-1.0

LLJ

(D -2.0

I 

O

-2.5

-3.0

DAM CLOSURE

1930 1935 1940 1945

WATER YEAR

FIGURE 36. Changes in mean streambed elevation with time 

at streamflow-gaging station on Colorado River 6.4 kilometers downstream from Parker Dam, Arizona. Plotted points 

represent elevation corresponding to a discharge of 90.6 

cubic meters per second, as determined from rating tables. 

No upstream control station available.

much as 100 percent or decreases of as much as 90 

percent were observed. At many cross sections, the 

changes in bed elevation and in channel width proceeded irregularly with time. At other cross sections, 

however, the average rates of degradation and also of 

changes in channel width can be described by a simple 

hyperbolic equation of the form:

(l/y)=C1 +C2(l/0
CONCLUSIONS 57

CC u '°

LiJ

LU

^

2 nn

.EVATION,

0 CUl C

FREAMBED El

I o

CO

z -1,5

^ 

Z

LU

3 -2f9

I I I I I

**" **^^ \ / DAM CLOSURE

^*~- \ - --« UPSTREAM GAGE ' - - ^_ _»-\ - " "" \t--* N -

i

-

DOWNSTREAM GAGE'

I I I I

IL

1 1 

X"

\

I

35 1940 1945 1950 1955 1960 19(

WATER YEAR

FIGURE 37. Changes in mean streambed elevation with time at 

streamflow-gaging station on Jemez River 1.3 kilometers 

downstream from Jemez Canyon Dam, New Mexico, and at the 

control station near Jemez 13 kilometers upstream from dam. Plotted points represent elevation corresponding to a discharge of 0.034 

cubic meter per second downstream from dam and 0.37 cubic meter 

per second upstream from dam, as determined from rating tables.

0.5

"1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 

WATER YEAR

FIGURE 38. Changes in mean streambed elevation with time at 

streamflow-gaging station on Missouri River 13 kilometers 

downstream from Fort Peck Dam, Montana. Plotted points represent elevation corresponding to a discharge of 85 cubic meters per 

second as determined from rating tables. No upstream control station available.

where

Y is either bed degradation in meters or relative change

in channel width;

Ci and C2 are empirical coefficients; and 

t is time in years after the onset of the particular channel change.

This model equation at present only describes observed channel changes. However, it perhaps could beQ 0.5

LU

< LU

LU hCC UJ

0,0

LU > 

C3 LU

O -1.0

1 I

DAM CLOSURE

1945 1950 1955 1960 

WATER YEAR

1965 1970

FIGURE 39. Changes in mean streambed elevation with time at 

streamflow-gaging station on Missouri River 11 kilometers 

downstream from Fort Randall Dam, South Dakota. Plotted points 

represent elevation corresponding to a discharge of 464 cubic 

meters per second, as determined from rating tables. No upstream 

control station available.

0.5

_2 5

'1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980

WATER YEAR

FIGURE 40. Changes in mean streambed elevation with time at 

streamflow-gaging station on Missouri River 8 kilometers 

downstream from Gavins Point Dam, South Dakota. Plotted points 

represent elevation corresponding to a discharge of 312 cubic 

meters per second, as determined from rating tables. No upstream 

control station available.

come usable for pre-construction estimates if a way 

could be found to predict the two coefficients, at least 

where subsurface and bank controls are absent. These 

coefficients probably are functions, at least, of flow releases and boundary materials. Research is needed to 

find a way of determining the coefficients prior to dam 

closure.

Without a predictive equation, estimates of expected 

degradation need to be based on sediment-transport 

equations. The applicability of sediment-feransport equations will depend on the channel-bed material, hydraulic 

characteristics, and depth to bedrock. The subsurface 

conditions are assessed best by detailed engineering 

and geologic surveys, such as excavations and core borings. It is difficult, however, to conduct such surveys
58 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

i

< UJ

0.5

£

UJ 0.0

^ -0.5

O 

i- 

£ -1.0

-1.5

I I I 

DAM CLOSURE

o 1940 1945 1950 1955 1960 1965 1970 1975 1980 

WATER YEAR

FIGURE 41. Changes in mean streambed elevation with time at 

streamflow-gaging station on Smoky Hill River 1.3 kilometers 

downstream from Kanopolis Dam, Kansas, and at the control station at Ellsworth 48 kilometers upstream from dam. Plotted points 

represent elevation corresponding to a discharge of 0.51 cubic 

meter per second downstream from dam and 0.43 cubic meter per 

second upstream from dam, as determined from rating tables.

0.5

-2.0

1950 1955 1960 1965 1970 

WATER YEAR

1975 1980

FIGURE 42. Changes in mean streambed elevation with time at 

streamflow-gaging station on Republican River 2.7 kilometers 

downstream from Milford Dam, Kansas, and at control station at 

Clay Center 49 kilometers upstream from dam. Plotted points represent elevation corresponding to a discharge of 1.2 cubic meters 

per second downstream from dam and 3.4 cubic meters per second 

upstream from dam, as determined from rating tables.

accurately. Core borings might fail to disclose coarse 

sediments at depth, and excavations or more detailed 

examinations may be required to find any controls. 

Even excavations may be insufficient if not suitably located.

Extrapolation of a fitted hyperbolic curve to estimate 

future bed degradation or changes in width at a site 

probably will give reliable estimates in a number of 

cases, assuming no major hydraulic changes are introduced. However, bed degradation at some (possibly 

many) cross sections will not be as deep as the predicted bed degradation because of unassessed subsurface controls (coarse sediment or bedrock). Similarly,

Q

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Z

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UJ

0

2 

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cc

UJ i 

UJ

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2

z~

gi 

<

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LU

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0

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i« GAGE

I i 44

^DOWNSTREAM GAGE

I I I I I I

1935 1940 1945 1950 1955 1960 1965 1970 

WATER YEAR

FIGURE 43. Changes in mean streambed elevation with time at 

streamflow-gaging station on North Canadian River 4.8 kilometers 

downstream from Canton Dam, Oklahoma, and at the control station near Seiling 45 kilometers upstream from dam. Plotted points 

represent elevation corresponding to a discharge of 0.031 cubic 

meter per second downstream from dam and 0.00057 cubic meter 

per second upstream from dam, as determined from rating tables.

-1.

1935 1940 1945 1950 1955 

WATER YEAR

1960 1965

FIGURE 44. Changes in mean streambed elevation with time at 

streamflow-gaging station on Red River 4.5 kilometers downstream 

from Denison Dam, Oklahoma, and at the control station near 

Gainesville, Texas, 106 kilometers upstream from dam. Plotted 

points represent elevation corresponding to a discharge of 3.7 cubic 

meters per second downstream from dam and 4.2 cubic meters per 

second upstream from dam, as determined from rating tables.

unassessed variations in bank erodibility can affect the 

predicted width changes.

In the sites studied here, rates of degradation during 

the initial period following dam closure are about 0.1 

to 1.0 m/yr, but ranged from negligible to as much as 

7.7 m/yr. (Such rapid rates generally did not last for 

more than a few months). Rates at many sites became 

very slow after 5 to 10 years.

The maximum depth of degradation varied considerably from one cross section to another and ranged from 

less than 1 m to as much as 7.5 m. On rivers having
CONCLUSIONS 59

0

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LJJ £.

CC QJ

£S

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19

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., DAM CLOSURE

j ^CONTROL GAGE

DOWNSTREAM GAGE^ *t 

' ' l ' ' l l ' '

35 1940 1945 1950 1955 1960 1965 1970 1975 19 

WATER YEAR

80

0

LJJ !<« *f CC

<C LJJ

LJJ hCC LJJ

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^ z

^0

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u3^

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o

FIGURE 45. Changes in mean streambed elevation with time at 

streamflow-gaging station on Neches River 0.5 kilometer 

downstream from Town Bluff Dam, Texas, and at the control station on Village Creek near Kountze in an adjacent drainage basin. 

Plotted points represent elevation corresponding to a discharge of 

4.2 cubic meters per second downstream from dam and 1.5 cubic 

meters per second at the control station, as determined from rating 

tables.

oo 0.5

2 CC

< UJ

UJ I0.0

S -0.5

LJJ >

-1.0

 -"*-

UPSTREAM GAGE

DOWNSTREAM

1940 1945 1950 1955 1960 

WATER YEAR

1965 1970 1975

FIGURE 46. Changes in mean streambed elevation with time at 

streamflow-gaging station on Chattahoochee River 4 kilometers 

downstream from Buford Dam, Georgia, and at the control station 

on the Chestatee River near Dahlonega 73 kilometers upstream 

from dam. Plotted points represent elevation corresponding to a 

discharge of 12.2 cubic meters per second downstream from dam 

and 3.4 cubic meters per second upstream from dam, as determined 

from rating tables.

CHANGE MEAN INSTREAMS ELEVATION, IN METERS

0.5

0.0

-O.b

i I i I I I 1 1 

^DAM CLOSURE

** « 

i i I I i i i i

1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 

WATER YEAR

FIGURE 47. Changes in mean streambed elevation with time at 

streamflow-gaging station on Rio Grande 1.3 kilometers 

downstream from Caballo Dam, New Mexico. Plotted points represent elevation corresponding to a discharge of 28.3 cubic meters 

per second, as determined from rating tables. No control station.

slopes of about 1 to 3 m/km, degradation of as little 

as 1 m significantly decreases the gradient.

0,5

o.o

-0.5

1

 ***5

I

1

I

1

""* ^ 

l

1 

1 CLOSURE

- **" \

I

1

X

1

DOWNSTREAM 

^GAGE

UPSTREAM GAGE 

 1

1945 1950 1955 1960 1965 1970 

WATER YEAR

1975 1980

FIGURE 48. Changes in mean streambed elevation with time at 

streamflow-gaging station on Marias River 3.2 kilometers 

downstream from Tiber Dam, Montana, and at the control station 

near Shelby 65 kilometers upstream from dam. Plotted points represent elevation corresponding to a discharge of 2.8 cubic meters 

per second downstream from dam and 4.0 cubic meters per second 

upstream from dam, as determined from rating tables.

o

LJJ 

CQ

SE oo

< f?T

Z Z

a> 

1-

< LJJ

O

-0.5

1945 1950 1955 1960 1965 

WATER YEAR

1970 1975 1980

FIGURE 49. Changes in mean streambed elevation with time at 

streamflow gaging station on Frenchman Creek 0.3 kilometer 

downstream from Enders Dam, Nebraska. Plotted points represent 

elevation corresponding to a discharge of 1.3 cubic meters per second, as determined from rating tables. No control station available.

Some of the rates and volumes of degradation in this 

study may appear small in the abstract. However, on 

a channel only 90 m wide and 15 km long, about 2 billion 

megagrams of sediment would be removed within 10 

years from the bed of the channel alone, at the rates 

described. The consequences of such degradation can 

include undermining of structures, abandonment of 

water intakes, reduced channel conveyance due to flatter gradients, and a decreased capacity for the transport of sediment contributed by tributaries.

Commonly, the section of maximum degradation in 

most cases was close to the dam, and degradation then 

decreased progressively downstream. However, large 

and small depths of degradation commonly were distributed somewhat irregularly with distance downstream 

from the dam. Also, the downstream location of zero 

degradation ranged from several to about 2,000 channel 

widths (4 to 125 km). For these reasons a smooth longitudinal profile is rare. In some cases not even the
60 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

anticipated downstream decline in degradation was observed within the distance covered by the cross sections. Further, although the longitudinal profile 

downstream from many dams tended to flatten with 

time as expected, this did not occur in all cases. 

Changes in channel elevation limited even to 1 or 2 

m can significantly affect the longitudinal profile on 

many rivers.

Many analyses were performed in seeking correlations of variables that would characterize conditions before and after dam closure. No simple correlations could 

be established between channel size, channel gradient, 

particle size, or quantities of flow, with the exception 

of a tentative relation for channel width. This reflects 

the number of variables and great variability of conditions in the sample.

In several of the rivers studied, bank erosion appears 

to account for more than 50 percent of the sediment 

eroded from a given reach. Bank erosion is related to 

bank composition. Erosion may be particularly severe 

where the river impinges on a bank of readily credible 

sand. Fine-grained cohesive sediments may slow the 

rate of erosion at specific points. In large rivers flowing 

on sand beds, such as those found in many areas of 

the western plains of the United States, the location 

of controls, discharge, and fluctuations of discharge appear to be principally responsible for varying rates of 

bank erosion.

Many large dams trap virtually all (about 99 percent) 

of the incoming sediment. The erosion of sediment immediately downstream from the dam, therefore, is not 

accompanied by replacement. Thus, although the rate 

of removal by the post-dam regulated flows may be less 

than that prevailing prior to regulation within a reach, 

the process does not result quickly in a new equilibrium. Both lateral erosion and degradation cease when 

the flow no longer transports the available sediments. 

Such cessation of net erosion may occur through local 

controls on boundary erosion, downstream base-level 

controls, decrease in flow competence (generally associated with armoring), infusion of additional transportable sediment, and through the development of 

channel vegetation. Armoring (increase in d50) appeared 

to be approximately proportional to the depth of bed 

degradation downstream from three dams for which 

data were available (fig. 15).

Hundreds of kilometers of river distance downstream 

from a dam may be required before a river regains, 

by boundary erosion and tributary sediment contributions, the same annual suspended load or sediment concentration that it transported at any given site prior 

to dam construction. On the North Canadian River 

downstream from Canton Dam, this distance is about 

200 to 500 km. On the Red River downstream from

Denison Dam, the distance is about the same or possibly longer. On the Missouri River, 1,300 km 

downstream from Gavins Point Dam, the post-dam average annual suspended loads are only about 30 percent 

of the pre-dam loads. The Missouri and some other rivers probably are not long enough for complete recovery.

Evaluation of the effects of dams on downstream 

channels is made difficult by the absence of adequate 

observations on the changes of natural channels in different climatic and physiographic regions under unregulated conditions. Natural variability that characterizes 

such changes (tables 1-3) may mask the response of 

the channel to flow regulation. To the extent that it 

is known, the geologic record indicates that small 

changes in climatic factors can produce significant alterations in channel morphology. This potential effect also 

complicates the identification of those changes in channel morphology and vegetation that can be ascribed solely to the effect of manmade structures. Some of the 

channel changes documented here might well have occurred during the period of observation even in the absence of human interference. However, several common 

trends should be noted, namely: (1) Frequent occurrence of major changes right after dam closure; (2) appearance in many cases of the greatest change just 

downstream from the dam with progressive decrease 

or recovery downstream; (3) progressive change toward 

an apparent new stability at a site, in the years after 

dam closure; (4) continuous or non-reversible character 

of the change at many locations; and (5) diversity of 

climatic and physiographic regions in which the process 

has been observed. These trends point to the installation of water-regulating dams and reservoirs and to the 

consequent elimination or significant decrease of sediment into downstream reaches as primary causes of the 

progressive channel change in a number of instances.

Vegetation generally increased in the reaches 

downstream from the dams studied here, covering as 

much as 90 percent of the channel bars and banks along 

some rivers. In some cases, part of this increased 

growth might have occurred even without the dam. 

That is, vegetation in the region may have proliferated 

as a result of climate changes or for other reasons not 

fully understood. Decreases of high flows by the dam 

seem to contribute to an increased downstream growth 

of riparian vegetation in many cases.

Most of the rivers investigated here are in a semiarid 

environment where the effective annual precipitation is 

between 20 and 40 cm. This is precisely the precipitation zone that Langbein and Schumm (1958, p. 1080) 

suggested is the critical point at which sediment yield 

may either decrease or increase, depending upon 

whether vegetation increases or decreases in response 

to a change in precipitation. The changes of the alluvial
REFERENCES 61

channels downstream from dams, and, in particular, the 

changes in vegetation and channel morphology observed 

at a number of locations, indicate the sensitivity of 

these relationships to small changes within short times, 

or to the effects of unusually large changes at a given 

moment in time. These effects may be mitigated or reversed in several decades. However, it is still difficult 

to predict what the effect of a persistent but small 

change in runoff, for example, would be on a given 

reach of channel. Interestingly, environmental-impact 

analyses require predictions of just such changes.

Where downstream channels are surveyed following 

dam construction, the usual method consists of topographic resurveys of fixed cross sections. These are 

measured either at predetermined, approximately equal 

time intervals (usually every 5 or 10 years), or on rare 

and sporadic occasions as funds permit. Such surveys 

instead need to be scheduled at frequent intervals (at 

least every 1 or 2 years) during the first 5 or 10 years 

after dam closure, because most of the channel changes 

occur during this period. Later surveys can be done 

much more infrequently, because much less change 

takes place during a unit time in these later years. The 

scatter on a plot of channel change versus time reflects, 

to some extent, the desirable frequency of resurveys. 

Where the scatter is large, shorter time intervals (more 

data) are needed to define a trend, and vice versa.

Although successive surveys of cross sections provide 

essential data for analyzing sediment and channel 

changes, repetitive aerial photography keyed to specific 

water stages might well provide more satisfactory data 

for some purposes at less cost. Consideration needs to 

be given to monitoring some major streams by means 

of aerial photography, perhaps using infrared techniques, where photographs can be taken at specific 

water stages and seasons to make successive sets of 

photography comparable. Such comparability is virtually non-existent at the present time.

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64 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

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TABLES 13, 14

TABLES 13, 14 67

TABLE 13. Data on channel features, as measured from resurveyed 

cross sections

[Footnotes on last page of table]

TABLE 13. Data on channel features, as measured from resurveyed 

cross sections Continued

datE

Year

Year of

collection

Years 

after dam 

closure

Distance of cross

section downstream 

(kilometers)

Colorado River, Arizona, Gl

1956

1959

1963

1965

1975

1956

1959

1963

1965

1975

1956

1959

1963

1965

1975

1956

1959

1963

1965

1975

1956

1959

1963

1965

1975

1956

1959

1963

1965

1975

1956

1959

1963

1965

1975

dat

Year

1956

1959

1965

1975

1956

1959

1965

1975

1956 

1959

1965

1975

1935 

1935

1936 

1937

1938

1939

1940 

1941

1942 

1943

1944

1945 

1946

1947 

1948

1935

1935

1936

1937 

1938

0

3

7

9

19

0

3

7

9"

19

0

3

7

9

19

0

3

7

9

19

0

3

7

9

19

0

3

7

9

19

0

3

7

9

19

Year of 

a collection

after dam

closure

Colorad

0

3

9

19

0

3

9

19

0 

3

9

19

0

.5

1 

2

3

4

5

6

7 

8

9

10 

11

12 

13

0

.5

1

2 

3

Year of dam closure

1.1

1.1

1.1

1.1

1.1

2.6

2.6

2.6

2.6

2.6

4.3

4.3

4.3

4.3

4.3

6.4

6.4

6.4

6.4

6.4

8.0

8.0

8.0

8.0

8.0

10.5

10.5

10.5

10.5

10.5

13.0

13.0

13.0

13.0

13.0

section downstream

from dam 

(kilometers)

Total change

in mean bed 

elevation 

(meters)

en Canyon Dam

1956i'

0

-1.75

-1.30

-1.70

-1.90

0

-1.00

-1.85

-2.00

-2.15

0

-2.10

-2.45

-3.65

-3.70

0

-.90

-1.20

-1.50

-1.60

0

-1.05

-1.70

-4.35

-4.10

0

-.35

-1.00

-1.90

-2.05

0

-.75

-.25

-4.05

-4.50

Total change

elevation 

(meters)

D River, Arizona, Glen Canyon Dam Contin

Year of dam closure

16.0

16.0

16.0

16.0

19.5

19.5

19.5

19.5

25

25

25

25

Colorado River, Arizona,

Year of dam closure

1.9 

1.9

1.9 

1.9

1.9

1.9

1.9 

1.9

1.9 

1.9

1.9

1.9 

1.9

1.9 

1.9

2.3

2.3

2.3

2.3 

2.3

19561'

0

-.85

-7.25

-7.00

0

-.45

-2.00

-2.20

0 

0

-5.20

-3.80

Hoover Dam

1935

0 

-1.20

-1.30 

-1.35

-.95

-1.15

-1.65

-1.55

-1.50 

-1.45

-1.35

-1.35 

-1.20

-1.30 

-1.50

0

-.10

-.60

-1.25 

-1.25

Channel width 

(meters)

-'(104)

(137)

(141)

(141)(?)

(141)

(183)

(183)

(183)

(183)

(183)

(169)

(167)

(167)

(167)

(167)

(272)

(272)

(272)

(272)

(272)

(140)

(143)

(146)

(146)

(146)

(252)

(252)

(280)

(285)

(285)

(97.5)

(99.0)

(95.5)

(99.0)

(99.0)

Channel width 

(meters)

ued

(95.5)

(95.5)

(95.5)

(95.5)

(189)

(189)

(189)

(189)

(109) 

(107)

(108)

(107)

2/

__

-

data

Year

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

Year of

collection

Years 

after dam 

closure

Coloradc

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

Year of 

data collection

Year

1944

1945

1946

1947

1948

1935

1935

1936

1937 

1938

1939

1940

1941

1942

1943

1944 

1945

1946 

1947

1948

1935

1935 

1936

1937 

1938

1939

1940 

1941

1942 

1943

1944

1945

1946

1947 

1948

Years 

after dam

closure

Colorado

9

10

11

12

13

0

.5

1

2 

3

4

5

6

7

8

9 

10

11 

12

13

0

.5

1

3

4

5 

6

7 

8

9

10

11

12 

13

Distance of cross

section downstream 

(kilometers)

Total change

in mean bed Channel width 

elevation (meters) 

(meters)

River, Arizona, Hoover Dam Continued

Year of dam closure

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.3

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

3.2

4.5

4.5

4.5

4.5

4.5

4.5

4.5

4.5

4.5

4.5

Distance of cross

section downstream 

from dam 

(kilometers)

River, Arizona, Hoover

Year of dam closure

4.5

4.5

4.5

4.5

4.5

5.5

5.5

5.5

5.5 

5.5

5.5

5.5

5.5

5.5

5.5

5.5 

5.5

5.5 

5.5

5.5

6.1

6.1 

6.1

6.1 

6.1

6.1

6.1 

6.1

6.1 

6.1

6.1

6.1

6.1

6.1 

6.1

1935

-1.20

-1.70

-1.90

-2.05

-2.05

-1.65

-1.50

-1.70

-1.65

-1.60

0

-1.95

-2.70

-3.60

-3.60

-3.65

-5.25

-5 . 10

-5 . 10

-4.90

-5 . 00

-4.70

-4.70

-4.90

-5 . 20

0

-1.60

-2 . 15

-2.15

-2.25

-2.25

-2.45

-2.45

-2.55

-2.50

Total change 

in mean bed Channel width

elevation (meters) 

(meters)

Dam Continued

1935

-2.75

-2.80

-2.80

-2.85

-2.75

0

-1.10

-1.20

-1.15 

-1.20

-1.15

-1.25

-1.25

-1.30

-1.45

-1.25 

-1.00

-.80 

-.95

-1.15

0

-1.35 

-1.45

-2.25 

-2 . 15

-1.75

 2 . 60 

-2.60

-2.80 

-2.65

-2.45

-2.20

-2.00

-2 . 15 

-2.35
68 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 13. Data on

Year of 

data collection

Year

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

Years 

after dam

closure

Colorado

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

channel features, as measured from, resurveyed

cross sections Continued

Distance of cross

section downstream 

from dam 

(kilometers)

River, Arizona, Hoov

Year of dam closur

7.1

7.1

7.1

7.1

7.1

7.1

7.1

7.1

7.1

7.1

7.1

7.1

7.1

7.1

7.1

8.0

8.0

8.0

8.0

8.0

8.0

8.0

.8.0

8.0

8.0

8.0

8.0

8.0

8.0

8.0

9.7

9.7

9.7

9.7

9.7

Total change 

in mean bed Channel width

elevation (meters) 

(meters)

er Dam Continued

e 1935

0

-1.20

-2.05

-2.70

-3.15

-3.05

-3.00

-3.05

-3.85

-3.20

-3.20

-3.25

-3.15

-3.15

-3.10

0

-2.20

-2.30

-2.70

-2.95

-3.35

-4.45

-4.65

-4.70

-4.55

-4.35

-4.35

-4.35

-4.35

-4.35

0

-.75

-1.60

-1.85

-2 . 35

TABLE 13 . Data on channel features, as measured from resurveyed

cross sections Continued

Year of 

data collection

Year

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

Years 

after dam

closure

Colorado

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

Distance of cross 

section downstream

from dam 

(kilometers)

River, Arizona, Hoo

Year of dam closu

11.0

11.0

11.0

11.0

11.0

12.5

12.5

12.5

12.5

12.5

12.5

12.5

12.5

12.5

12.5

12.5

12.5

12.5

12.5

12.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

13.5

Total change 

in mean bed

elevation 

(meters)

ver Dam Continued

re 1935

-1.90

-1.85

-1.85

-1.90

-1.90

0

-1.75

-2.15

-2.55

-2.95

-2.90

-4.35

-3.95

-5.25

-5.70

-5.90

-6.20

-6.35

-6.90

-7.45

0

-1.90

-1.70

-1.90

-1.85

-1.70

-3.30

-3.55

-3.45

-3.65

-3.65

-3.70

-3.65

-3.70

-3.70

Channel width

(meters)

__

"

~

Ye,

data c

Year

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

ir of

Election

Years 

after dam

Colorado

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

section downstream 

from dam 

(kilometers)

River, Arizona, Hoover

Year of dam closure

9.7

9.7

9.7

9.7

9.7

9.7

9.7

9.7

9.7

9.7

10.5

10.5

10.5

10.5

10.5

10.5

10.5

10.5

10.5

10.5

10.5

10.5

10.5

10.5

10.5

11.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

Total change

in mean bed Channel width 

elevation (meters) 

(meters)

Dam Continued

1935

-2.75

-3.75

-3.70

-4.40

-4.50

-4.55

-4.50

-4.35

-4.35

-4.35

0

-.50

-1.20

-1 . 10

-1.05

-.95

-1.15

-1.15

-1.15

-1.15

-1.05

-1.05

-1.15

-1.15

-1.15

0

-.60

-1.00

-1.20

-1.40

-1,35

-1,70

-1.80

-1.85

-2.00

dat

Year

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

Year of 

a collection

Years 

after dam 

closure

Colo'rc

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

Distance of c

section downst

from dam 

(kilometers

ido River, Arizon

Year of dam

15.5

15.5

15.5

15.5

15.5

15.5

15.5

15.5

15.5

15.5

15.5

15.5

15.5

15.5

15.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

18.0

18.0

18.0

18.0

18.0

ross Total change

ream in mean bed Channel width

elevation (meters) 

) (meters)

a, Hoover Dam Continued

closure 1935

0

-1.50

-2.15

-2.50

-3.30

-4.40

-4.35

-4.45

-5 . 50

-5 . 10

-5 . 20

-5 . 10

-5 . 15

-5.25

-5.25

0

-.25

-1.50

-1.90

-2.00

-2.25

-3.05

-3.05

-3.10

-3.15

-3.10

-3.10

-3.10

-3.35

-3.65

0

-.25

-1.15

-1.85

-2.00
TABLES 13, 14 69

TABLE

Ye 

data c

Year

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1 937

1938

1939

1940

1941

1942

1943

Ye 

data c

Year

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

13. Data on

ar of 

ollection

Years 

after dam 

closure

Colorado

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

ollection

Years 

after dam 

closure

Colorado

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

channel features, as measured from resurveyed

cross sections Continued

Distance of cross

section downstream 

(kilometers)

River, Arizona, Hoover

Year of dam closure

18.0

18.0

18.0

18.0

18.0

18.0

18.0

18.0

18.0

18.0

19.5

19.5

19.5

19.5

19.5

19.5

19.5

19.5

19.5

19.5

19.5

19.5

19.5

19.5

19.5

21

21

21

21

21

21

21

21

21

21

Distance of cross 

section downstream 

from dam 

(kilometers)

River, Arizona, Hoover

Year of dam closure

21

21

21

21

21

28

28

28

28

28

28

28

28

28

28

28

28

28

28

28

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

Total change

in mean bed Channel width 

elevation (meters) 

(meters)

Dam Continued

1935

-2.15

-2 . 60

-2.55

-2.45

-2.60

-2.60

-2 . 50

-2.40

-2.45

-2.55

0

-.20

-1.65

-1.65

-1.85

-2.55

-3.55

-3.65

-4 . 15

-4.30

-4.40

-4.50

-4.50

-4 . 65

-4 . 80

0

0

-1.00

-1.50

-1.65

-2.05

-2.40

-2.55

-2.60

-2.60

Total change 

in mean bed Channel width 

elevation (meters) 

(meters)

Dam Continued

1935

-2.60

-2.60

-2.65

-2.75

-2.75

l/o" ~

-.10

-.35

-.75

-1.00

-1.75

-1.95

-3.10

-3.10

-2.95

-2.95

-3.25

-3.25

-3.35

__

0 I'o

-.45

-.90

-1.20

-1.20

-1.40

-2.15

-2.30

-2.30

-2.40

-2.30

-2 . 30

-2.30

TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued

data

Year

Year of 

collection

Years 

after dam 

closure

Colo

Distance of cross

section downstream 

from dam 

(kilometers)

rado River, Arizona, Hoover

Total change 

in mean bed

elevation 

(meters)

Dam Continued

Channel width

(meters)

Year of dam closure 1935

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1.1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1.1

2

3

Year of 

data collection

Year

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

Years 

after dan 

closure

Cole

4

5

6

7

8

9

10

11

12

13

0

.5

1.1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

42

42

42

42

42

42

42

42

42

42

42

42

42

42

42

51

51

51

51

51

51

51

51

51

51

51

51

51

51

51

57

57

57

57

57

Distance of cross 

section downstream

" (kilometers)

jrado River, Arizona, Hoover

Year of dam closure

57

57

57

57

57

57

57

57

57

57

63

63

63

63

63

63

63

63

63

63

63

63

63

63

63

70

70

70

70

70

70

70

70

70

70

_

 0

-.30

-.50

-.75

-1.00

-1.20

-1.05

-2.70

-2.60

-2.60

-2.90

-3.30

-3.40

-3.40

 0

-.15

-.30

-.60

-.85

-1.05

-2.30

-2.20

-2.75

-2.80

-2.80

-3.00

-3.10

_

3/ o"

 u

-.30

-.65

Total change 

in mean bed 

elevation 

(meters)

Dam Continued

1935

-1.00

-1.50

-1.35

-2.05

-2.95

-2.95

-3.10

-3.55

-3.00

-3.00

 0

-.10

-.75

-.90

-1.35

-1.50

-3.55

-3.30

-3.15

-3.60

-3.50

-2.80

-4.50

0

+ .05

-.15

-.90

-1.20

-1.45

-1.95

-2.20

-3.15

-3.55

_

~

__

__

Channel width 

(meters)

_

-_
70 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 13. Data on channel features, as measured from resurveyed TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued cross sections Continued

Year of

data collection

Year

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

dat

Year

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

after dam 

closure

Colorado

9

10

11

12

13

0

.5

1

2.6

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2.6

3

4

5

6

7

8

9

10

11

12

13

Year of 

a collection

Years 

after dam

Colorado

0

.5

1

2.6

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3.2

Distance of cross

section downstream 

from dam 

(kilometers)

Total change

in mean bed Channel width 

elevation (meters) 

(meters)

River, Arizona, Hoover Dam Continued

Year of dam closure

70

70

70

70

70

77

77

77

77

77

77

77

77

77

77

77

77

77

77

77

87

87

87

87

87

87

87

87

87

87

87

87

87

87

87

Distance of cross

section downstream 

(kilometers)

River, Arizona, Hoover

Year of dam closure

94

94

94

94

94

94

94

94

94

94

94

94

94

94

94

104

104

104

104

104

104

104

104

104

104

104

104

104

104

104

110

110

110

110

110

1935

-3.95

-4.15

-4.35

-4.35

-4.40

_

_

_

1/0

-.20

-.55

-.70

-.90

-1.65

-1.70

-2.45

-2.45

-2.30

-2.45

-2.50

_

_

3/ 0" -

-.10

-.25

-.50

-.75

-.85

-1.20

-2 . 05

-2 . 10

-2 . 15

-2 . 20

-2 . 40

Total change

in mean bed Channel width 

elevation (meters) 

(meters)

Dam Continued

1935

__

_

_

1/0

-.25

-.60

-.65

-1.00

-1.70

-2 . 10

-2.55

-2.75

-2 . 80

-3 . 00

-3.10

_

_

_

__

I/O

-.65

-.55

-1.05

-1.70

-2 . 05

-2.15

-2 . 60

-3.10

-3.30

-3.10

__

I/O

Year of

data collection

Year

Years 

after dam 

closure

Distance of cross

section downstream

from dam 

(kilometers)

Colorado River, Arizona, Hoover

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1935

1935

1936

1937

1938

1939

1940

1941

1942

1943

1944

1945

1946

1947

1948

1948

1948

1949

1950

1951

dat

Year

1952

1953

1954

1955

1956

1957

1958

1959

1960

1961

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1948

1949

1950

1951

1952

1953

1954

1955

1956

1957

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3.2

4

5

6

7

8

9

10

11

12

13

0

.5

1

2

3

Year of 

a collection

Years 

after dam 

closure

Cole

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

0

1

2

3

4

5

6

7

8

9

Year of dam closure

110

110

110

110

110

110

110

110

110

110

117

117

117

117

117

117

117

117

117

117

117

117

117

117

117

Colorado River, Arizona,

Year of dam closure

1.1

1.1

1.1

1.1

1.1

section downstream

from dam 

(kilometers)

>rado River, Arizona, Davis

Year of dam closure

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

1.1

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

Total change

in mean bed

elevation 

(meters)

Dam Continued

1935

-0.35

-.60

-1.15

-1.80

-1.85

-2.40

-2.55

-2.65

-2.75

-2.95

3/ 

- 0

0

-.50

-.85

-1.85

-1.85

-2.45

-2.65

-3.05

-3.55

-3.70

Davis Dam

4/ 1948-'

0

-.65

-1.20

-2.20

-2.45

in mean bed

elevation 

(meters)

Dam Continued

-2.60

-3.25

-3.40

-3.80

-4.85

-4.85

-4.40

-4.95

-5.05

-5.10

-5.05

-4.90

-4.90

-5.05

-5.10

-5.05

-5.10

-5.35

-5.65

-5.65

-5.50

-5.60

-5.75

-5.65

0

-.20

-.45

-.50

-.65

-1.40

-1.45

-1.45

-1.55

-1.70

Channel width

(meters)

--

~

!/

Channel width

(meters)

-- 
TABLES 13, 14 71

TABLE 13. Data on channel features, as measured from resurveyed 

cross sections Continued

TABLE 13. Data on channel features, as measured from resurveyed 

cross sections Continued

Year of

data

Year

collection

Years 

after dam 

closure

Distance of cross

section downstream 

from dam 

(kilometers)

Total change

in mean bed Channel width 

elevation (meters) 

(meters)

Colorado River, Arizona, Davis Dam Continued

1958

1959

1960

1961

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

1972

1973

1974

1975

1938

1939 

1940

1941 

1942

1943

1944

1945

1947

1949

1951

1955

1960

1965

1970

1975

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

Co

0

1 

2

3 

4

5

6

7

9

11

13

17

22

27

32

37

Year of 

data collection

Year

1938

1939

1940

1941

1942

1943

1944 

1945

1947

1949

1951

1955

1960

1965 

1970 

1975

1938

1939 

1940 

1941 

1942

1943 

1944 

1945 

1947 

1949

1951 

1955 

1960 

1965

1970

1975

1938

1939

1940

1941

1942

Years 

after dam 

closure

Colorado

0

1

2

3

4

5

6 

7

9

11

13

17

22

27 

32 

37

0

1 

2 

3 

4

5 

6 

7 

9 

11

13 

17 

22 

27

32

37

0

1

2

3

4

Year of dam closure

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

8.8

lorado River, Arizona,

Year of dam closure

27

27 

27

27 

27

27

27

27

27

27

27

27

27

27

27

27

Distance of cross 

section downstream

(kilometers)

River, Arizona, Parker

Year of dam closure

39

39

39

39

39

39

39 

39

39

39

39

39

39

39 

39 

39

46

46 

46 

46 

46

46 

46 

46 

46 

46

46 

46 

46 

46

46

46

66

66

66

66

66

1948^/

-2 . 00

-2.00

-2.05

-2.05

-2 . 15

-2 . 15

-2 . 40

-2 . 60

-2.70

-2.75

-2.80

-2.65

-2 . 40

-2.30

-2.45

-2.60

-2.70

-2.75

Parker Dam

1938i/o y

-.65 

-1.00

-1.50 

-1.80

-2 . 05

-2.30

-2.45

-2.60

-2.55

-2 . 60

-2.85

-2 . 95

-3.00

-3.05

-3 . 15

Total change 

in mean bed Channel width 

elevation (meters) 

(meters)

Dam Continued

1938

0

-.05

-.10

-.75

-1.80

-2 . 05

-2.60 

-2.45

-2.75

-2 . 80

-3.50

-3.80

-3.85

-3.65 

-4.35 

-4.35

0

-.15 

-.65 

-1.30 

-2.25

-2.45 

-2.60 

-2 . 70 

-2.85 

-2.85

-2.85 

-3.65 

-4.20 

-4.60

-4 . 15

-4 . 25

Year of

data collection

Year

1943

1944

1945

1947

1949

1951

1955

1960

1965

1970

1975

1938

1939

1940

1941

1942

1943

1944

1945

1947

1949

1951 

1955

1960 

1965

1970 

1975

1938

1939

1940

1941

1942

1943

1944

1945

1947

1949

dat

Year

1951

1955

1960

1965

1970

1975

1952

1959

1965

1975

1952 

1959 

1965

1975

1952 

1959 

1965 

1975

1952 

1959 

1965 

1975

1952 

1959 

1965 

1975

1952

1959

1965

0 ~ 1975 -.30

-.60

-1.35

-2.25

Years 

after dam 

closure

Colorado

5

6

7

9

11

13

17

22

27

32

37

0

1

2

3

4

5

6

7

9

11

13 

17

22 

27

32 

37

0

1

2

3

4

5

6

7

9

11

Year of 

a collection

Years 

after dam 

closure

Colorado

13

17

22

27

32

37

Jemez

0

6

12

22

0 

6 

12

22

0 

6

12 

22

0 

6

12 

22

0 

6

12 

22

0

6

12

22

Distance of cross

section downstream 

(kilometers)

River, Arizona, Parker

Year of dam closure

66

66

66

66

66

66

66

66

66

66

66

80

80

80

80

80

80

80

80

80

80

80 

80

80 

80

80 

80

95

95

95

95

95

95

95

95

95

95

Distance of cross 

section downstream 

from dam 

(kilometers)

Total change

in mean bed 

elevation 

(meters)

Dam Continued

1938

-2.30

-2.55

-2.90

-2.85

-2.70

-3.00

-2.75

-3.30

-2.70

-3.30

-3.45

0

-.10

-.20

-1.05

-1.40

-1.75

-1.50

-1.50

-1.40

-1.05

-1.60 

-1.50

-1.70 

-2.05

-2.05 

-2.40

0

+.05

-.35

-.65

-.80

-1.50

-1.90

-1.35

-1.20

-1.25

Total change 

in mean bed 

elevation 

(meters)

Channel width 

(meters)

__

_

_

__

_

Channel width 

(meters)

River, Arizona, Parker Dam Continued

Year of dam closure

95

95

95

95

95

95

1938

-1.35

-1.00

-1.95

-2.05

-2.20

-2.05

_

River, New Mexico, Jemez Canyon Dam

Year of dam closure

1.0

1.0

1.0

1.0

1.3 

1.3 

1.3

1.3

1.6 

1.6 

1.6 

1.6

1.8 i.'s

1.8 

1.8

2.4 

2.4 

2.4 

2.4

2.7

2.7

2.7

2.7

1953

0

-2.7

-2.7

-1.7

0

-2.4 

-2.7

-1.5

0 

-1.8 

-2.8 

-2.1

0

-2.2 

-3.0 

-1.9

0 

-1.4 

-2.7 

-1.6

0

-1.8

-2.1

-1.3

142

49.0

11.5

31.0

272 

70.0 

17.0

24.0

270 

138 

21.5 

20.0

216

105 

48.5 

49.5

190 

133 

18.5 

29.5

220

39.5

42.0

59.0
72 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

PABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued

data

Year

Year of 

collection

Years 

closure

Distance of cross 

section downstream 

f rom dam 

(kilometers)

Jemez River, New Mexico, Jemez

Total change 

in mean bed 

elevation 

(meters)

Canyon Dam Continued

Channel width 

(meters)

Year of dam closure 1953

1952

1959

1965

1975

1952

1959

1965

1975

12/43-

2/44

1951

1966

1972

12/43-

2/44

1951

1966

1972

12/43-

2/44

1951

1966 

1972

12/43-

2/44

1951 

1966 

1972

12/43-

2/44

1951

1966

1972

data

Year

12/43-

2/44

1951

1966

1972

12/43-

2/44

1951

1966 

1972

12/43-

2/44

1951 

1966 

1972

12/43-

2/44

1951

1966 

1972

12/43- 

2/44

1951

1966 

1972

12/43-

2/44

1951

1966

1972

0

6

12

22

0

6

12

22

A

1

9

24

30

1

9

24

30

1

9

24 

30

1

9 

24 

30

1

9

24

30

Year of 

collection

Years

after dam 

closure

Arkansa

1

9

24

30

1

9

24 

30

1

9 

24 

30

1

9

24 

30

1

9

24 

30

1

9

24

30

3.1

3.1

3.1

3.1

3.4

3.4

3.4

3.4

rkansas River, Colorado,

Year of dam closu

3.5

3.5

3.5

3.5

5.0

5.0

5.0

5.0

8.5

8.5

8.5 

8.5

12.0

12.0 

12.0 

12.0

15.5

15.5

15.5

15.5

Distance of cross 

section downstream

from dam 

(kilometers)

s River, Colorado, John

Year of dam closu

19.0

19.0

19.0

19.0

22

22

22 

22

26

26 

26 

26

29

29

29

29 

33

33

33 

33

36

36

36

36

0

-1.5

-1.8

-1.0

0

-1.1

-1.3

-.6

John Martin Dam

re 1942

1/0

-.10

-1.95

-.40

3/0

-.10

-1.05

-.35

I/O

-.30

-.80 

-.35

I/O

-.20 

-.80 

-.85

I/O

-.25

-.85

-.90

Total change 

in mean bed

elevation 

(meters)

Martin Dam Continued

re 1942

I/O

-.05

-.15

-.50

I/O

-.60

-1.15 

-.95

I/O

-.25 

-.85 

-.75

I/O

+ .20

+.25

+1.30 

I/O

-.65

-.40 

-.45

0

-.15

-.20

-.20

214

75.5

74.5

47.0

178

74.5

100

110

146

142

30.5

27.0

128

131

46.5

44.0

76.0

69.5

39.5 

34.0

100

95.5 

30.0 

35.0

157

88.0

40.5

38.5

Channel width

(meters)

144

144

96.5

43.5

288

165

74.0 

72.5

230

241 

127 

86.5

168

165

46.0

50.0 

201

130

99.5 

59 0

110

75.5

56.5

59.0

TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued

Y 

data

Year

1936

1950

1955

1956

1958

1960

1966

1973

1936

1950

1955

1956

1958

1960

1966

1973

1936

1950

1955

1956

1958

1960

1966

1973

1936 

1950

1955

1956

1958 

1960 

1966

1973

collection

Years 

after dan 

closure

0

13

18

19

21

23

29

36

0

13

18

19

21

23

29

36

0

13

18

19

21

23

29

36

0 

13

18

19

21 

23 

29

36

Year of 

data collection

Year

1936

1950

1955

1956

1958

1960

1966

1973

1936 

1950

1955

1956

1958 

1960 

1966

1973

Y

after dam 

closure

Missou

0

13

18

19

21

23

29

36

0 

13

18

19

21 

23 

29

36

section downstream 

(kilometers)

Missouri River, Montana,

Year of dam closur

9.2

9.2

9.2

9.2

9.2

9.2

9.2

9.2

13.0

13.0

13.0

13.0

13.0

13.0

13.0

13.0

16.5

16.5

16.5

16.5

16.5

16.5

16.5

16.5

23 

23

23

23

23 

23 

23

23

Distance of cross 

section downstream

from dam 

(kilometers)

ri River, Montana, Fort P

Year of dam closure

45

45

45

45

45

45

45

45

75 

75

75

75

75 

75 

75

75

Missouri River, North Dakota

1946 

1954

1960

1964

1970

1976

1946

1954

1960

1964

1970

1976

(0) 

1

7

11

17

23

(0)

1

7

11

17

23

Year of dam closure

2.7 

2.7

2.7

2.7

2.7

2.7

6.4

6.4

6.4

6.4

6.4

6.4

Total change 

in mean bed 

elevation 

(meters)

Fort Peck Dam

e 1937'/

0

-.80

-.65

-.70

-.70

-.65

-.75

-.90

0

-.65

-.80

-.75

-.60

-.75

-1.00

-1.05

0

-1.00

-1.20

-1.00

-1.00

-1.05

-1.15

-1.75

0 

-.50

-.70

-1.00

-1.05 

-1.15 

-1.15

-1.50

Total change 

in mean bed

elevation 

(meters)

eck Dam Continued

1937^

0

-.15

-.45

-.60

-.10

-.20

-.40

-.75

0 

-.20

-.40

-.25

+.05 

-.10 

-.20

-.25

, Garrison Dam

1953

0 

-.20

1/-1.35

1/-1.60

1/-2.30

-/-2.80

0

-.45

1/-2.10

1/-2.75

1/-3.65

1/-3.95

Channel width 

(meters)

348

398

402

402

408

408

408

408

234

238

238

238

236

236

238

238

248

304

336

336

336

336

340

340

256 

262

268

268

272 

272 

272

274

Channel width

(meters)

190

202

212

212

212

216

238

238

274 

286

286

288

290 

292 

292

298

530 

550

505

505

505

500

450

525

388

390

392

402
TABLES 13, 14 73

TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued

data

Year

Year of 

collection

Years 

after da 

closure

 section downstream 

from dam 

m (kilometers)

Missouri River, North Dakota, G

1946

1954

1960

1964

1970

1976

1946

1954

1960

1964

1970

1976

1946

1954

1960

1964

1970

1976

1946

1948

1954

1960

1964

1970

1976

1946

1954

1960

1964

1970

1976

1946

1954

1960

1964

1970

1976

data

Year

1946

1954

1960

1964

1970

1976

1949

1954

1960

1964

1970

1976

1949 

1954

1960

1964 

1970 

1976

1949

1954

1960 

1964 

1970 

1976

1949 

1954 

1960 

1964

1970

1976

1949 

1954

1960

1964

1970

1976

(0)

1

7

11

17

23

(0)

1

7

11

17

23

(0)

1

7

11

17

23

(0)

(0)

1

7

11

17

23

(0)

1

7

11

17

23

(0)

1

7

11

17

23

Year of 

collection

Years

after da 

closure

Missou

(0)

1

7

11

17

23

(0)

1

7

11

17

23

(0)

1

7

11 

17 

23

(0)

1

7 

11 

17 

23

(0) 

1 

7 

11

17

23

(0)

1

7

11

17

23

Year of dam closu

8.0

8.0

8.0

8.0

8.0

8.0

10.5

10.5

10.5

10.5

10.5

10.5

12.0

12.0

12.0

12.0

12.0

12.0

15.0

15.0

15.0

15.0

15.0

15.0

15.0

17.5

17.5

17.5

17.5

17.5

17.5

21

21

21

21

21

21

Distance of cross

 section downstream

from dam 

(kilometers)

Total change 

in mean bed 

elevation 

(meters)

arrison Dam Continued

re 1953

0

-.65

-1.70

-2.20

-2.70

-3.25

0

-.75

-1.35

-1.60

-2.35

-2.75

0

+.05

-1.00

-1.35

-1.50

-2.10

0

-.35

+ .05

-.65

-.75

-1.15

-1.25

0

0

-.65

-.15

-1.45

-.90

0

-.35

+ .65

+ .45

+ .10

+ .05

Total change

in mean bed

(meters)

Channel width 

(meters)

458

424

428

428

428

428

585

520

525

540

555

565

492

520

520

530

545

540

505

535

580

630

715

790

835

310

570

600

610

680

705

895

915

925

930

945

960

Channel width

(meters)

ri River, North Dakota, Garrison Dam Continued

Year of dam closu

24

24

24

24

24

24

28

28

28

28

28

28

32 

32

32

32 

32 

32

36

36

36 

36 

36 

36

38 

38 

38 

38

38

38

44 

44

44

44

44

44

re 1953

0

-.80

-1.60

-1.60

-1.90

-1.95

0

-1.00

-.70

-1.05

-2.15

-3.05

0 

-.40

-.35

-.50 

-.65 

-.80

0

-.35

-1.00 

-.90 

-1.60 

-1.50

0 

-.20 

-.30 

-.40

-.70

-1.50

0 

+ .10

-.20

-.05

-.20

-.85

_

1,295

1,300

1,305

1,310

1,315

300

300

296

298

300

306

1,290 

1,395

1,425

1,430 

1,435 

1,430

1,325

865

855 

885 

955 

1,005

448 

520 

525 

540

555

595

462 

505

685

740

790

TABLE 13. Data on channel features as measured from resurveyed

cross sections Continued

data

Year

1949

1954

1960

1964

1970

1976

1949

1954

1960

1964

1970

1976

1949

1954

1960

1964

1970

1976

1949

1954

1960

1964

1970

1976

1949

1954

1960

1964

1970

1976

1949

1954

1960

1964

1970

1976

Year of 

collection

Years 

after dam 

closure

Missouri

(0)

1

7

11

17

23

(0)

1

7

11

17

23

(0)

1

7

11

17

23

(0)

1

7

11

17

23

(0)

1

7

11

17

23

(0)

1

7

11

17

23

Year of

data -- 1n --*--' 

Year

1949

1954

1960

1964

1970

1976

1946

1954

1960

1964

1970

1976

1952 

1954 

1957

1960

1962 

1967 

1970 

1975

1952 

1954 

1957 

1960

1962

1967

1970 

1975

1952

1954

1956

805

CUA-LeC LiUU

Years

after dam 

closure

Missouri

(0)

1

7

11

17

23

(0)

1

7

11

17

23

Missm

0 

2 

5

8

10 

15 

18 

23

0 

2 

5 

8

10

15

18 

23

0

2

4

section downstream 

from dam 

(kilometers)

Total change 

in mean bed 

elevation 

(meters)

Channel width 

(meters)

River, North Dakota, Garrison Dam Continued

Year of dam closi

47

47

47

47

47

47

51

51

51

51

51

51

54

54

54

54

54

54

58

58

58

58

58

58

61

61

61

61

61

61

70

70

70

70

70

70

section downstream

from dam 

(kilometers)

River, North Dakota, G

Year of dam closu

78

78

78

78

78

78

87

87

87

87

87

87

jri River, South Dakota

Year of dam closu

1.6 

1.6 

1.6

1.6

1.6 

1.6 

1.6 

1.6

3.1 

3.1 

3.1 

3.1

3.1

3.1

3.1 

3.1

4.2

4.2

4.2

re 1953

0

+ .45

+ .45

+ .25

-.35

-1.05

0

+ .05

-1.05

-.85

-1.05

-1.65

0

+ .90

+.65

+ .95

+ .35

-.23

0

+ .25

-.45

-.15

-.35

-.50

0

0

+ .30

+ .30

-.45

-.30

0

+ .05

+ .25

-.15

-.45

-.50

Total change

in mean bed

elevation 

(meters)

arrison Dam Continued

re 1953

0

+ .45

+ .45

+ .25

-.20

-.35

0

+ .75

+ .65

+ .60

+ .10

+ .20

, Fort Randall Dam

re 1952

0 

-1.00 

-1.30

-.85

-.80 

-.90 

-.25 

-.45

0 

-.35 

-.70 

-.60

-.95

-.90

-1.00 

-1.45

0

-.25

-.65

525

710

930

1,140

1,145

1,150

840

845

488

550

600

625

376

645

690

700

725

790

635

680

715

725

740

765

565

510

595

620

635

670

416

420

424

422

428

430

Channel width

(meters)

448

490

505

525

545

560

434(?)

595

605

605

610

615

484 

484 

472

448

458 

458 

585 

590

645 

645 

650 

650

650

655

655 

655

675

675

675
74 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 13. Data on channel features, as measured from resurveyed TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued cross sections Continued

dat

Year

1962

1967

1970

1975

1952

1954

1957

1960

1962

1967

1970

1975

1952

1954

1956

1960

1962

1967

1970

1975

1952

1954

1957

1960

1962

1967

1970

1975

1952 

1954

1956

1960

1962 

1967

1975

Year of

i collection

Years 

after dam 

closure

Missouri Rive

10

15

18

23

0

2

5

8

10

15

18

23

0

2

4

8

10

15

18

23

0

2

5

8

10

15

18

23

0 

2

4

8

10 

15

23

Year of 

data collection

Year

1952

1954

1956

1960

1962 

1967

1975

1952

1954

1957

1960

1962

1967

1970

1975

1952 

1954

1957

1960

1962 

1967 

1975

1952 

1954 

1957 

1960

1962 

1967 

1975

1952

1954

1956

1960

Years 

after dam 

closure

Missouri Riv

0

2

4

8

10 

15

23

0

2

5

8

10

15

18

23

0 

2

5

8

10 

15 

23

0 

2 

5 

8

10

15 

23

0

2

4

8

Distance of cross

section downstream 

(kilometers)

r, South Dakota, Fort

Year of dam closur

4.2

4.2

4.2

4.2

5.1

5.1

5.1

5.1

5.1

5.1

5.1

5.1

6.6

6.6

6.6

6.6

6.6

6.6

6.6

6.6

7.7

7.7

7.7

7.7

7.7

7.7

7.7

7.7

11.0 

11.0

11.0

11.0

11.0 

11.0

11.0

Distance of cross 

section downstream 

from dam 

(kilometers)

er, South Dakota, Fort

Year of dam closur

12.5

12.5

12.5

12.5

12.5 

12.5

12.5

14.5

14.5

14.5

14.5

14.5

14.5

14.5

14.5

19.0 

19.0

19.0

19.0

19.0 

19.0 

19.0

24 

24 

24 

24

24 

24 

24

29

29

29

29

Total change

in mean bed 

elevation 

(meters)

Randall Dam Contin

5 1952

-0.90

-.90

-1.05

-1.35

0

+.20

-.40

-.40

-.60

-.60

-.80

-.80

0

-.20

-.50

-1.15

-1.30

-1.15

-1.15

-1.85

0

-.25

-.75

-1.10

-1.35

-1.15

-1.05

-1.60

0 

-1.30

-1.75

-1.50

-1.50 

-1.65

-2.60

Total change 

in mean bed 

elevation 

(meters)

Channel width 

(meters)

ued

685

690

695

690

720

745

730

735

740

750

755

755

1,060

1,075

1,095

1,115

1,130

1,130

1,135

1,165

1,070

1,115

1,130

1,145

1,160

1,245

1,260

1,280

404 

406

406

408

410 

420

462

Channel width 

(meters)

Randall Dam Continued

e 1952

0

0

-.60

-.75

-.80 

-.80

-1.40

0

+.50

+ .20

+.25

+ .35

+ .20

+ .10

0

0 

-.10

-.05

-.45

-.20 

+ .35 

-.15

0 

-.30 

+.10 

+.30

+.25 

-.35 

-.75

0

-.45

-.50

-.50

565

565

570

570

575 

585

605

1,080

1,070

1,060

1,065

1,065

1,065

1,065

1,035

366 

366

406

645

700 

735 

750

640 

640 

650 

645

645 

645 

645

1,040

1,070

1,075

1,070

Y

data

Year

ear of

collection

Years 

after dai

Missouri

Distance of cross

section downstream 

from dam 

° (kilometers)

River, South Dakota, Fort

Total change

in mean bed 

elevation 

(meters)

Randall Dam Contint

Channel 

(mete

led

width 

rs)

Year of dam closure 1952

1962

1967

1970

1975

1952

1954

1957

1960

1962

1967

1970

1975

1952

1954

1957

1960

1962

1965

1967

1970

1975

1952

1954

1957

1960

1962

1965

1967

1970

1975

1952

1954

1957

1961

Y 

data

Year

1962

1967

1970

1975

1955

1960

1965

1970

1974

1955

1960

1965

1970 

1974

1955

1960

1965 

1970 

1974

1955 

1960 

1965 

1970 

1974

1955 

1960 

1965

1970

1974

10

15

18

23

0

2

5

8

10

15

18

23

0

2

5

8

10

13

15

18

23

0

2

5

8

10

13

15

18

23

0

2

5

9

ear of 

collection

Years 

after dan 

closure

Missouri

10

15

18

23

Mis

0

5

10

15

19

0

5

10

15 

19

0

5

10 

15 

19

0 

5 

10 

15 

19

0 

5 

10

15

19

29

29

29

29

35

35

35

35

35

35

35

35

43

43

4j

43

43

43

43

43

43

53

53

53

53

53

53

53

53

53

58

58

58

58

Distance of cross 

section downstream 

from dam 

1 (kilometers)

River, South Dakota, Fort

Year of dam closure

58

58

58

58

souri River, South Dakota,

Year of dam closure

2.3

2.3

2.3

2.3

2.3

3.4

3.4

3.4

3.4 

3.4

4.3

4.3

4.3 

4.3 

4.3

5.3 

5.3 

5.3 

5.3 

5.3

6.8 

6.8 

6.8

6.8

6.8

-0.50

-.50

-.75

-.65

0

+ .05

+ .10

+.05

-.10

-.25

0

-.45

0

+.35

-.55

-.45

-.05

+.20

+.10

+.70

+.10

0

+.35

+.60

+.60

+.70

+.75

+.70

+1.00

+.60

0

+.50

+.25

+.50

Total change 

in mean bed 

elevation 

(meters)

Randall Dam Continu

1952

+0.70

+ .75

+ .75

+ .85

Gavins Point Dam

o /

1955^

0

-1.30

-1.50

-2.15

-2.50

0

-1.00

-1.50

-2.00 

-2.30

0

-.25

-1.20 

-1.45 

-1.90

0 

-.55 

-1.20 

-J..50 

-2.00

0 

-.40 

-.60

-.80

-1.25

1,065

1,075

1,075

1,075

695

695

695

695

695

695

695

695

760

895

1,035

1,050

1,055

1,060

1,060

1,070

1,115

685

690

690

700

705

705

705

710

710

810

835

835

845

Channel 

(mete

ed

845

860

870

885

374

374

380

380

374

525

525

525

525 

525

344

416

420 

420 

426

630 

645 

650 

655 

660

980 

1,155 

1,160

1,170

1,175

width 

rs)
TABLES 13, 14 75

TABLE 13. Data on channel features, as measured from resurveyed 

cross sections Continued

TABLE 13. Data on channel features, as measured from resurveyed 

cross sections Continued

Year of

data

Year

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

collection

Years 

after dam 

closure

Missouri River,

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

Year of 

data collection

Year

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

Years 

after dam 

closure

Missouri River

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

Distance of cross

ection downstream 

(kilometers)

South Dakota, Gavin

Year of dam closure

7.9

7.9

7.9

7.9

7.9

8.4

8.4

8.4

8.4

8.4

8.5

8.5

8.5

8.5

8.5

9.5

9.5

9.5

9.5

9.5

11.0

11.0

11.0

11.0

11.0

12.5

12.5

12.5

12.5

12.5

14.5

14.5

14.5

14.5

14.5

Distance of cross 

ection downstream

(kilometers)

South Dakota, Gavin

Year of dam closure

16.5

16.5

16.5

16.5

16.5

18.0

18.0

18.0

18.0

18.0

22

22

22

22

22

23

23

23

23

23

26

26

26

26

26

27

27

27

27

27

28

28

28

28

28

Total change

in mean bed 

elevation 

(meters)

Channel width 

(meters)

3 Point Dam Continued

1955-/

0

-.55

-.80

-1.35

-1.50

0

-1.10

-.65

-1.05

-1.80

0

-.25

-.80

-1.70

-2.05

0 

-.45

-.65

-1.15

-1.55

0

-.50

-.50

-.90

-1.05

0

-.45

-.65

-.50

-1.35

0

0

+.45

-.45

-.50

Total change 

in mean bed 

elevation 

(meters)

s Point Dam Co

1955^

0

-1.00

-.50

-.45

-.90

0

+.05

-.20

-.20

-.25

0

+.10

+.15

-.25

-.60

0

+.05

+.50

+.05

+.10

0

+.65

+.50

+1.30

+.80

0

+.40

+.10

-.20

-.05

0

-1.05

-.05

-.10

-.25

885

885

885

885

885

478

478

478

478

478

366

368

366

368

362

464

456

464

466

466

880

1,020

1,035

1,060

1,065

348

412

438

446

470

790

880

880

1,045

1,050

Channel width 

(meters)

ntinued

845

850

1,125

1,160

1,190

905

905

920

920

930

615

625

635

645

645

520

605

780

950

975

326

466

480

675

690

960

1,165

1,215

1,215

1,220

805

975

1,050

1,080

1,095

data

Year

Year of

collection

Years 

after dam 

closure

Distance of cross

section downstream 

from dam 

(kilometers)

Missouri River, South Dakota, Gavins

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

1955

1960

1965

1970

1974

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

0

5

10

15

19

Year of 

data collection

Year

Years 

after dam 

closure

Missouri Ri

Year of dam closure

30

30

30

30

30

32

32

32

32

32

34

34

34

34

34

36

36

36

36

36

38

38

38

38

38

39

39

39

39

39

41

41

41

41

41

Distance of cross 

section downstream 

from dam 

(kilometers)

ver, South Dakota, Gavii

Total change

in mean bed 

elevation 

(meters)

Point Dam Continu

1955*/

0

+.15

-.15

+.35

+.35

0

+.25

+.50

+.60

+.05

0

-.25

+.05

+.05

-.20

0

-.60

-1.20

-.80

-1.30

0

+.05

-.10

+.30

+.10

0

+.25

+.40

+.35

-.10

0

-.10

-.15

-.20

-.60

Total change 

in mean bed 

elevation 

(meters)

is Point Dam Contin

Channel width 

(meters)

ed

460

570

575

575

580

505

585

600

620

625

790

845

880

910

980

1,780

1,785

1,815

1,835

1,840

655

660

665

670

680

368

378

380

380

380

890

890

905

925

935

Channel width 

(meters)

ued

8/ Year of dam closure 1955^'

1955

1960

1965

1970

1974

1957

1960

1965

1970

1974

1957

1960

1965

1970

1974

1958

1960

1965

1970

1974

1957

1960

1965

1970

1974

1957

1960

1965

1970

1974

1958

1960

1965

1970

1974

0

5

10

15

19

2(0?)

5

10

15

19

2

5

10

15

19

3

5

10

15

19

2

5

10

15

19

2

5

10

15

19

3

5

10

15

19

44

44

44

44

44

46

46

46

46

46

48

48

48

48

48

52

52

52

52

52

55

55

55

55

55

57

57

57

57

57

61

61

61

61

61

0

-.25

-.35

-.45

-.45

I/O

-.45

-.45

-.35

-1.00

I/O

+.15

+.35

-.10

+.15

I/O

-.10

+.15

-.05

-.60

I/O

-.05

+ .20

+.25

-.30

I/O

+.20

+.10

-.35

-.50

I/O

-.35

-.20

-.20

-.80

1,600

1,600

1,600

1,605

1,605

945

960

970

975

975

895

1,080

1,145

1,180

1,190

1,040

1,125

1,290

1,415

1,440

675

755

1,030

1,105

1,130

865

865

925

935

940
76 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued

data

Year

Year of 

collection

Years 

after dam 

closure

Missouri Ri^

Distance of cross

section downstream 

from dam 

(kilon^ters)

rer, South Dakota, Gavins

Total change

elevation 

(meters)

Point Dam Contim.

Channel width

(meters)

ed

Year of dam closure 1955-/

1959

1960

1965

1970

1974

1958

1960

1965

1970

1974

1959 

1960

1965 

1970

1974

1959

1960

1965

1970

1974

1959

1960

1965

1970

1974

1959

1960

1965

1970

1974

1959

1960

1965

1970

1974

dat

Year

1959

1960

1965

1970

1974

1950

1952

1962

1963

1971

1977

1950

1952

1962

1963

1971

1977

1950

1952

1962

1964

1971

1978

1950

1952

1962

1964

1971

1978

4

5

10

15

19

3

5

10

15

19

4 

5

10 

15

19

4

5

10

15

19

4

5

10

15

19

4

5

10

15

19

4

5

10

15

19

Year of 

i collection

Years 

after dam 

closure

Missouri Ri

4

5

10

15

19

rledic

(0)

3

13

14

22

28

(0)

3

13

14

22

28

(0)

3

13

15

22

29

(0)

3

13

15

22

29

64

64

64

64

64

69

69

69

69

69

72 

72

72 

72

72

78

78

78

78

78

82

82

82

82

82

85

85

85

85

85

89

89

89

89

89

Distance of cross

section downstream 

from dam 

(kilometers)

ver, South Dakota, Gavins

Year of dam closure

93

93

93

93

93

ine Creek, Nebraska, Medi

Year of dam closure

0.8

.8

.8

.8

.8

.8

13.0

13.0

13.0

13.0

13.0

13.0

16.0

16.0

16.0

16.0

16.0

16.0

16.5

16.5

16.5

16.5

16.5

16.5

1/ 0

+ .05

+ .05

+ .10

+ .75

1/ 0

-.45

-1.00

-1.85

-1.45

3/0 

+ .10

-.20 

-.35

-.65

3/0

-.25

-.30

-.65

-1.15

3/0

-.05

+.05

+ .25

+ .20

3/0

+ .20

+.65

+ .40

-.10

 0

+ .30

-.35

+ .25

+ .35

Total change

in mean bed 

elevation 

(meters)

Point Dam Contin

1955*/

3/0

-.15

+1.70

+1.15

+ .85

cine Creek Dam

1949

0

-.10

-.20

-.20

-.20

-.20

0

-.05

+ .50

+ .30

+.45

+ .40

0

-.55

-.30

-.25

+.20

+ .25

0

-.25

+ .30

-.10

+ .45

+ .15

960

975

1,110

1,140

1,140

755

785

785

795

840

1,415 

1,445

1,555 

1,640

1,645

535

535

535

545

545

1,300

1,300

1,335

1,365

1,390

1,355

1,535

1,905

1,925

1,935

680

685

765

865

895

Channel width 

(meters)

ued

330

392

710

740

755

83.5

93.0

107(7)

91.5

99.0

107

30.5

32.0

38.5

36.5

36.5

35.5

_

20.5

21.0

20.5

21.5

21.0

25.5

25.5

26.0

25.5

25.5

25.5

TABLE 13. Data on channel features , as measured from resurveyed

cross sections Continued

Year of 

data collection

Year

Years 

after dam 

closure

Distance of cross 

section downstream

from dam 

(kilometers)

Middle Loup River, Nebra

1950

1955

1956

1957

1957

1957

1961

1961

1962

1964

1964 

1967

1969 

1971

1961

1962

1964

1964

1967

1969

1971

1964

1964

1967

1969

1971

1961

1962

1964

1964

1967

1969

1971

1967

1969

1971

(0)

.2

1.3

2.3

2.7

2.3

6.3

6.6

7.5

9.3

9.7 

12.3

14.3 

16.3

6.3

7.5

9.3

9.7

12.3

14.3

16.3

9.3

9.7

12.3

14.3

16.3

6.3

7.5

9.3

9.7

12.3

14.3

16.3

12.3

14.3

16.3

Year of 

data collection

Year

Years 

after dam 

closure

Year of dam clos

0.2

.2

.2

.2

.2

.2

.2

.2

.2

.2

.2 

.2

.2

1.6

1.6

1.6

1.6

1.6

1.6

1.6

3.1

3.1

3.1

3.1

3.1

5.6

5.6

5.6

5.6

5.6

5.6

5.6

7.4

7.4

7.4

Distance of cross 

section downstream

from dam 

(kilometers)

Smoky Hill River, Kansas

1946

1951

1952

1961

1971

1946

1951

1952

1961

1971

1946

1951

1952

1961

1971

1946

1951

1952

1961

1971

1946

1951

1952

1961

1971

1946

1951

1952

1961

1946

1951

1952

1961

(0)

3

4

13

23

(0)

3

4

13

23

(0)

3

4

13

23

(0)

3

4

13

23

(0)

3

4

13

23

(0)

3

4

13

(0)

3

4

13

Year of dam closi

0.8

.8

.8

.8

.8

2.9

2.9

2.9

2.9

2.9

4.8

4.8

4.8

4.8

4.8

6.8

6.8

6.8

6.8

6.8

3.7

8.7

8.7

8.7

8.7

13.0

13.0

13.0

13.0

16.5

16.5

16.5

16.5

Total change 

in mean bed

elevation 

(meters)

ska, Milburn Dam

ure 1955

9/0

-.60

-1.20

-2.30

-2.30

-1.35

-1.45

-1.80

-1.90

-2.15

-2.15 

-2.65

-2.15 

-2.40

3/0

-.35

-.60

-.60

-1.00

-1.10

-1.20

-/(O)

0

-.20

-.15

-.25

-/(O)

-.40

-.65

-.70

-.90

-.90

-1.05

3 /

-.05

-.25

Channel width

(meters)

I/

7.6

29.0

44.5

44.5

57.5

96.5

96.5

109

110

111 

114

117 

124

230

234

232

238

234

234

234

118

118

120

118

123

90.5

91.0

91.0

91.0

92.0

91.5

92.0

163

166

174

Total change 

in mean bed Channel width

elevation (meters) 

(meters)

, Kanopolis Dam

ire 1948

0

-.80

-1.10

-1.30

-1.45

0

-.20

-.35

-1.05

-1.05

0

+ .05

+ .15

-.50

-.50

0

-.05

0

-.45

-.20

0

+ .25

0

-.25

-.20

0

-.05

-.10

-.25

0

+ .35

+.25

+.35

46.5

45.0

44.5

45.0

48.0

41.0

42.0

42.0

41.0

45.0

40.0

40.5

39.0

39.5

42.0

50.0

49.5

49.5

46.5

47.0

39.5

41.5

46.0

47.0

50.0

34.5

38.5

39.0

39.5

39.5

41.0

41.0

40.5
TABLES 13, 14 77

TABLE 13. Data on channel features, as measured from resurveyed 

cross sections Continued

TABLE 13. Data on channel features, as measured from resurveyed 

cross sections Continued

data

Year

1946

1951

1952

1961

1946

1951

1952

1961

1946

1951

1952

1961

1946

1951

1952

1961

1946

1951

1952

1961

1946

1951

1952

1961

1946

1951

1952

1961

1946

1951

1952

1961

da a

Year

1946

1951

1952

1961

1946

1951

1952

1961

1967

1974

1967

1975

1944

1949

1961

1969

1944

1949

1961

1969

1944

1949

1961

1969

1944

1949

1961

1969

Year of

collection

Years 

after dam 

closure

Smoky

(0)

3

4

13

(0)

3

4

13

(0)

3

4

13

(0)

3

4

13

(0)

3

4

13

(0)

3

4

13

(0)

3

4

13

(0)

3

4

13

Year of 

collection

Years 

after dam 

closure

Smoky

(0)

3

4

13

(0)

3

4

13

0

7

0

8

2

7

19

27

2

7

19

27

2

7

19

27

2

7

19

27

Distance of cross

from dam 

(kilometers)

Hill River, Kansas, Kanop

Year of dam closure

18.5

18.5

18.5

18.5

23

23

23

23

25

25

25

25

35

35

35

35

42

42

42

42

50

50

50

50

56

56

56

56

73

73

73

73

section downstream

from dam 

(kilometers)

 lill River, Kansas, Kanopo

Year of dam closure

92

92

92

92

108

108

108

108

Republican River, Kansas,

Year of dam closure

2.7

2.7

4.0

4.0

Wolf Creek, Oklahoma, For

Year of dam closure

.3

.3

.3

.3

1.0

1.0

1.0

1.0

1.3

1.3

1.3

1.3

1.6

1.6

1.6

1.6

Total change

elevation 

(meters)

oils Dam Continued

1948

0

+ .10

+ .15

+ .10

0

-.05

+ .15

-.10

0

-.05

-.15

-.25

0

+ .05

+ .20

+ .15

0

+.05

0

-.10

0

-.05

-.25

0

+ .05

_

+ .05

0

+.05

-.15

-.20

Total change

in mean bed 

elevation 

(meters)

lis Dam Continued

1948

0

0

-.20

0

+ .25

+ .20

+ .25

Milford Dam

1967

0

-.85

0

-.15

t Supply Dam

1942

0

-2.05

-3.15

-3.40

0

-1.90

-2.20

-2.00

0

-1.40

-2.10

-2.60

0

-.25

-1.50

-2.45

Channel width

(meters)

39.5

39.5

35.5

38.0

35.5

38.0

37.0

36.5

38.5

39.5

40.0

41.5

36.5

36.0

34.5

37.0

30.0

30.0

33.5

32.5

36.5

36.5

35.0

54.5

34.0

63.0

33.0

34.0

38.0

39.0

39.5

Channel width 

(meters)

35.0

35.0

40.0

30.0

30.0

29.5

31.0

156

165

98.0

116

242

26.5

32.5

23.0

137

30.5

56.0

57.5

158

46.0

63.5

28.5

172

163

90.0

15.0

dat

Year

1944

1949

1961

1969

1944

1949

1961

1969

1944

1949

1961

1969

1944

1949

1961

1969

1944

1949

1961

1969

1944

1949

1961

1969

1947

1949

1951

1951

1959

1966

1976

dat

Year

1947

1949

1951

1951

1959

1966

1976

1947

1949

1951

1951

1959

1966

1976

1947

1949

1951

1951

1959

1966

1976

1947

1949

1951

1951

1959

1966

1947

1949

1951

1951

1959

1966

Year of

a collection

Years 

after dan 

closure

Wolf

2

7

19

27

2

7

19

27

2

7

19

27

2

7

19

27

2

7

19

27

2

7

19

27

N

0

1

2.8

3.4

11

18

28

Year of

a collection

Years 

after dair 

closure

North C

0

1

2.8

3.4

11

18

28

0

1

2.8

3.4

11

18

28

0

1

2.8

3.4

11

18

28

0

1

2.8

3.4

11

18

0

1

2.8

3.4

11

18

section downstream

from dam 

(kilometers)

Creek, Oklahoma, Fort Si

Year of dam closi

1.8

1.8

1.8

1.8

2.6

2.6

2.6

2.6

2.9

2.9

2.9

2.9

3.9

3.9

3.9

3.9

4.7

4.7

4.7

4.7

6.6

6.6

6.6

6.6

Drth Canadian River, Okl

Year of dam clos

1.8

1.8

1.8

1.8

1.8

1.8

1.8

Distance of cross

from dam 

(kilometers)

anadian River, Oklahoma,

Year of dam closu

3.1

3.1

3.1

3.1

3.1

3.1

3.1

5.0

5.0

5.0

5.0

5.0

5.0

5.0

5.6

5.6

5.6

5.6

5.6

5.6

5.6

10.5

10.5

10.5

10.5

10.5

10.5

12.0

12.0

12.0

12.0

12.0

12.0

in mean bed

elevation 

(meters)

pply Dam Continued

re 1942

0

-1.35

-2.30

0

-.45

-1.05

0

-.80

-1.60

-1.60

0

-1.20

-1.45

-2.00

0

-.45

-1.15

-1.50

0

-.35

-1.15

-1.30

ahoma, Canton Dam

are 1948

0

-.90

-1.20

-1.55

-2.90

-2.75

-3.00

Total change

in mean bed 

(meters)

Canton Dam Contini

re 1948

0

-.60

-1.00

-1.30

-1.50

-1.20

-1.50

0

-.60

-.65

-1.05

-.80

-.95

-1.65

0

-.35

-1.05

-1.35

-.85

-1.60

-1.50

i°/o

+ .75

+1.05

+1.05

+ .85

+ .90

0

-.15

-.65

-.75

-.55

-1.45

Channel width

(meters)

296

191

20.0

107

88.5

120

242

81.5

52.5

55.0

246

79.0

84.5

24.5

272

166

97.0

26.0

240

121

38.0

30.0

64.5

61.0

62.0

67.0

29.5

17.5

18.5

Channel width 

(meters)

aed

65.0

53.0

47.0

56.5

55.5

56.5

48.0

47.0

48.5

46.5

45.5

49.0

27.5

17.5

45.5

44.0

44.0

35.0

29.5

16.5

20.0

^35.5

53.5

91.5

91.5

97.0

76.0

75.0

77.0

63.5

61.0

85.0

12.5
78 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued

dat

Year

Year of 

a collection

Years 

after dam 

closure

North Can

section downstream 

(kilometers)

adian River, Oklahoma

Total change 

in mean bed 

elevation 

(meters)

, Canton Dam Contin

Channel width 

(meters)

ued

Year of dam closure 1948

1947

1949

1951

1951

1959

1966

1947 

1949

1951

1951

1959

1966

1947

1949

1951

1951

1959

1966

1947

1949

1951

1951

1959

1966

1947

1949

1951

1951

1959

1966

1947

1949

1951

data

Year

1951

1959

1966

1947

1949

1951

1951

1959

1966

1947

1949

1951

1951

1959

1966

1947

1949

1951

1951

1959

1966

1947 

1949

1951

1951

1959

1966

1947 

1949

1951

1951

1959

1966

0

1

2.8

3.4

11

18

0 

1

2.8

3.4

11

18

0

1

2.8

3.4

11

18

0

1

2.8

3.4

11

18

0

1

2.8

3.4

11

18

0

1

2.8

Year of 

collection

Years 

after dam 

closure

North Cana

3.4

11

18

0

1

2.8

3.4

11

18

0

1

2.8

3.4

11

18

0

1

2.8

3.4

11

18

0

1

2.8

3.4

11

18

0 

1

2.8

3.4

11

18

14.5

14.5

14.5

14.5

14.5

14.5

27 

27

27

27

27

27

35

35

35

35

35

35

50

50

50

50

50

50

58

58

58

58

58

58

68

68

68

section downstream 

from dam 

(kilometers)

dian River, Oklahoma,

Year of dam closu

68

68

68

92

92

92

92

92

92

104

104

104

104

104

104

114

114

114

114

114

114

125 

125

125

125

125

125

134 

134

134

134

134

134

0

-.10

-.35

-.45

-.85

-.70

0 

+.20

+.05

+ .05

-.65

-.55

0

-.20

-.25

-.45

-.35

-.45

0

-.10

-.10

+ .05

+ .45

-.05

0

-.30

-.10

-.15

-1.05

-.85

0

+.45

+.15

Total change

in mean bed 

elevation 

(meters)

Canton Dam Continu

re 1948

-0.65

+ .05

+.35

0

+.10

+.10

+.30

-.30

+ .25

0

-.25

-.20

-.10

-.50

-.55

0

-.10

-.20

-.05

-.05

-.20

0 

-.30

-.50

-.50

-.65

-.70

0 

0

+ .10

-.05

-.35

-.35

93.0

93.5

74.0

72.0

42.0

23.0

105 

116

105

106

39.5

40.0

52.5

57.5

57.5

56.5

49.5

30.0

76.0

70.5

69.0

99.5

100

59.5

96.5

95.0

113

106

40.5

39.5

__

_

~

Channel width 

(meters)

ed

__

_

_

_

__

_

_

_

_

_

44.5

45.0

44.0

35.5

31.5

27.5

38.5 

38.0

38.5

34.0

26.5

30.0

40.0 

33.0

36.5

33.5

33.0

27.5

TABLE IB. Data on channel features, as measured from resurveyed

cross sections Continued

Y 

data

Year

1947

1949

1951

1951

1959

1966

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

Ye 

data c

Year

ear of 

collection

Years 

after dam 

closure

North C

0

1

2.8

3.4

11

18

1

6

14

1

6

14

1

6

14

1

6

14

1

6

14

1

6

14

1

6

14

1

6

14

ar of 

ollection

Years 

after dam 

closure

Distance of cross 

section downstream 

from dam 

(kilometers)

anadian River, Oklahoma,

Year of dam closur

154

154

154

154

154

154

Canadian River, Oklahoma

Year of dam closur

.8

.8

.8

2.1

2.1

2.1

3.4

3.4

3.4

4.7

4.7

4.7

6.6

6.6

6.6

8.0

8.0

8.0

11.5

11.5

11.5

14.0

14.0

14.0

Distance of cross

section downstream 

from dam 

(kilometers)

Canadian River, Oklahoma, Eufau

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

1964

1969

1977

1

6

14

1

6

14

1

6

14

1

6

14

1

6

14

1

6

14

1

6

14

Year of dam closure

16.0

16.0

16.0

18.5

18.5

18.5

20

20

20

23

23

23

34

34

34

37

37

37

40

40

40

Red River, Oklahoma-Texas,

Total change 

in mean bed 

elevation 

(meters)

Canton Dam Contim

e 1948

0

-.05

+ .05

+ .05

-.35

-.50

, Eufaula Dam

e 1963

0

-5.05

-4.95

0

-2.30

-3.20

0

-2.10

-2.80

0

-1.15

-2.15

0

-.65

-1.20

0

-.35

-1.00

0

-.35

-1.40

0

-.35

-1.15

Total change

in mean bed 

elevation 

(meters)

la Dam--Continued

1963

0

0

-.60

0

-.90

-1.10

0

-.70

-1.35

0

-.45

-.60

0

-.30

+ .40

0

-.70

0

0

+ .05

+1.20

Denison Dam

Channel width 

(meters)

ed

39.0

39.5

39.5

38.5

28.5

31.0

252

177

234

460

208

234

284

183

220

560

560

400

218

280

284

362

515

402

446

462

420

505

620

494

Channel width

(meters)

422

438

374

149(?)

198

280

446

454

428

400

460

580

260

260

187

360

350

346

406

470

478

Year of dam closure 1942

1942

1945

1948

1958 

1969

1942

1945

1948

1958

1969

0

3

6

16

27

0

3

6

16

27

.6

.6

.6

.6 

.6

1.1

1.1 

1.1

1.1

1.1

0

-1.25

-1.35

-1.45 

-1.60

0

-1.40

-3.00

-2.40

228

244

278

280 

282

236

230

i!/

W
TABLES 13, 14 79

TABLE IS. Data on channel features, as measured from resurveyed

cross sections Continued

data

Year

1942

1945

1948

1958

1969

1942

1945

1948

1958 

1969

1942

1945

1948

1958 

1969

1942

1945 

1948 

1958 

1969

1942 

1945 

1948

1958 

1969

1942 

1945 

1948

1958

1969

1942

1945

1948

1958

1969

data

Year

1942

1945

1948

1958

1969

1942

1945

1948

1953

1969

1942

1945

1948

1958

1969

1946

1948

1958

1969

1946

1948

1958

1969

1946

1948

1958

1969

1946

1948

1958

1969

1946

1948

1958

1969

Year of

Years

closure

Red

0

3

6

16

27

0

3

6

16

27

0

3

6

16

27

0

3 

6 

16 

27

0 

3 

6

16

27

0 

3 

6

16

27

0

3

6

16

27

Year of 

collection

Years 

after da 

closure

Red

0

3

6

16

27

0

3

6

16

27

0

3

6

16

27

4

6

16

27

4

6

16

27

4

6

16

27

4

6

16

27

4

6

16

27

 section downstream 

from dam 

m (kilometers)

River, Oklahoma-Texas, Denis

Year of dam closure

2.1

2.1

2.1

2.1

2.1

3.2

3.2

3.2

3.2 

3.2

5.1

5.1

5.1

5.1 

5.1

7.2

7.2 

7.2 

7.2 

7.2

8.4 

8.4 

8.4

8.4 

8.4

11.5 

11.5 

11.5

11.5

11.5

15.0

15.0

15.0

15.0

15.0

Distance of cross

 section downstream 

from dam 

(kilometers)

Total change

elevation 

(meters)

on Dam Continued

1942

0

-1.35

-2.45

-2.40

0

-.80

-1.60

-2.40 

-2.00

0

-.45

_

-2.20 

-1.65

0

-.10 

-.75 

-1.30 

-1.30

0 

-1.15

-1.45

-1.75

0 

-1.20

-1.85

-2.10

0

-1.45

-1.85

-2.45

-3.25

Total change

in mean bed 

elevation 

(meters)

Channel width 

(meters)

228

230

240

238

i^/284

284

284

284 

284

210

218

_

222 

214

274

280 

280 

280 

296

396 

398

400 

400

151 

135

__

140

224

149

149

151

152

Channel width 

(meters)

River, Oklahoma-Texas, Denison Dam Continued

Year of dam closure

18.5

18.5

18.5

18.5

18.5

22

22

22

22

22

27

27

27

27

27

34

34

34

34

41

41

41

41

48

48

48

48

65

65

65

65

80

80

80

80

1942

0

+.15

-1.40

-.60

0

-.20

-1.20

-.70

0

-.30

-.45

-.40

-.20

I/O

+ .05

-.25

-.20

I/O

+.35

-.45

-.30

I/O

+ .10

+.75

+ .75

I/O

-.10

-.10

+.10

I/O

+1.05

-.10

+ .35

318

324

__

336

342

244

244

256

246

282

328

360

382

372

218

228

292

296

308

308

292

300

376

530

775

780

480

488

630

640

705

1,050

990

1,025

TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued

Year of 

data collection

Year

1946

1948

1958

1970

1946

1948

1958

1970

1946 

1948

1958

1970

1946 

1948 

1958

1970

1946 

1948 

1958 

1970

1946 

1948

1958 

1970

1946 

1948 

1958

1970

Years 

after dam

closure

Red Ri\

4

6

16

28

4

6

16

28

4 

6

16

28

4 

6 

16

28

4 

6 

16 

28

4 

6

16 

28

4 

6 

16

28

section downstream 

from dam 

(kilometers)

er, Oklahoma-Texas, Deni

Year of dam closure

90

90

90

90

101

101

101

101

109 

109

109

109

122 

122 

122

122

132 

132 

132 

132

142 

142

142 

142

150 

150 

150

150

Total change 

in mean bed 

elevation 

(meters)

son Dam Continued

1942

I/O

-.80

-.70

-.55

I/O

-.20

+.45

-.10

I/O 

-.35

-.65

-.55

I/O 

-1.00 

+.20

-1.35

I/O 

-.05 

+ .30 

-.50

I/O 

-.35

-.10 

-.65

I/O 

+.40 

+ .10

-.25

Channel width 

(meters)

191

198

226

214

368

382

408

360

234 

268

262

266

1,085 

1,195 

910

1,025

312 

322 

366 

374

464 

336

452 

384

270 

294 

324

258

Neches River, Texas, Town Bluff Dam

1951

1965

0

14

Year of 

data collection

Year

1951

1960

1965

1951 

1960

1965

1951

1960

1965

1951

1960

1965

1951

1960

1965

after dam 

closure

Neches

0

9

14

0 

9

14

0

9

14

0

9

14

0

9

14

Year of dam closure

.2

.2

Distance of cross

section downstream 

from dam 

(kilometers)

River, Texas, Town Bluff

Year of dam closure

1.4

1.4

1.4

2.9 

2.9

2.9

4.7

4.7

4.7

6.3

6.3

6.3

8.0

8.0

8.0

1951

0

-2.25

94.5

127

Total change

in mean bed Channel width 

elevation (meters) 

(meters)

Dam Continued

1951

0

-.20

-.90

0 

-.10

-.60

0

-.90

+.50

0

-.60

-.65

0

-.25

-.05

111

127

127

101 

100

100

90.5

97.5

101

117

133

145

121

151

157

Des Moines River, Iowa, Red Rock Dam

1962

1978

1962

1978

1962

1978

1962

1978

1962

1978

1962

1978

1962

1978

(0)

9

(0)

9

(0)

9

(0)

9

(0)

9

(0)

9

(0)

9

Year of dam closure

2.3

2.3

4.7

4.7

6.1

6.1

12.0

12.0

14.0

14.0

22

22

25

25

1969

0

-1.00

0

-1.15

0

-1.05

0

-1.75

0

-.60

0

+ .20

0

+ .05

185

214

155

162

180

171

184

131

145

199

212

158

168
80 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 13. Data on channel features, as measured from resurveyed

cross sections Continued

data

Year

1962

1978

1962 

1978

1962 

1978

1962

1978

1962

1978

1962

1978

1962 

1978

1962 

1978

1962 

1978

1962

1978

1962

1978

1962

1978

1962

1978

1956 

1963

1964

ear of 

collection

Years 

after dam 

closure

Des r

(0)

9

(0) 

9

(0) 

9

(0)

9

(0)

9

(0)

9

(0) 

9

(0) 

9

(0) 

9

(0)

9

(0)

9

(0)

9

(0)

9

Ch

0

7

8

Year of 

data collection

Year

1965

1968

1971

1956

1963

1964

1965

1968

1971

1956

1963

1964

1965

1968

1971

1956

1963

1964

1965

1968

1971

1956

1963

1964

1965

1968

1971

1956

1963

1964

1965

1968

1971

Years 

after dam 

closure

Chattah

9

12

15

0

7

8

9

12

15

0

7

8

9

12

15

0

7

8

9

12

15

0

7

8

9

12

15

0

7

8

9

12

15

Distance of cross 

section downstream 

from dam 

(kilometers)

Total change 

in mean bed Channel width 

elevation (meters) 

(meters)

oines River, Iowa, Red Rock Dam Continued

Year of dam closur

29

29

33 

33

36 

36

38

38

40

40

42

42

48 

48

50 

50

52 

52

55

55

62

62

68

68

72

72

attahoochee River, Georg

Year of dam closur

.5

.5

.5

section downstream

from dam 

(kilometers)

oochee River, Georgia, B

Year of dam closur

0.5

.5

.5

1.9

1.9

1.9

1.9

1.9

1.9

2.9

2.9

2.9

2.9

2.9

2.9

4.0

4.0

4.0

4.0

4.0

4.0

5.8

5.8

5.8

5.8

5.8

5.8

7.6

7.6

7.6

7.6

7.6

7.6

e 1969

0

+ .65

0 

+ .10

0

-.45

0

-.65

0

-1.85

0

-.35

0 

-.45

0 

-.60

0 

-1.30

0

-.05

0

+.25

0

+1.00

0

+1.05

ia, Buford Dam

e 1956

0 

-.95

-1.00

153

185

160 

181

154

165

146

154

200

146

171

172

148 

155

108 

110

141 

119

129

152

206

208

179

185

162

164

95.0 

98.0

98.5

Total change 

in mean bed Channel width

elevation 

(meters)

uford Dam Continued

e 1956

-0.95

-1.10

-1.00

0

-1.40

-1.50

-1.80

-2.15

-2.55

0

-.90

-.95

-1.30

-1.60

-1.85

0

-.75

-.60

-.90

-1.35

-1.45

0

-.30

-.20

-.45

-.75

-.70

0

+ .05

+ .10

-.05

-.20

-.30

(meters)

99.0

100

103

76.0

77.0

74.0

76.0

77.5

75.5

71.5

74.0

79.5

74.0

74.5

76.0

63.0

67.0

68.0

68.5

67.0

68.5

67.0

68.0

67.0

69.5

69.0

91.0

98.0

97.0

95.5

98.0

TABLE 13. Data on channel features, as measured from

cross sections Continued

Year of 

data collection Distance of cross Total change 

 section downstream in mean bed

Years , . . from dam elevation

Closure"1 (kilometers) (meters)

Chattahoochee River, Georgia, Buford Dam Continued

Year of dam closure 1956

1956 0 11.0 0

1963 7 11.0 -.90

1964 8 11.0 +.05

1965 9 11.0 +.05 

1971 15 11.0 +.10

1957 1 13.5 3/ 0 

1963 7 13.5 -.20

1964 8 13.5 -.10

1965 9 13.5 -.10

1968 12 13.5 -.35

1971 15 13.5 + .05

1957 1 16.5 3/ 0

1964 8 16.5 -.40

1965 9 16.5 -.60 

1968 12 16.5 -.55

1957 1 18.0 1/0 

1963 7 18.0 +.20 

1964 8 18.0 +.20

1965 9 18.0 +.25 

1968 12 18.0 +.15

1971 15 18.0 +.30

1957 1 21 3/0

1963 7 21 +.25

1964 8 21 +.15

1965 9 21 +.05

1968 12 21 +.15

1971 15 21 +.25

1957 1 24 3/0

1963 7 24 +.15

1964 8 24 +.25

1965 9 24 +.15

1968 12 24' +.10 

1971 15 24 +.15

FOOTNOTES TO TABLE 13 

 ""/Cofferdam closure 1956; official closure of Glen Canyon Da

- Channel confined in rock-walled canyon. Widths, if listed 

only for general order of magnitude.

 First measurement of this cross section was later than yea

closure. Total changes in bed elevation were measured from this

-/Year of initial diversion. Official closure was in 1951.

 A diversion dam (Headgate Rock Dam) , located about 24 kilo

Parker Dam and closed in 1942, may have some unknown influence on

sections listed here rom a out

-'Year storage began. Dam completed in 1939.

-/Not all is bed degradation; arrival of bar or spit near le

part of previous channel.

-/Storage began 1952.

9 /

 All data for this cross section apply only to the thalweg

to the entire channel. During dam construction most of the flow

into the thalweg, and it gradually grew to become the new main ch

in/

 Pronounced lateral migration of channel at this section at

1947 through 1966.

 /Right bank washed out by tributary changing course in 1957

 Bridge section; width constrained.

resurveyed

Channel width 

(meters)

70.5

70.5

70.5

71.5

69.5 

64.0

67.5

68.0

72.0

74.0

59.0

69.0

67.0 

68.0

57.5 

60.0 

61.0

63.5 

62.0

57.0

54.0

55.0

54.0

55.0

60.5

58.5

59.0

61.0

60.0 

58.5

m was 1963.

, meaningful

r of dam

later year.

meters below

the cross

ft bank severed

rather than

was diverted

annel .

least from
TABLES 13, 14 81

TABLE 14. Changes in streambed elevations as estimated from

streamflow-gaging-station rating tables

Name of

downstream gaging stati 

and control station

Co

Colorado River below

Parker Dam

Jemez

Jemez River below

Jemez Canyon Dam

Jemez River near

[Footnotes on last page of table]

River , distance Reference

on of station , d "char Se 

, , (cubic meters irom dam .

(kilometers) per SeCOnd)

Period

streambed 

elevation =

change from 

initial

gage height

(meters)

lorado River, Arizona, Parker Dam

Year of dam closure 1938

6.4 i/90.6

tion

10/34-11/35

12/35-2/37

2/37-12/37

12/37-3/38

10/38-1/39

1/39-12/39

1/40-12/40

1/41-9/41

10/41-5/42

0

+ .18

+ .061

+ .18

-.91

-1.92

-1.74

-2.07

-2.59

River, New Mexico, Jemez Canyon Dam

Year of dam closure 1953

1.3 -/.034

43 .37

Jemez (control station)

Name of

downstream gaging stati 

and control station

Miss

Missouri River below

Fort Peck Dam

No suitable control

station

Missouri

Missouri River below

Fort Randall Dam

distance Reference

m of station , dlschar §e 

, , (cubic meters 

rrom dam /, .-, >, per second) (kilometers) r

Duri River, Montana, Fort Peck

Year of dam closure 1937

13 -/85.0

River, South Dakota, Fort Rand

Year of dam closure 1952

11 -/464

4/51-3/52

3/55-7/55

 8/55-9/55

10/55-2/56

2/56-8/56

8/56-5/57

5/57-10/57

10/57-3/58

3/58-6/58

6/36-9/36

10/36-5/37

6/37-9/38 

10/38-9/39 

10/39-5/41

8/49-5/52

4/58-4/60

10/60-5/61

10/61-3/63

Period

Dam

4/38-10/38

10/38-4/39

10/39-9/40

10/40-9/41

10/41-10/44

10/44-9/45

10/47-9/48

10/48-9/51

10/51-9/52

2/54-9/55

10/55-2/56 

3/56-9/56

10/57-9/58

10/59-2/61

10/61-9/65

10/65-11/66

11/66-9/79

all Dam

5/47-9/51

10/52-11/52

3/53-5/53

7/53-11/53

5/54-9/54

10/54-3/55

3/55-9/55

10/55-9/56

10/56-9/59

10/59-9/60

0

-.55

-.95

-.98

-.73

-.98

-1.49

-2.07

-2.32

0

-.18

-.12 

-.061 

-.12

-.40

-.30

-.43

-.37

Change in 

streambed 

elevation =

change from 

initial

gage height

(meters)

0

-.030

-.21

-.24

-.30

-.52

-.67

-.85

-1.01

-1.19

-1.25 

-1.37

-1.31

-1.34

-1.37

-1.46

-1.49

0

+ .030

+ .030

-.30

-.30

-.37

-.24

-.30

-.24

-.34

TABLE 14. Changes in streambed elevations as estimated from

streamflow-gaging-station rating tables Continued

Change in 

River streambed 

Name of distance dischar" elevation *

downstream gaging station of station , , . Period change from 

. . . ,- . (cubic meters . .^. -, and control station from dam ,, initial

(kilometers) per secona ' gage height

(meters)

Missouri River, South Dakota, Fort Randall Dam Continued

Year of dam closure 1952

10/60-12/63 -0.40

12/63-9/64 -.43

10/64-11/65 -.40

11/65-10/67 -.43

10/67-6/69 -.61

No suitable control

station

Missouri River, South Dakota, Gavins Point Dam

Year of dam closure 1955

Missouri River at 8 -/312 3/32-9/33 0

Yankton 10/33-7/34 -.15 

8/34-3/37 -.061

3/37-9/38 -.18

10/38-5/39 -.15

5/39-3/40 -.27

3/40-3/41 -.15

3/41-6/41 -.091

6/41-9/41 -.061

10/41-5/42 -.12

5/42-3/43 -.34

6/43-3/44 -.49

10/45-3/47 -.61

3/47-9/48 -.61

10/48-3/51 -.37

3/51-3/52 -.49 

5/53-11/54 -.58 

11/54-4/55 -.61

5/55-9/55 -.46

10/55-9/56 -.55

10/56-9/57 -.67

10/57-1/59 -.70

1/59-12/60 -.76

12/60-9/61 -.82

10/61-9/62 -.88

Change in 

River streambed 

Name of distance "discharge elevation =

downstream gaging station of station . ^ Period change from 

and control station from dam 1C me tf8 initial

(kilometers) per secona} gage height

(meters)

Missouri River, South Dakota, Gavins Point Dam Continued

Year of dam closure 1955

10/62-12/63 -0.98

12/63-9/65 -1.01

10/65-9/68 -1.19

10/68-9/71 -1.25

10/73-9/76 -1.92

10/76-9/79 -2.16

No suitable control

station

Smoky Hill River, Kansas, Kanopolis Dam

Year of dam closure 1948

Smoky Hill River near 1.3 0.51 10/40-9/41 0

Langley 10/41-9/42 +.12

10/42-10/46 +.30

10/46-5/47 +.12

5/47-9/47 -.030

10/47-3/49 0

10/49-9/50 -.24

9/50-3/51 -.40

it/52-12/52 -.91

10/53-6/54 -.91

6/54-9/54 -.88

10/54-10/55 -.91

10/55-7/57 -.88

10/59-4/60 -1.01

4/60-6/61 -1.04

10/61-9/64 -1.04

10/64-9/68 -1.07

10/68-5/70 -1.10

10/70-10/71 -1.10

10/71-3/73 -1.13

3/73-10/73 -1.19

10/73-10/74 -1.34

10/74-9/76 -1.40

10/76-12/77 -1.37
82 DOWNSTREAM EFFECTS OF DAMS ON ALLUVIAL FANS

TABLE 14. Changes in streambed elevations as estimated from

streamflow-gaging-station rating tables Continued

Fiver 

, , . Reference Name of distance

downstream gaging station of station M meters Period 

and control station from dam ^

(kilometers) per seCOtld)

Smoky Hill River, Kansas, Kanopolis Dam Continued

Year of dam closure 1948

Smoky Hill River at 48 0.43 7/40-9/45

Ellsworth (control 10/45-7/46

station) 7/46-9/49

10/49-8/50

8/50-4/51

4/51-9/51

10/51-9/53

10/53-6/55

10/56-7/57

10/57-9/61

10/61-9/62

10/62-9/63 

10/63-6/64

6/64-5/65 

7/65-11/65

2/66-8/66 

7/67-11/68 

11/68-4/69 

6/69-12/69

1/70-6/70

6/70-10/70 

10/70-3/71 

7/71-1/72 

1/72-10/73

2/75-?

Republican River, Kansas, Milford Dam

Year of dam closure 1967

Republican River below 2.7 1.2 10/63-9/64

Milford Dam 10/64-7/65

7/65-2/66

2/66-7/67

7/67-11/67

Mr River,. Reference Name of distance

downstream gaging station of station discharge Period 

and control station from dam <.cubic meters 

(kilometers) per second)

Republican River, Kansas, Milford Dam Continued

Year of dam closure 1967

11/67-2/69

2/69-5/70

10/70-6/72

6/72-4/73

4/73-11/73

11/73-4/74

4/74-6/75

6/75-1/76

1/76-9/77

10/77-6/78

6/78-3/79

3/79-1/80

Republican River at 49 3.4 10/53-2/55

Clay Center 2/55-6/55

(control station) 6/55-9/55

10/55-9/56

10/56-9/58

10/58-2/59

2/59-9/59

10/59-3/60

3/60-9/62

10/62-9/63

10/63-1/68

2/68-7/69 

10/69-5/71 

5/71-5/72

5/72-9/73

10/73-9/77

10/77-1/80

North Canadian River, Oklahoma, Canton Dam

Year of dam closure 1948

North Canadian River 4.8 .031 10/37-9/41

at Canton 10/42-9/43

11/46-5/47

2/48-5/49

6/49-9/50

Change in 

streambed 

elevation =

change from 

initial

gage height

(meters)

0

+ .030

0

+ .030

-.061

+ .030

0

+ .030

-.030

-.061

-.030

+ .030 

-.030

0 

+ .15

0 

+.030 

0 

.12

.061

.21 

.18 

.061 

0

+ .091

0

+ .27

+ .15

+ .12

+.091

Change in 

streambed

elevation =

change from 

initial 

gage height

(meters)

0

-.27

-.49

-.58

-.95

-1.25

-1.34

-1.37

-1.40

-1.43

-1.49

-1.59

0

-.030

-.061

-.030

-.061

-.21

-.18

-.15

 . 18

-.061

-.091

-.12 

-.15 

-.12

-.15

-.27

-.34

0

+ .21

+ .21

+ .18

-.43

TABLE 14. Changes in streambed elevations as estimated from

streamflow-gaging-station rating tables Continued

, v ve Reference Name of distance discharge

downstream gaging station of station , cubic meters 

and control station from dam ,

(kilometers) Per Sec°nd)

North Canadian River, Oklahoma, Canton D

Year of dam closure 1948

North Canadian River 45 0.00057

near Selling

(control station)

Change in 

streambed 

elevation =

Period change from 

initial

gage height

(meters)

am Continued

10/50-9/51 -0.70

10/51-9/53 -1.01

10/53-5/54 -.88

5/54-9/54 -1.01

7/46-2/48 0

2/48-9/50 -.030

5/51-3/53 -.030

4/53-9/53 0

10/53-9/54 -.27

10/54-5/55 -.27

5/55-9/65 -.40

Red River, Texas-Oklahoma, Denison Dam 

Year of dam closure 1943

Red River near Colbert, 4.5 3.7

Oklahoma

River

Name of ' distance disc 

downstream gaging station of station , j^ me ters 

and control station from dam ^ c ,. (kilometers) per sec°nd)

7/42-10/42 0

11/42-4/44 -.37 

4/44-3/45 -.40 

10/45-6/46 -.88 

6/46-7/47 -1.01

10/47-1/48 -.91

1/48-7/48 -.98 

7/48-9/48 -1.04 

10/48-6/49 -1.16 

10/49-9/51 -1.16

10/52-8/54 -1.13

8/54-3/55 -1.16

3/55-9/55 -1.19

10/55-7/57 -1.13

10/57-8/58 -1.31

8/58-11/58 -1.34

11/58-4/59 -1.28

9/59-2/60 -1.28

2/60-4/60 -1.34

4/60-7/60 -1.37

Change in 

streambed

elevation =

Period change from 

initial 

gage height

(meters)

Red River, Texas-Oklahoma, Denison Dam Continued

Year of dam closure 1943

Red River near 106 4.2

Gainesville, Texas

(control station)

Neches River, Texas, Town Bluff

Year of dam closure 1951

Neches River at Town .5 4.2

Bluff

Village Creek near Kountze 1.5 

(control station)

10/36-5/38 0

5/38-5/40 +.21

5/40-9/40 +.30

10/40-5/41 +.37

4/43-5/43 -.061

10/43-4/44 -.061

6/44-1/46 0

6/46-7/47 +.24

7/47-6/48 +.030

7/48-10/49 +.27

10/49-? +.15 

10/50-5/51 +.30

6/52-6/57 +.091

6/57-11/57 +.37

5/58-? +.73

10/58-5/59 +.70

6/59-11/59 +.95

12/59-5/62 +.70

Dam

3/51-5/52 0

5/52-11/54 -.061

11/54-9/55 -.15 

10/55-9/58 -.37 

10/58-12/59 -.46

12/59-9/62 -.55

10/62-9/63 -.67

10/63-9/70 -.73

10/70-10/71 -.85

10/71- -.95

4/39-12/47 0 

12/47-9/49 -.061

1/51-9/51 -.030

10/51-2/56 0

2/56-11/61 -.061

11/61-9/66 -.15
TABLES 13, 14 83

TABLE 14. Changes in streambed elevations as estimated from

streamflow-gaging-station

River 

Name of distance 

downstream gaging station of station 

and control station from dam 

(kilometers)

Chattahoochee River,

Year of dam

Chattahoochee River near 4.0

Buford

Chestatee River near 73

Dahlonega (control

station)

River 

Name of distance

downstream gaging station of station

(kilometers)

rating tables Continued

Change in

Reference streambed 

,. , elevation =

(cubic meters Perlod chanSe from 

, N initial 

per second) , . ,

gage height 

(meters)

Georgia, Buford Dam

closure 1956

12.2 10/50-9/53 0

10/53-9/55 -.061

10/55-5/57 0

5/57-9/57 -.091

10/57-10/58 -.061

10/58-9/59 -.12

10/59-12/60 -.15

1/61-5/61 -.24

5/61-9/62 -.27

10/62-11/63 -.40

11/63-1/64 -.49

1/64-2/64 -.40

2/64-7/64 -.49

7/64-3/65 -.55

3/65-4/65 -.58

4/65-5/65 -.52

6/65-8/65 -.58 

8/65-10/66 -.67

10/66-9/68 -.76

10/68-1/70 -.88 

1/70-4/71 -.98

3.4 4/40-12/40 0

3/41-12/42 +.15

12/42-3/43 0 

3/43-10/43 -.061

10/43-2/44 -.15

2/44-11/44 -.24

11/44-12/45 -.27

1/47-11/53 -.31

11/53-9/54 -.37

10/54-9/71 -.40

10/71-9/73 -.37

Change in 

 ,- streambed 

*efe elevation -

discharge . , , ,. 

(cubic meters Perlod ^"^lal""1

per Sec°nd) gaslight

(meters)

Rio Grande, New Mexico, Caballo Dam

Year of dam

Rio Grande below Caballo 1.3

Dam

No suitable control

station

Marias River, He 

Year of dam

Marias River near 3.2

Chester

Marias River near 65

Shelby (control

station)

closure 1938

-28,3 2/38-10/38 0

10/38-12/39 0

1/40-9/40 -.061

1943 -.091

1944 -.12

1945 -.15

1946-48 -.46

3/55-12/55 -.40

1957 -.43

1958 -.40

1959-60 -.64

1961-62 -.70

1963-64 -.73

1965 -.70

1966 -.76

1967 -.70

1972 -.67

1974 -.76

1979 -.76

closure 1955

2.8 10/55-9/79 0

4.0 6/48-4/49 0

4/49-4/50 -.030

10/51-6/53 -.061

10/54-? -.15

10/57-9/59 -.21

10/59-9/61 -.24

10/61-9/63 -.27

10/63-6/64 -.21

6/64-3/76 -.18

TABLE 14. Changes in streambed elevations as estimated from

streamflow-gaging-station rating tables Continued

Change in

River _ streambed 

, . Reference , Name of distance . elevation =

downstream gaging station of station . *f Period change from 

and control station from dam <. cublc meters initial

(kilometers) per Sec n gage height 

(meters)

Frenchman Creek, Nebraska, Enders Dam

Year of dam closure 1950

Frenchman Creek near 0.3 - 1.3 2/46-9/48 0

Enders 10/48-1/50 -.061

1/50-9/51 -.15

10/51-9/54 -.12

10/54-1/59 -.18

1/59-9/60 -.21

10/60-4/62 -.27

4/62-9/62 -.30

10/62-9/63 -.34

10/63-4/67 -.37

4/67-5/68 -.43

5/68-9/72 -.46

9/72-10/78 -.49

Mo suitable control

station

 Lowest discharge common to all rating tables.

-The flow exceeded 75 percent of the time.

 The flow exceeded about 85 percent of the time.

 The flow exceeded about 68 percent of the time.

 The flow exceeded about 83 percent of the time.

 In adjacent drainage basin.

 The flow exceeded about 40 percent of the time.

A U.S. GOVERNMENT PRINTING OFFICE: 1985 576-049/20,01 9 REGION NO. 8