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 Page Page Abstract ......................... 1 Downstream effects of dams continued Introduction ....................... 1 Mean bed elevation Continued Scope of study ................... 1 Longitudinal-profile changes ............. 26 Study sites and selection criteria ......... 1 Bed material and degradation ................ 29 Acknowledgments .................. 2 Theoretical expectations ................. 29 Methods of analysis and data sources ......... 2 Variations in bed-material sizes with time at a cross sec­ Water discharge .................. 2 tion ........................... 29 Sediment load .................... 2 Variations in bed-material sizes with distance Bed and bank materials .............. 2 downstream ....................... 31 Mean bed elevation ................. 4 Channel width ......................... 31 Measured cross sections ............ 4 General nature of width changes ............ 31 Published graphs of bed-elevation changes . 4 Distance affected ..................... 35 Gage height-discharge records (rating tables) 4 Factors affecting changes in channel width ...... 36 Channel width .................... 5 Alluvial-bank materials ............... 36 Time origin of channel changes .......... 5 Bedrock-bank controls and downstream effects . 37 Vegetation ...................... 5 Water flow ...................... 37 Variability of natural channels Width-depth ratio .................. 39 Time trends of channel widening at a site ...... 39 Downstream effects of dams ................. 7 Time trends of channel narrowing at a site ...... 42 Water discharges ..................... 7 Prediction of post-dam channel-width changes ..... 43 Sediment loads ...................... 8 Role of a dam in effecting a change in channel width . 47 Mean bed elevation .................... 14 Sediment volumes removed and channel equilibrium .... 49 General nature of changes in bed elevation . 14 Vegetation ........................... 50 Degree of change attributable to dams ...... 15 Observed changes in vegetation ............. 50 Degraded reach downstream from a dam ..... 17 Possible causes of vegetation changes ......... 52 General function of degradation with time at a Separating flow regulation from other factors affecting site ....................... 17 vegetation change ................... 54 Maximum degradation and associated time . 22 Effects of vegetation growth .............. 55 Standardized degradation-time plot ...... 23 Conclusions ........................... 56 Length of degraded reach ........... 24 References ........................... 61 Zone of variable bed changes ........... 26 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 kilome­ ters 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 kilome­ ters 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