1. Hoover Dam – Colorado River

4. Reasons for Dams Flood Control 39,000 dams worldwide higher than 15 m (ICOLD, 1988)

6. Reasons for Dams Flood Control Irrigation

8. Reasons for Dams Flood Control Irrigation Power Generation

10. Reasons for Dams Flood Control Irrigation Power Generation Recreation

12. Reasons for Dams Flood Control Irrigation Power Generation Recreation Navigation

14. Impacts of Dams - Cross section of a graded stream and tributary in equilibrium - Dam creates reservoir for hydropower and flood control - What happens?

15. If sediment supply from tributary is low: - Deposition in reservoir and further upstream due to rise in base level and lowered slope - Preferential erosion downstream due to decreased load

16. If sediment supply from tributary is high: - Deposition in reservoir and further upstream due to rise in base level - Deposition downstream due to decreased capacity of peak discharges – insufficient to remove tributary inputs

17. Change in Flow Below Trinity Dam, CA

18. Data from Williams and Wolman (1984)

19. Reduction in Sediment Supply Hoover Dam – Colorado River From Williams and Wolman (1984)

20. Reduction in Sediment Supply Gavins Pt. Dam – Missouri River From Williams and Wolman (1984)

22. Channel Narrowing Downstream of Black Butte Dam, CA

28. Habitat Impacts of Dams Lower flows Migration barriers – block fish passage New ecosystems develop in reservoirs behind dams – out compete native species Channel incision downstream – armoring Beach erosion

29. How Mitigate for These Channel Adjustments and Habitat Losses?

30. Dam Mitigation Dam removal – 500 dams removed in U.S.

37. White House gives thumbs down on removal of Pacific Northwest dams CNN – 7/19/2000: White House gives thumbs down on removal of Pacific Northwest dams

38. -Sediment accumulates behind dam limiting its water retention capacity Impacts of Dam Removal -Dam is removed -What happens?

39. -Erosion of sediment accumulated in reservoir is redeposited downstream Impacts of Dam Removal

40. Habitat Impacts of Aggradation Shallower Flows –Increased water temperatures Pools Filled –Loss of cover and resting habitat Increased Fines –Lower egg survival and fry emergence –Loss of juvenile cover Destabilizes Banks –Bank erosion

41. Issues Surrounding Removal Sediment management Release of contaminants Loss of benefits of dams Loss of established (nonnative) ecosystem Historic preservation Potential to damage infrastructure

43. Dam Mitigation Dam removal Provide fish passage – fish ladders

49. Dam Mitigation Dam removal Provide fish passage – fish ladders Sediment management downstream

50. Rhine River - Germany Adding Gravel Below Dam

51. Dam Mitigation Dam removal Sediment management downstream Provide fish passage – fish ladders Artificial flood releases

55. Unaltered River and Floodplain

61. Levees

62. Breached Levees

68. Levelling Levees

69. Breached Levees

71. Overtopping Levees

73. Setback Levees

75. Buyouts

76. S. Fork Nooksack River, WA