Hoover Dam – Colorado River

Reasons for Dams

• Flood Control

39,000 dams worldwide higher than 15 m (ICOLD, 1988)

Reasons for Dams

• Flood Control

• Irrigation

Reasons for Dams

• Flood Control

• Irrigation

• Power Generation

Reasons for Dams

• Flood Control

• Irrigation

• Power Generation

• Recreation

Reasons for Dams

• Flood Control

• Irrigation

• Power Generation

• Recreation

• Navigation

Impacts of Dams

- Cross section of a graded stream and tributary in equilibrium

- Dam creates reservoir for hydropower and flood control

- What happens?

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

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

Change in Flow Below Trinity Dam, CA

Average Annual Peak Discharge

0

1000

2000

3000

4000

5000

6000

7000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41

Dam

Dis

char

ge

(cm

s)

Pre-dam Post-damData from Williams and Wolman (1984)

Reduction in Sediment SupplyHoover Dam – Colorado River

Fro

m W

illia

ms

and

Wol

man

(19

84)

Reduction in Sediment Supply

Gavins Pt. Dam – Missouri River

From Williams and Wolman (1984)

From Williams and Wolman (1984)

Channel Narrowing

Dow

nst

ream

of

Bla

ck B

utte

Dam

, C

A

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

How Mitigate for These ChannelAdjustments and Habitat Losses?

Dam Mitigation

• Dam removal – 500 dams removed in U.S.

 

  

CNN – 7/19/2000:

White House gives thumbs down on removal of Pacific

Northwest dams

-Sediment accumulates behind dam limiting its water retention capacity

Impacts of Dam Removal

-Dam is removed-What happens?

-Erosion of sediment accumulated in reservoir is redeposited downstream

Impacts of Dam Removal

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

Issues Surrounding Removal

• Sediment management

• Release of contaminants

• Loss of benefits of dams

• Loss of established (nonnative) ecosystem

• Historic preservation

• Potential to damage infrastructure

Dam Mitigation

• Dam removal

• Provide fish passage – fish ladders

Dam Mitigation

• Dam removal

• Provide fish passage – fish ladders

• Sediment management downstream

Rhi

ne R

iver

- G

erm

any

Adding Gravel Below Dam

Dam Mitigation

• Dam removal

• Sediment management downstream

• Provide fish passage – fish ladders

• Artificial flood releases

Unaltered River and Floodplain

Levees

Breached Levees

Levelling Levees

Breached Levees

Overtopping Levees

Setback Levees

Buyouts

S. Fork Nooksack River , WA