Geomorphic Effects of Dams on Rivers

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Geomorphic Effects of Dams on Rivers

• Gordon Grant

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Hydrologic Regime

• The hydrologic regime
• Geology
• Climate
• Topography
• Changes in flow (Q) due to manipulation
• Direct dams, diversions
• Indirect climate shift, forest practices
• Flows are influenced in magnitude, duration,
  frequency and time
• There are consequences for manipulation

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Hydrograph McKenzie River, OR
Flood control dams
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Run of the River Dam-No Flood Control Capacity
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Sediment Regime

• Much harder to get sediment data than hydrologic
  data
• Use reservoir sedimentation rates Japan surveys
  once a year
• Influenced by other dams
• Landuse history
• Ratio of sediment supply above and below dam
• Sediment budgets

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Channel Response

• Incision occurs immediately downstream of dam
• Bed incises to equilibrium then stops
• Channel narrowing in which peak flows are
  affected
• Loss of secondary channel
• Colonization of bars by vegetation (traps
  sediment)
• Alluvial fans stabilize

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How to bring hydrologic and sediment transport
effects together?

• How is the dam affecting the river?
• Channel response
• Flow and sediment

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Geologic Influence

• Geology plays a major role in hydrology
• High Cascades vs. Western Cascades streams

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Dam Effects

• High cascades
• Very little sediment derived
• Little sediment accumulated behind dams
• W. Cascades
• Deeply dissected
• More sediment accumulation

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Summary

• Dams alter
• Sediment regime
• Hydrologic regime
• River morphology