What do we know?

What is known about sediment removal from reservoirs following dam removal is based primarily on geomorphic analogies, 5-10 field studies from Wisconsin, Pennsylvania and Oregon with varying degrees of data quality and resolution, and a suite of physical and numerical modeling experiments. These approaches have revealed the following insights:

1. Field studies show that channels respond to dam removal in ways that are at least qualitatively similar to channels responding to other forms of base level lowering.
2. Much of the sediment stored in a reservoir (60-85%) is retained in the reservoir over temporal scales of years to decades.
3. Rate and magnitude of sediment removal from reservoirs is highly dependent on the stratigraphy of reservoir sediment.
4. Groundwater levels in reservoir sediment is a strong control on the rate and magnitude of sediment removal.
5. Headcuts appear to be a prominent mechanism of sediment erosion, as opposed to gradual river incision across the entire length of the reservoir (i.e. a diffusional type of river lowering)

1. What controls the rate and timing of sediment removal from reservoirs following removal? (days to months)
   • What is the role of grain size distribution and stratigraphy in reservoir in controlling the initial release and mechanism of sediment removal from reservoirs?
   • What is the role of timing and method of removal process?
   • What is the role of the geometry of the system?
2. What is the stability of reservoir sediment during and after dam removal? (years to decades)
   • What are the stratigraphic controls on sediment removal from reservoirs?
   • Are there critical geometries of reservoir and channels in controlling sediment removal?
   • Are there hydrologic controls on long-term sediment removal from reservoirs?
3. What controls the ultimate geometry of the channel in the reservoir?
   • Do available models predict hydraulic geometry of channels forming in reservoirs following dam removal?  
   • How do stratigraphy, hydrology, reservoir geometry affect the final channel geometry?
   • Do various management decisions made during the removal process affect the final channel geometry outcome?

A proposed research agenda:

The workshop attendees defined the “handoff” between the upstream system and the downstream system as the timing, magnitude and character of the sediment delivered from the reservoir to the below-dam system. The Upstream Group organized their proposed research agenda in terms of the key variables that may control that sediment delivery:

1. Grain size distribution and stratigraphic variables
   • Non-cohesive: unimodal
   • Non-cohesive: bimodal
   • Mixture of cohesive and non-cohesive (at different portions of fines)
   • Effect of stratigraphic variability
     • Hand-placed mixtures
     • Fluvially-deposited mixtures:
       1. Sand over gravel
       2. Gravel over sand
       3. Lenses/packets of fines
       4. Jurassic Tank?
       5. Constrained from various field sites
2. Hydrologic variables
   • Hydrographic variability
     • Timing of storm relative to stage of removal
     • Constant discharge  
   • Groundwater variability
     • Rapid drawdown of groundwater relative to rate of incision
     • Graduate drawdown of groundwater relative to rate of incision
3. Geometry variables
   • Geometry of reservoir basin
   • Delta geometry
   • Ratio of sediment volume to water volume
   • Initial channel position
4. Management variables
   • Vegetation
     • Types
     • Timing
   • Stone bank stabilization
   • Stepped-riffles
   • Stability of C4 channel in reservoir through time