The sediment that makes up the bed and banks of the Trinity River ranges in size from silt and sand to gravel, cobbles and boulders. Since 1964, the dams have trapped all sediment that was normally delivered to the lower river from the upper watershed. Additionally, infrequent high flows of the past few decades have allowed sediment supplied from the tributaries in the first 17 miles below the dams to accumulate in deltas rather than be transported and distributed downstream as historically occurred prior to the TRD. Overall, the changes in the coarse sediment regime have reduced the natural gravel transport processes, reduced gravel bar deposits, and reduced salmon spawning and rearing habitat.
The overall restoration strategy for managing coarse sediment is to increase coarse sediment storage in the river (gravel/cobble bars), improve coarse sediment transport (distribute gravel/cobble downstream), and restore a balance between coarse sediment supply and coarse sediment transport using high flows and mechanical gravel introduction. If the river were considered a conveyor belt periodically transporting and depositing coarse sediment, the proposed restoration strategy would add coarse sediment at the upstream end of the conveyor belt at a rate equal to what the conveyor is moving, such that accumulations or deficits (channel incision and armoring) are minimized. This strategy can be referred to as restoring the coarse sediment budget.
Specific recommendations consist of the following:
- release flows up to 11,000 cfs to transport and distribute cobbles and gravels from tributary deltas to create bars, riffles, and floodplains in downstream reaches;
- replenish cobble and gravel storage in the reach immediately downstream of Lewiston Dam that is most impacted by the loss of upstream coarse sediment supply;
- introduce long-term periodic gravel and cobble supply in first fifteen miles below Lewiston Dam at a rate equal to that transported by high flow releases to maintain cobble and gravel storage, enabling the river to create and maintain complex instream habitat.
Recommended flow releases will be larger in wetter water years than drier water years. Therefore, larger volumes of cobble and gravel will need to be introduced immediately downstream of Lewiston Dam during wetter years. Based on preliminary sediment transport computations, the following estimates of gravel and cobble introduction needs for each water year are as follows:
|Water Year Type||Annual Introduction Volume|
|Extremely Wet||31,000-67,000 cubic yards|
|Wet||10,000-18,000 cubic yards|
|Normal||1,800-2,200 cubic yards|
|Dry||150-250 cubic yards|
|Critically Dry||0 cubic yards|
Actual introduction rates for a given year would be based on sediment transport estimates specific for the high flow releases for that year.