Sediment Management
| Sediment Transport and Monitoring | |
| Coarse Sediment | |
| Fine Sediment Control | |
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.
Chinook salmon alevin and eggs in clean spawning gravel
Sediment Transport and Monitoring
Video of sediment monitoring - bedload sampler
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[MP4 file - 10mb]
Coarse Sediment
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.
Recent Projects
Gravel Injection Project Completed: May 2009
The Trinity River Restoration Program performed high flow gravel injections at two locations in Lewiston along the Trinity River between April 30 and May 2, 2009. Approximately 2,500 tons of gravel was put into the river at the Diversion Pool site just upstream of the new Lewiston bridge on Trinity Dam Boulevard and 1,000 tons of gravel was put into the river at the California Department of Fish and Game Saw Mill Wildlife Area site at the end of Cemetery Road.
The gravel was being placed into the river to replace gravel that is now trapped behind Trinity Dam. As the gravel moves downstream with the high spring river flows, it creates the juvenile rearing and adult spawning habitat that is crucial to salmon and steelhead.
These injections were accomplished by a truck mounted conveyor system that is able to extend up to 130 feet across the river. The rock conveyed into the river ranged in size from 3/8” to 5”.
Lewiston Spawning Gravel Introduction Project: September 2003
Immediately downstream of the Lewiston dam gravels are depleted as flows move them toward the ocean. This project immediately increased the quality and quantity of salmonid spawning and rearing habitat in the Upper Trinity River by spreading 2,000 cubic yards of gravel over approximately 325 feet of the river’s bed 1.7 miles downstream of Lewiston Dam. Turbidity was monitored during placement according to permit requirements established by the North Coast Regional Water Quality Control Board. Numerous redds were constructed by spawning salmon shortly after the project was completed. The project required only three weeks to implement for immediate spawning use, but also provided long term salmonid benefits by providing river building materials needed to sustain geomorphic processes of river bed scour, transport, and redeposition. This gravel will in turn create natural river channel features (e.g., point and alternate bars) as high flows move it downstream. These river channel features will, along with upcoming channel rehabilitation projects and flow, provide and maintain needed rearing habitat for juvenile salmonid life stages and their food resources.
Fine Sediment Control
What does watershed land use have to do with salmon and steelhead? Fine sediment (sand and silt) delivery from tributary watersheds to the Trinity River increases with roads, logging, and other land uses. These fine sediments accumulate in the Trinity River, and pose a severe threat to the survival and development of salmon and steelhead eggs. By improving land use practices and removing unused logging roads (as shown in the photo below), fine sediment supply to the Trinity River decreases, which increases salmon and steelhead production.
Re-contoured logging road in the Little Grass Valley Creek watershed.
Proposed restoration actions include the following:
- Continue watershed rehabilitation work to reduce fine sediment supply to Trinity River;
- Continue using sedimentation ponds at the mouth of Grass Valley Creek, and improve the maintenance schedule to reduce fine sediment supply to the Trinity River;
- Increase high flow releases to transport fine sediment through the Trinity River;
- Remove sand-laden riparian berms as part of bank rehabilitation sites, and relocate the fine sediment outside the floodway;
- Create “functional” floodplains as part of bank rehabilitation projects such that high flows can scour fine sediment from the low flow channel and deposit it onto floodplain surfaces.