Gravel Augmentation

Reason for Action

Spawning Salmon Dams in rivers block more than just water; sediments are also prevented from passing dams.  While the trapping of fine sediments (silt and sand) is generally considered beneficial to fisheries in the Trinity River, trapping of coarse gravels has lead to a deficit in the upper reaches (below the dams).  These coarse gravels are a necessary characteristic of spawning beds, so a gravel deficit leads to a reduction in spawning habitats.  Gravel is also a mobile material, so is important for maintaining geomorphic processes that lead to a dynamic and diverse river channel.

TRRP Gravel Augmentation

Gravel Augmentation TRRP adds gravel to the river  to make up for the deficit caused by the dams. The amount gravel injected into the river is based on scientific analyses and calculation of a gravel budget for the river.  Gravels injected are of a size appropriate for use by spawning salmon.

Augmentation occurs at several locations in the Trinity River above the confluence of Weaver Creek.  Below Weaver Creek, it is thought that tributaries provide sufficient gravel to river processes; gravel may be used at constructed rehabilitation sites for specific purposes, though such gravels generally come from on-site sources such as old dredge-piles. Augmentation may occur during high flow releases or by placement during summer and early fall, typically at rehabilitation sites.

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Gravel Augmentation Gravel injection during high flows at Lowden, spring of 2010.

Gravel Augmentation Resulting gravel bar formed by the injection at Lowden in spring of 2010 (shown at lower summer flows).