Application of a 2D hydrodynamic model to design of reach-scale spawning gravel replenishment on the Mokelumne River, California

In-stream chinook salmon (Oncorhynchus tschawytscha) spawning habitat in California's Central Valley has been degraded by minimal gravel recruitment due to river impoundment and historic gravel extraction. In a recent project marking a new direction for spawning habitat rehabilitation, 2450 in 3 of gravel and several boulders were used to craft bars and chutes. To improve the design of future projects, a test was carried out in which a commercial modelling package was used to design and evaluate alternative gravel configurations in relation to the actual pre- and post-project configurations. Tested scenarios included alternate bars, central braid, a combination of alternate bars and a braid, and a flat riffle with uniformly spaced boulders. All runs were compared for their spawning habitat value and for susceptibility to erosion. The flat riffle scenario produced the most total, high, and medium quality habitat, but would yield little habitat under flows deviating from the design discharge. Bar and braid scenarios were highly gravel efficient, with nearly 1 m(2) of habitat per 1 m(3) of gravel added, and yielded large contiguous high quality habitat patches that Were superior to the actual design. At near bankfull flow, negligible sediment entrainment was predicted for any scenario. Copyright (C) 2004 John Wiley Sons, Ltd.