The loss of a natural gravel supply to the Trinity River downstream from Lewiston Dam has been implicated as contributing to decreases in salmonid populations following dam construction. A supply of mobile gravel is necessary to sustain the fluvial processes that create diverse physical habitats that support all salmon life stages, as well as a wide range of other riverine species. Artificial gravel augmentation is among the strategies employed by the Trinity River Restoration Program (TRRP) to recover salmonid populations in the river. However, the rate and manner in which augmented gravel will alter downstream habitats, as well as the potential risks associated with gravel augmentation, are imperfectly known. This report presents the results of site-specific monitoring performed at two sites where gravel augmentations were implemented in water year (WY) 2015, as well as an investigation of photographic methods for assessing system-wide changes in substrate conditions over time.
In 2015, repeat topographic surveys were conducted to assess geomorphic responses to high-flow gravel injections at the Diversion Pool, where 1000 yd3 of gravel was injected during the spring high-flow release, and at Lowden Ranch, where 680 yd3 of gravel was injected. Earlier studies have indicated that most of the gravel injected at the Diversion Pool prior to 2015 was deposited on the inside of a bend a short distance downstream from the injection point.