Reduction of Fine Sediment Input: What is Practical and at What Scale?

In the Trinity River basin, northern California, salmon and steelhead populations declined following the construction of dams and diversion of flow. A large-scale restoration program includes goals to augment coarse gravel, reconfigure the floodplain, improve stream temperature regimes, alter stream flows through controlled flow releases, and reduce fine sediment in the mainstem Trinity River. Questions that must be addressed are: what are the current conditions, what is possible to correct, what is practical to correct, and on what scale can restoration be effective? This study assesses restoration effectiveness of one aspect of the program: reduction of fine sediment. To date, techniques used include construction of sediment detention basins and road upgrades or decommissioning. To assess the probable effectiveness of these efforts, the mainstem Trinity River was divided into cells, defined by locations of confluences of large tributaries. In each river cell, fine sediment in mainstem storage was quantified for several sediment mobility categories: active channel bed, gravel bars, floodplain, deltas and riparian berms. Probable fine sediment input from tributaries, primarily from landslides and roads, was compared to the mainstem fine sediment storage volumes and to measured sediment yield from tributaries. For example, in a river cell with 30,000 Mg of fine sediment stored in the active channel bed, the major tributary in that reach delivers about 700 Mg of fine sediment per year. This tributary basin has 160 km of roads with more than 400 stream crossings. The costs to control the fine sediment from this tributary basin may not be justified in terms of making a biologically significant difference in mainstem aquatic habitat quality. In contrast, in other cells, fine sediment input from tributaries is significant when compared with mainstem sediment storage. Transport and storage of fine sediment are dependent on other restoration activities, such as controlled flow releases, and in turn, fine sediment dynamics influence bed mobility and riparian vegetation. Consequently, results from the current study need to be integrated with other aspects of the restoration program on a watershed scale.