Reservoir Delta Morphodynamics and Implications For Fish Passage In The Yakima River Basin

Reservoir deltas form at the confluence of a stream with a reservoir pool, which acts as a base level for each stream segment. These delta surfaces are associated with deposition, especially large sediment clasts, and can be highly dynamic during flood events with considerable changes in channel location and form. While the formation of deltas within lakes is a natural process, deltas in reservoirs are also subjected to large fluctuations in the managed reservoir pool water level. In the Yakima River basin in the state of Washington, USA, the Endangered Species Act (ESA)-listed bull trout and other salmonids are prevented or impaired from migrating between the reservoirs and upstream spawning habitat because of the delta morphology and associated hydrology of the tributary and reservoir pool. These fish passage issues include partial or complete dewatering of the delta stream channels, wide channels with insufficient flow depths for fish passage, lack of channel cover leading to increased predation, and channels flowing over bedrock ledges creating large hydraulic drops that fish are unable to jump over. In the western United States, where climatic variability is common, these conditions are even more pronounced during dry years. Current efforts to promote fish passage are on an annual basis in the Yakima basin, but the present solutions are labor intensive and unsustainable.

This study had two main objectives: 1) to develop a conceptual understanding of the geomorphic processes that contribute to reservoir delta evolution, and 2) quantify the specific conditions at select reservoir deltas across five Yakima River basin reservoirs (Keechelus, Kachess, Cle Elum, Bumping, Rimrock). Two types of delta tributaries were identified and are the focus of the research: mainstem alluvial streams that enter the reservoir at the upstream extent, and lateral tributaries that flow into to the reservoir but are not aligned with the downstream valley geometry. Using a combination of historical elevation datasets and newly collected field data, several topographic analyses were conducted to inform the conceptual model of reservoir delta fish passage at the tributary locations. Analyses included evaluating the evolution of long profiles through time, characteristic channel depths, sedimentation patterns, and inundation dynamics across each of the deltas. All have important implications for fish passage. Results indicate the importance of event-based understanding of reservoir delta surface dynamics where changes in channel conditions for fish passage can be improved or hindered. Surfaces were also found to be relatively stable over other longer periods of time as well. Results from this study will be used to inform future modeling efforts to develop and test longterm solutions to fish passage.