Dams, drought, fog, and fish [electronic resource] : an analysis of hydrologic and geomorphic factors affecting salmonids
Keywords: Civil and Environmental Engineering.

The majority of the work presented here is motivated by the widespread decline and Endangered Species Act (ESA) listing of anadromous salmonid stocks in the western United States. Common factors limiting the abundance of anadromous salmonids include man-made barriers (i.e., dams) to migration and inhospitable low-water conditions during the precipitation-limited dry season. Geomorphic and hydrologic issues related to the health and recovery of these stocks are addressed in the studies included here. The erosion and fate of exposed sediment deposits within reservoirs following dam removal is often the most contentious issue for removal projects. To assess how sediment deposits evolve and erode following small dam (< 15 m) removal, a database of recent removal projects detailing erosion rates and volumes was developed. Empirical relationships between the fraction of deposit material eroded and descriptive site data are evaluated. Results indicate that, in general, sites with deposits composed of primarily fine material (often cohesive), staged removals (progressive dam lowering over time), low bed slopes, or high deposit width to incoming channel width ratios tend to retain a significant proportion of deposit material (> 80%). Conversely, deposits with non-staged removals, or high bed slopes and low deposit width to channel width ratios tend to release a significant proportion of deposit material (30-65%). Watershed sediment yield and upstream channel characteristics also provide good predictions of deposit erosion volumes. Combined, these results can be used to produce estimates of erosion characteristics of future small dam removals. As flows recede over the typically precipitation-free West Coast dry season, available aquatic habitat within streams decreases, thermal capacities decline, stream temperatures increase, and dissolved oxygen content decreases. All of these factors, along with multiple other secondary effects, negatively impact cold-water salmonids. To evaluate if climate-change induced trends in dry-season low flows and rates of recession exist in rain-dominated West Coast watersheds, discharge time-series of minimally-developed basins in the region were analyzed. Of the 57 gages analyzed, 47% have trends in summer low flows and rates of recession, 81% of which correspond to decreasing summer streamflows. Few trends in springtime conditions are identified, indicating that decreasing late summer flow trends are primarily due to increased rates of summertime baseflow recession. A second low-flow oriented study was conducted to evaluate the use of readily-available remotely-sensed and digital data as predictors of dry-season flow variability within a region. The study included 33 headwater catchments within the Santa Cruz Mountains of California. Results indicate that such data are well suited to provide indications of dry-season flow variability within a defined region. Within the study area, indices of vegetation and dry-season climate (potential evapotranspiration and marine fog occurrence) provide the best indicators of dry-season flow conditions. Marine fog plays a particularly prominent role in vegetation distribution and dry-season flow conditions. Fog tends to reduce potential evapotranspiration, promote conifers (coast redwood and Douglas fir), reduce rates of baseflow recession, increase dry-season discharge, and promote perennial flow in coastal watersheds. To better quantify the effects of fog on dry-season hydrologic budgets, a final field study was conducted in the Santa Cruz Mountains to measure fog, fog drip, soil moisture, and ultimately the impacts of fog on dry-season streamflow. A unique dataset documenting fog drip, infiltration, and streamflow fluctuations due to fog events is presented. Fog-drip totals of up to 961 mm were recorded over a period 77 days. In addition to a general decrease in the rate of baseflow recession with increasing fog cover, individual evapotranspiration-reducing and fog-drip-producing marine layer events are shown to coincide with increases in streamflow of up to 200%. Such increases in flowrates, along with concurrent reductions in daily maximum and range of streamflow temperatures, indicate that in some coastal watersheds marine fog promotes stream conditions beneficial to salmonids.