The loss of the natural flow regime due to dams in northern California watersheds can result in lower algal abundance through the summer, supporting fewer macroinvertebrates, especially mobile grazers, which make up a disproportionate part of fish diets. The salmonid populations of the Eel River are directly impacted by the Potter Valley Project, a dam and diversion system on its upper mainstem. Flow releases from the dams exhibit a steep receding limb in spring and summer with lower base flows compared to modeled unimpaired flows. Yet, the effects of the dams on the greater ecosystem are not known. As the Potter Valley Project comes up for relicensing in 2022, it is critical for management to examine possible effects of the current flow regime on the downstream environment. This project is a comparative observational study of the algal and benthic macroinvertebrate communities of the mainstem Eel River below Cape Horn Dam in a dry (2015) and wet (2017) year as well as relative to unregulated rivers within the same watershed. The percent cover of algae and relative abundance of macroinvertebrate taxa were measured at one site directly below the dam and another further downstream with similarly located sites on the unregulated river.
Results/Conclusions
In 2015, the hydrograph showed only a single scouring flood event and flows dropping below 200 cfs by early spring. The algae community was dominated by Cladophora with a high abundance of diatoms in the early summer then transitioning to Spirogyra with fewer diatoms present. On average, there were 20 families of aquatic macroinvertebrates present. Summer composition (in terms of % relative abundance) was dominated by filtering collectors (33%) such as the net spinning Hydropsychidae and gathering collectors (41%) such as Elmidae. Cladophora mats were also smaller in size and less dense than those found on the unregulated South Fork Eel River with fewer grazers (8%). This pattern suggests the ecosystem of the mainstem Eel River in a dry year is heterotrophic and relying heavily on upstream and terrestrial resources.
The wet 2017 water year had several scouring floods along with late season spates keeping flows above 700 cfs through the first half of spring. Preliminary data suggests the growth cycle of algae has been delayed with lower abundances of many macroinvertebrate taxa, especially grazers. With sampling sites on the unregulated Middle Fork Eel River in 2017, shifts in the food web due to flow alterations were further explored.