Managing cyanobacteria with a water quality control curtain in Iron Gate Reservoir, California

Mejica BN, Ebert DA, Tanaka SK, Deas ML. 2023. Managing cyanobacteria with a water quality control curtain in Iron Gate Reservoir, California. Lake Reserv Manage. XX:XXX-XXX.Iron Gate Reservoir, a eutrophic monomictic reservoir on the Klamath River, a Pacific coast river within the United States, stratifies thermally during summer and is subject to cyanobacteria blooms that produce microcystin toxins, causing public health concerns. An impermeable curtain was installed in Iron Gate Reservoir upstream of the powerhouse intake to control the withdrawal depth and improve water quality of releases to the downstream Klamath River. Thermal stratification minimizes vertical downward movement of cyanobacteria-laden near-surface waters, and the curtain was hypothesized to reduce cyanobacteria and toxin concentrations downstream of the curtain and dam. The Wedderburn number, which indicates unstable (mixed) or stratified conditions, was used to calculate mixed layer depths and define conditions under which the curtain would be likely to be effective. Changes in chlorophyll a, cyanobacteria gene, Microcystis spp. gene, and microcystin concentrations were used to quantify curtain effectiveness for different curtain depths and mixed layer depths. When curtain depth extended below the mixed layer, concentrations of total chlorophyll a, total cyanobacteria, total Microcystis spp., and total microcystin in waters downstream of the curtain were statistically significantly reduced by an average of 87, 90, 78, and 64%, respectively. Concentrations were not consistently reduced downstream of the curtain under other configurations. Findings indicate the curtain can be managed to take advantage of stratification within the epilimnion and address issues pertaining to downstream cyanobacteria and associated toxins, and should improve water quality and reduce public health notices for the 305.7 km of river downstream of Iron Gate Dam.