Water quality ramifications of temporary drawdown of Oregon reservoirs to facilitate juvenile Chinook salmon passage

Abstract Hamilton SK, Murphy CA, Johnson SL, Pollock A. 2022. Water quality ramifications of temporary drawdown of Oregon reservoirs to facitiliate juvenile Chinook salmon passage. Lake Reserv Manage. 38:165–179. Several high-head dams on the Willamette River system in Oregon create reservoirs that support the growth and survival of anadromous salmonids but may impede downstream passage of juvenile fishes, including the dam that creates the Fall Creek Reservoir (also known as Fall Creek Lake). Managers have been conducting brief “deep drawdowns” of Fall Creek Reservoir to promote downstream passage of juvenile threatened Chinook salmon (Oncorhynchus tshawytscha), and to reduce reservoir populations of invasive fish species that prey on or compete with salmon. Over 4   years we studied downstream water quality changes during these drawdowns, which reach the original stream level for short periods in late autumn. In other regions, drawdown of lakes and reservoirs can cause nutrient release and eutrophication, often in conjunction with erosion of within-reservoir sediment and export of fine sediment downstream. In this case, although large amounts of sediment were released at the lowest water levels, there were only modest increases in available nutrient concentrations that were ephemeral and unlikely to cause downstream eutrophication. In addition, although the oxygen demand of the released sediments was not high enough to further reduce dissolved oxygen downstream, very brief (1–2 h) drops in dissolved oxygen occurred immediately downstream as the reservoir water level approached streambed but did not appear to harm fishes. Sedimentation of downstream habitats is of concern, but careful management of the drawdown could mitigate sediment export because most export occurred at the lowest reservoir water levels. Suggestions for predicting and minimizing impacts from reservoir drawdowns are offered. Supplemental data for this article is available online at https://doi.org/10.1080/10402381.2021.2017082 .