Reservoir Project Overview

Study Background
The majority of the 13 Willamette Valley Project (WVP) dams and reservoirs in Willamette Basin are located on rivers with anadromous salmonids (NOAA Fisheries 2008). The WVP Biological Opinion (BiOp) requires improvements to operations and structures to reduce impacts on Upper Willamette River (UWR) Chinook Salmon (Oncorhynchus tshawytscha), listed under the Endangered Species Act (ESA). Recent efforts to minimize the effects of dams and reservoirs on these salmonids has resulted in both new infrastructure (e.g. fish passage, temperature control) as well as changes in reservoir management operations, including outflow and increased drawdown (USACE 2012; Reasonable and Prudent Alternative measures 4.8, 4.11, 4.12.1, 9.3).

There are limited data that can be used to predict the possible implications of modified drawdown regimes on reservoir food web, productivity and nutrient dynamics, but substantial tradeoffs might be expected (ISAB 2011). We are working to quantify the extent to which changes in food and nutrient resources due to these modified drawdowns are likely to propagate through the food web, influencing the ability of the system to support survival and growth of listed salmonids.

The configuration of WVP dams in concert with reservoir lengths and shoreline complexity proves a challenging environment to provide downstream fish passage for all salmon life history stages present in a reservoir. These WVP reservoirs are annually drawn down to conservation pools during winter months to provide benefits from flood damage reduction and refilled during spring. To improve passage for juvenile Chinook Salmon, deep drawdown to the streambed and elimination of the conservation pool began at Fall Creek Reservoir during the winter of 2011.

An additional proposed benefit of deep drawdown includes displacement of introduced fish species that are predators of Chinook Salmon. However, deep drawdowns also lead to export of sediment (Schenk and Bragg 2014), nutrients bound to the sediment and zooplankton (Larson 2000). With these exports of nutrients, plankton and fish, there is the potential for major modifications to the reservoir Chinook Salmon environment upon refilling. A better understanding of the long-term effects of modified draw-downs, such as that occurring at Fall Creek Reservoir, on juvenile Chinook Salmon growth and survival will aid development of best management practices for WVP reservoirs.

In spring 2013, we started a project that evaluates if deep or extended reservoir drawdowns initiate cascading effects on reservoir conditions, aquatic food webs and productivity. We are quantifying reservoir nutrient availability, Chinook Salmon prey availability, and food web dynamics in several upper Willamette Basin reservoirs with differing drawdown regimes. This project will inform managers about possible direct and indirect implications of seasonal drawdowns on habitat availability, rearing, and survival of juvenile Chinook Salmon. Improving the understanding of potential tradeoffs of management activities is crucial for evaluating the success of extended drawdowns as a management practice in reservoirs.

In fall 2018, we started a project to identify the location of the infectious stage (copepodid) gill-maggot parasite Salmincola californiensis. There is concern that the survival of reservoir rearing juvenile Chinook Salmon is being limited by heavy infection burdens. We are working to model infection and linkages to ecological conditions that may reveal operational methods of control.

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