Dworshak Dam: Corps revives Nutrient Supplementation program

Fertilized reservoir reduces harmful blue-green algae, provides more plankton for fish     When the Idaho Department of Fish and Game (IDFG) and U.S. Army Corps of Engineers (Corps) partnered to test whether nutrient restoration can work to benefit fishing and […]

Fertilized reservoir reduces harmful blue-green algae, provides more plankton for fish    

Fertilized reservoir reduces harmful blue-green algae, provides more  plankton for fish

The Walla Walla District supplies and applies the fertilizer by loading it on a work boat and distributing it across the reservoir and has a contract with a limnologist who reviews water quality samples and provides weekly fertilizer application prescriptions. (U.S. Army Corps of Engineers photo)

When the Idaho Department of Fish and Game (IDFG) and U.S. Army Corps of Engineers (Corps) partnered to test whether nutrient restoration can work to benefit fishing and other recreational use on Dworshak Reservoir, it began as a pilot project began in 2007. What started as a pilot project is now bearing fruit. The nutrient project has increased the carrying capacity of the reservoir. There is more edible algae for all organisms to eat, which, translates into more food for fish.

The idea behind adding nutrients to the reservoir is to restore nitrogen (the limiting nutrient) and offset the effects of declining nutrient levels. Excessive amounts are not added, but instead small amounts of nitrogen are added that can readily be used up by organisms low on the food chain. Benefitting organisms low on the food chain provides more food for those higher up the food chain. This eventually should provide more food for Kokanee that, in turn, can be eaten by larger fish like Bull Trout and Smallmouth Bass.

Reservoirs go through a natural aging process after they are created. When a reservoir is first filled it submerges trees, grasses, and other vegetation. The breakdown of this vegetation releases nutrients into the water. The first several years after a reservoir is filled are typically the most nutrient rich conditions in a reservoir. Eventually there will be less vegetation below the high water line to provide nutrients. In Dworshak Reservoir, there is almost no vegetation below the high water line.

As a reservoir ages, eventually, the rivers and streams that flow into a reservoir become the main source of nutrients. Each spring the North Fork Clearwater and other streams flowing into Dworshak provide a nutrient pulse to the reservoir. But, these nutrients only last for a while and nitrogen is typically used up by late-July. Afterwards, nutrients decrease rapidly and reservoir productivity declines. Low reservoir productivity leads to less food for kokanee and other fish.

Kokanee measuring between 10 and 14” with a few fish measuring 17 inches in length populate Dworshak’s Reservoir. (Idaho Dept. of Fish and Wildlife photos)

Kokanee measuring between 10 and 14” with a few fish measuring 17 inches in length populate Dworshak’s Reservoir. (Idaho Dept. of Fish and Wildlife photos)

“We originally planned to evaluate the project over a five year period, reaching a decision by the end of 2011,” said Natural Resource Manager Paul Pence. “ But in 2010, we were required to stop nutrient additions due to changes in permitting requirements. Because it can take several years to begin to see the effects of nutrient restoration on fish populations, we did not have enough information to make a decision at this time. However, we started a second pilot phase in 2012 and continued it through 2016, giving us a full five years of data. ” he said. “After completing the fifth and final year of the pilot phase the evidence continues to show that nutrient restoration has improved the ecology of the reservoir.” 

In 2017 the program was converted from a pilot project to routine reservoir maintenance. DWA took over all operations with limited support from Walla Walla District. The Corps partnered with the Idaho of Fish and Game. The Corps supplies and applies the fertilizer and has a contract with a limnologist who reviews water quality samples and provides weekly fertilizer application prescriptions. Also, the Corps provides data reporting, lab analysis, water sampling materials and boat fuel for IDFG while IDFG provides monthly water quality sampling and reviews our data.
It has led to a reduction in harmful bluegreen algae, coupled with an increase in beneficial algae. This change in algae means that there has been more plankton for fish to eat, safer water for recreation, and essentially no change in water clarity. Based on these results, we are currently laying the groundwork for long-term implementation while we continue to monitor and evaluate the project, he added.

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