AHSAHKA, Idaho – The U.S. Army Corps of Engineers’ Walla Walla District issued a final Finding of No Significant Impact (FONSI) for the Corps' “Dworshak Long-Term Nutrient Supplementation Program.”
The Corps plans to implement a long-term nutrient supplementation program at the Dworshak Dam and Reservoir Project on the North Fork of the Clearwater River near Ahsahka, Idaho. The purpose of the program is to enhance the biological productivity of Dworshak reservoir, primarily to improve the kokanee fishery, and to decrease the growth of undesirable blue-green algae.
The Corps started a Dworshak Nutrient Supplementation Pilot Project study in 2007 in conjunction with the Idaho Department of Fish and Game (IDFG) to assess the feasibility of increasing the biological productivity of Dworshak Reservoir and abundance of edible phytoplankton (plant-like plankton) by adding inorganic nitrogen liquid fertilizer. The Corps and IDFG agreed to suspend the pilot study in July 2010, restarted it in 2012, and continued it from spring 2012 to fall 2016. Monitoring data collected during this period indicated the effects of the nutrient supplementation were positive compared with data collected from the reservoir before the addition of the nitrogen. A summary report prepared after the 2015 growing season indicated the amount of edible phytoplankton, zooplankton biomass, primary productivity, and kokanee size increased while the amount of toxin-forming blue-green algae decreased. Drinking water has not been affected by nutrient supplementation.
The North Fork of the Clearwater River is naturally nutrient-poor because of the underlying geology, soils and land use. In the years immediately following Dworshak Dam construction completion in the 1970s, the reservoir initially contained plentiful nutrients from the decomposition of organic material that was inundated by rising reservoir water. However, the amount of nutrients declined after the initial decomposition. The water in the reservoir eventually returned to a nutrient-poor state.
The Corps solicited public comments on its draft FONSI and EA Jan. 27 through Feb. 27, 2017, and all comments submitted were considered. The signed FONSI and EA are available in electronic form on the District’s website https://nww.usace.afpims.mil/Missions/Environmental-Compliance/.
For additional information or questions about the Nutrient Supplementation Program at Dworshak, please contact natural resources staff at 208-476-1255.
Release no. 17-038