PNNL

Abstract

Estimates of Chinook salmon (Oncorhynchus tshawytscha) spawning habitat upstream of Grand Coulee Dam were made as part of a phased approach to evaluate the feasibility of reintroducing anadromous salmon to this blocked reach of the Columbia River. A 2-dimensional depth-averaged hydrodynamic model was developed for a 76-km reach between Kettle Falls, Washington, and the U.S.-Canada international border and used to predict water velocities and depths at 10%, 50%, and 90% flow exceedance values that represented historical (1967–2017) high, median, and low river flow, respectively. The outputs from the hydrodynamic model were combined with calculated bed slopes as well as empirical and modeled data on substrate availability to estimate salmon habitat availability. A probabilistic spawning model was developed to estimate salmon spawning probability based on habitat characteristics at the exceedance flow levels as well as three categories of substrate classification. A majority of the highest probability salmon spawning habitat was located between Northport, Washington, and the U.S.-Canada international border. The model predicted 17.6 ha of spawning habitat at the 50% exceedance flow level for areas with pebble and cobble substrates, and estimates considered inter-redd spacing; this equates to a redd capacity in the 76-km study reach varying from 2,893–4,091. Spawner capacity estimates ranged from 5,786–32,728 with adults per redd varying from 2–8, respectively. We conclude that suitable Chinook salmon spawning habitat is available upstream of Grand Coulee Dam. The results of this study will be useful to fisheries managers considering salmon reintroduction and ecosystem function under potential changes to the Columbia River Treaty.