A WATER QUALITY MODEL OF THE SPOKANE RIVER-LONG LAKE SYSTEM USING CE-QUAL-W2

A water quality model of the Upper Spokane River system was developed by Portland State University for the Washington Department of Ecology. The model will be used to develop Total Maximum Daily Loads (TMDL). The Upper Spokane River consists of multiple river and reservoir sections and is located in the Northeastern part of Washington State. The modeled section stretches from the Washington-Idaho Stateline at River mile (RM) 96.0 downstream to Long Lake dam at RM 32.5. The goals of the modeling effort were to gather data to construct a computer simulation model of the Spokane River system including Long Lake Reservoir and the pools behind Nine Mile dam, Upper Falls dam and Upriver dam; and to ensure that the model accurately represents the system hydrodynamics and water quality (flow, temperature, dissolved oxygen, phytoplankton, periphyton and nutrient dynamics). The hydrodynamic and water quality model CE-QUAL-W2 Version 3 was used to model the Spokane River system. CE-QUAL-W2 is a two dimensional (longitudinal-vertical), laterally averaged, hydrodynamic and water quality model that has been under development by the Corps of Engineers Waterways Experiments Station. The Upper Spokane model grid consists of 187 longitudinal segments and 47 vertical layers that are divided into 12 branches and 4 water bodies. Included in the model are all major dischargers and tributaries. The ability to simulate multiple CBOD compartments in CE-QUAL-W2 permitted the simulation of discharger specific CBOD compartments and decay rates. The Upper Spokane River has significant gains and losses to the underlying aquifer that were simulated using distributed tributaries. In general, the model reproduces the river and reservoir responses to the known boundary conditions. Root mean square (RMS) error of temperature predictions was typically less than 1 degree Celsius. The development of a periphyton model was essential in calibrating dissolved oxygen, pH, and nutrients. RMS error of dissolved oxygen and pH predictions averaged 1 mg/1 and 0.45, respectively. The model is well suited for evaluating the impacts of management strategies to improve water quality in the Spokane River Long Lake region.