Projects & Programs
Temperature and Water Quality Modeling of the Lower Deschutes River, Oregon
PNNL developed hydrodynamic and temperature mass balance models and water quality models for Lake Billy Chinook, Oregon on the main stem of the Deschutes River and applied to provide a preliminary evaluation of various fish passage enhancement alternatives for the Pelton Round Butte Hydroelectric Project. Study results showed that surface/selective withdrawal operation provided major benefits. Blending of surface and bottom waters would allow alteration of outlet temperatures such that compliance with temperature criteria immediately downstream could be achieved. The surface withdrawal phase of operation would also enhance surface currents and fish motion towards the collectors. The impact of the proposed modifications on water quality parameters such as dissolved oxygen (DO), pH and algal biomass in the waterbodies downstream was not clear. Additionally, it was uncertain how the lower Deschutes River would respond to temperature; that is, how far downstream temperature changes would extend.
The objective of this study was to develop a predictive water quality model of the lower Deschutes River to confirm that water quality in the river would not be adversely affected due to upstream alterations. This was accomplished using the finite element models RMA-2 and RMA-4q and the available data collected by Portland General Electric. The water quality model included the periphyton as the primary driver of water quality variations. Model calibration was done using 1999 water column data, while verification was done with 1997 water column data. The calibrated model was then used to evaluate the effect of various reservoir modifications on reservoir and downstream water quality, particularly blending of surface and bottom waters of Lake Billy Chinook.
Preliminary results indicate that temperature changes extend to the mouth of the river since residence time is less than one day. These same results also indicate no adverse impact on DO and only slight increases in pH. Since the residence time is very short, nutrient changes have little effect on water column algae growth. Changes in DO and pH are primarily caused by changes in periphyton growth.
Project Highlights:
- PNNL developed hydrodynamic and mass balance models on the main stem of the Deschutes River
- Water quality modeling was accomplished using RMA-2 and RMA-4q finite element models, including the periphyton as the primary driver of water quality variations
- PNNL calibrated a model used to evaluate the effect of reservoir modifications, particularly the blending of surface and bottom waters of Lake Billy Chinook
- The study showed significant benefits to temperature and DO and a relatively minor increase in pH due to proposed upstream modifications.