The objective of the study was to compare water temperatures in the Lower Snake River for current (impounded) and unimpounded conditions using a mathematical model of the river system. A long-term analysis was performed using the MASS1 one-dimensional (1D) hydrodynamic and water quality model. The analysis used historical flows and meteorological conditions for a 35-year period spanning between 1960 and 1995. Frequency analysis was performed on the model results to calculate river temperatures at various percent of time exceeded levels. Results were are also analyzed to compute the time when, during the year, water temperatures rose above or fell below various temperature levels. The long-term analysis showed that the primary difference between the current and unimpounded river scenarios is that the reservoirs decrease the water temperature variability. The reservoirs also create a thermal inertia effect which tends to keep water cooler later into the spring and warmer later into the fall compared to the unimpounded river condition. Given the uncertainties in the simulation model, inflow temperatures, and meteorological conditions the results show only relatively small differences between current and unimpounded absolute river temperatures.
Revised: October 27, 2005 |
Published: February 15, 2001