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Regional Scale Simulation of Water Temperature in the Columbia River Basin

Application of MASS1 Model in a Study of the Lower Snake River

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Geographic extent of the MASS1 model for the Columbia and Snake River Systems (shown as a red line). Click for a larger version.

PNNL is using a PNNL-developed Modular Aquatic Simulation System 1D (MASS1) code to simulate hydrodynamics and water temperature on a regional scale in the Columbia River Basin. The MASS1 modeling tool is a one-dimensional, unsteady hydrodynamic and water quality model for river systems.

In the Lower Snake River, the MASS1 model was used to simulate the water temperature with the dams in place (current impounded conditions) and based on the premise the dams were removed (unimpounded conditions) using historical main stem inflows, tributary inflows, and meteorological conditions for a 35-year period spanning 1960 to 1995. Frequency analysis was performed on the results (saved values of daily average and maximum temperatures) to compare simulated water temperatures for impounded and unimpounded conditions. The unimpounded river warms up faster and has higher mid-summer temperatures than the impounded river, but it cools down more quickly starting in September.

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Median of the daily average water temperatures at Ice Harbor Dam for current impounded conditions (solid line) and for unimpounded (dashed line) conditions. The values were computed from the 35-year MASS1 model simulations. Click for a larger version.

Results and Benefits

The long-term analysis has shown 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 that tends to keep water cooler later into the spring and warmer later into the fall compared to the unimpounded river condition.


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