The Changing Walla Walla River: A 200 Year Perspective, with Emphasis on Inundation due to the Construction of McNary Dam on the Columbia River

Heidi Van Auken

Department of Geology, Whitman College, Walla Walla, WA 99362

Lewis and Clark were the first white men to take notice of the Walla Walla River in 1805. Missionaries soon followed the explorers, and in 1837, Narcissa and Marcus Whitman set up their mission, harnessing the Walla Walla River to power a grist mill, as well as for watering their crops. Immigrants following the Oregon Trail arrived in waves, and grazing and farming became important activities in the Walla Walla River basin. The small town of Walla Walla grew in size, especially with the discovery of gold in Idaho and northeastern Washington and Oregon. In 1904, a small dam was constructed on the Walla Walla River to supply the city with electricity. When floods ravaged the town in the 1930s, flood control measures in the form of channelization and a diversion reservoir were constructed. The idea for the McNary Dam on the Columbia River was proposed to supply irrigation, power, and navigation to the region.

The lower Walla Walla River was greatly impacted by the construction of the McNary Dam from 1947 to 1953. McNary Dam is located adjacent to Umatilla, Oregon, about 38 km by water southwest of the mouth of the Walla Walla River. The resulting reservoir, Lake Wallula, inundated large areas including about 7 km of the Walla Walla River. This caused many changes, including greater width and depth, slower velocity, different vegetation, and extensive sedimentation in the form of a new delta.

The soils within the Walla Walla watershed are highly erodible; Walla Walla tributaries drain part of the Palouse Hills, which historically have suffered approximately 1 cm per year soil loss. In the area, suspended sediment concentrations as high as 383,000 mg/l have been recorded. With such a high load, the new delta advanced rapidly as the water flowed into stagnant ponds. The delta filled the embayment along the lower Walla Walla River, and extended well into Lake Wallula by 1980, only 27 years after dam completion. The delta continues to grow today; at present, the subaerial portion of the delta extends about 700 m across the 3.25 km wide Lake Wallula; the subaqueous portion, having a low angle of repose, extends even farther across the reservoir.


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