Hydrology, Hydraulics, and Geomorphology of the Bonneville Flood

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Abstract
Approximately 14,500 yr ago, Pleistocene Lake Bonneville discharged 4,750 km3 of water over the divide between the closed Bonneville basin and the watershed of the Snake River. The resulting flood, released near Red Rock Pass, Idaho, followed the present courses of Marsh Creek, the Portneuf River, and the Snake and Columbia Rivers before reaching the Pacific Ocean. For 1,100 km between Red Rock Pass and Lewiston, Idaho, the Bonneville Flood left a spectacular array of flood features that have allowed for geologic reconstruction and quantitative evaluation of many aspects of the flood hydrology, hydraulics, erosion, and sediment transport. Geologic evidence of maximum flood stages in conjunction with step-backwater flow modeling has furnished estimates of peak discharge and local hydraulic flow conditions for 10 separate reaches along the flood route. Peak discharge was approximately 1.0 million m3·sec−1 at the Lake Bonneville outlet near Red Rock Pass. Downstream, near Lewiston, Idaho, the maximum discharge attenuated to 0.57 to 0.62 million m3·sec−1. In part, attenuation of the peak discharge was probably the result of flow storage in the wide alluvial valleys of the western Snake River Plain. Diverse geologic and geomorphic environments along the flood route resulted in large spatial variations in the channel geometry as the flood moved downstream. Consequently, the local hydraulic conditions (flow depth, velocity, boundary shear stress) of the Bonneville Flood varied substantially within and between the study reaches. The rate of energy expenditure was also highly varied; local calculated stream power magnitudes ranged from less than 10 watts·m−2 in ponded reaches to more than 100,000 watts·m−2 in major constrictions. More than 50% of the total energy loss at peak discharge was expended over a distance that encompassed less than 6% of the flood route. These spatial variations in local hydraulic conditions were profoundly important in controlling the distribution of flood processes and the resulting flood features. The deposition of tractively transported cobbles and boulders (measured clast diameters ranged from less than 10 cm to more than 10 m) were deposited in reaches of decreasing flow competence within quantitatively definable limits. The hydraulic conditions at areas of erosion were more difficult to precisely evaluate; however, erosion of basalt bedrock was primarily in reaches that had stream power magnitudes that exceeded 20,000 watts·m−2. Cavitation was probably an important erosional process in areas of most intense flow conditions.

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