Paleodischarge of the late Pleistocene Bonneville Flood, Snake River, Idaho, computed from new evidence
The Bonneville Flood resulted from catastrophic outflow from Pleistocene Lake Bonneville about 15,000 yr ago, when the lake overtopped its rim at Red Rock Pass in southeastern Idaho and discharged a vast volume of water down the Snake River. This paper provides revised estimates of the paleodischarge, volume, and duration of the Bonneville Flood, based on new evidence of its height and on current understanding of the amount of lowering of Lake Bonneville. Evidence for the revised height of the flood is derived from the altitude of erosional features and flood deposits at the head of a constricted reach of the Snake River Canyon at the mouth of Sinker Creek and from the altitudes of flood deposits at several places about 53 km upstream. Using the step-backwater method, we estimate that peak discharge for the Bonneville Flood through the constricted reach was from 793,000 to 1,020,000 m 3 /s and most likely was 935,000 m 3 /s. This discharge is 2.2 times the discharge previously reported and is the second largest flood known to have occurred in the world. At this rate of discharge, the shear stress for the flood would have been 2,500 N/m 2 , and the unit stream power would have been 75,000 N/m/s, as compared with values of 6 to 10 N/m 2 and 12 N/m/s for recent floods on the Mississippi and the Amazon. Other recent studies of the history of Lake Bonneville show that the volume of water released was 4,700 km 3 , or about 3 times greater than the volume previously inferred. Although this volume indicates a flood duration of 8 weeks at constant peak discharge, an accurate estimate of the duration would require dam-break modeling at Red Rock Pass. From a dam-break model, flood hydrographs at Red Rock Pass and the hydraulics of the flood wave along the Snake River could be computed.