PALEOFLOW VELOCITES OF CATASTROPHIC FLOODING ON THE CLARKS FORK OF THE YELLOWSTONE RIVER, NORTHWESTERN WYOMING

A boulder-strewn outwash fan at the mouth of the Clarks Fork River in northwestern Wyoming records multiple catastrophic floods from glacially dammed lakes during the Pinedale and Bull Lake glaciations. These lakes formed several kilometers up valley in tributaries dammed by the Clarks Fork and Crandall glaciers of the Yellowstone Ice Sheet (Carson et al., 1996). The vast majority of the deposits on the outwash plain are composed of granite and granitic gneiss boulders, sourced from exposed Precambrian basement up valley and along the southern edge of the Beartooth Mountains.

The goal of this study was to reconstruct paleoflow conditions based on boulder diameter. A study completed several decades ago (Ballard, 1976) provides a useful comparison, although GIS offers new analytical tools and techniques. The dimensions of 303 boulders were measured in the field. The largest recorded diameter was 3.37 m and, on a separate boulder, the largest recorded circumference 17.5 m. Following techniques of Costa (1983) and O’Connor (1993), the maximum velocity of the boulders was found to be 7.89 m/s and 9.52 m/s respectively. Ballard (1976) calculated a maximum velocity of 9.14 m/s; O’Connor calculated higher velocities for the Bonneville Flood of Idaho. After an analysis of the four terrace systems through GIS, it was determined that only the uppermost terrace (and presumably oldest) has not been significantly reworked. An analysis of the upper terrace system shows a trend of noticeably decreasing boulder dimensions with increasing distance from the terminal moraine, with estimated velocities varying from 9.52 m/s proximal the moraine to 4.3 m/s 9.5 km downstream from the moraine. These results, using boulder dimensions, are similar to previously calculated velocities of 9.14 m/s for the Clarks Fork site (Ballard 1976), but less than the calculated velocities of 14.3 m/s for the Bonneville Flood of Idaho (O’Connor 1993) and using O’Connor’s equation, 19 m/s for the lakes of the Altai Mountains in Siberia (Herget 2005) due to significant discharge differences of the considerably larger glacial lakes of Bonneville and the Altai Mountains.