FLOW DYNAMICS AND DEPOSIT DISTRIBUTION OF THE DONNER GLACIATION JOKULHLAUP, TRUCKEE, CALIFORNIA

Locations of large granitic boulders were mapped throughout the Upper Truckee River Valley, a region of extensive Pleistocene glacial activity in northeast California, to determine the flow dynamics and the flood path of a jokulhlaup produced during a pre-Tahoe glaciation. The glacier that occupied Squaw Valley during this time dammed up the Truckee River causing the lake levels of Lake Tahoe to rise 182 m. Once the glacial dam was breached a catastrophic jokulhlaup flowed down the narrow Truckee River Valley 13.5 km into the city of Truckee where the canyon widens to 2 km and makes an abrupt 90o turn to the east. Ninety-six boulders, ranging in mass from 3,390 to 4,557,975 kg, with a mean of 197,305 kg, were found and measured throughout the Upper Truckee River Valley in an effort to map the flow path of the jokulhlaup. The study only considered boulders that were Squaw Valley granite (the assumed source rock) in composition, had a tan weathering surface, and that were too large and/or at too high an elevation for a normal flood to carry. After using a handheld GPS unit to precisely determine the locations of the boulders, the path of the jokulhlaup was mapped through the Upper Truckee River Valley and Truckee.

A jokulhlaup is denser than an average flood as it contains a greater amount of boulders and sediment. Thus to calculate the velocity of an event that could carry such large boulders, we used the Chezy Equation (1):

V = Ch * sqrt(ghI) (1)

where V is the average velocity of the flow, Ch is a constant of 5.16, g is the acceleration due to gravity, h is the depth of the flow, and I is the slope of the channel. Using this equation, the velocity of the jokulhlaup before it entered the wider section of the canyon was calculated to be at most 120 kilometers per hour, and the velocity at the precise moment the ice dam broke (assuming the break was instantaneous) was calculated to be from 154 to 218 kilometers per hour.