Mean Velocities and Boulder Transport During Tahoe–Age Floods of the Truckee River, California–Nevada

In Truckee River glacial outwash deposits of the Tahoe Glaciation, boulders and gravel bars indicate discharges and velocities far greater than those of the present river. Collapse of unstable glacier dams near Lake Tahoe's outlet would have been necessary to release quickly the large volumes of impounded water that produced the above features. A mean flood velocity, using the Manning formula, near 30 feet per second, probably was attained along the valley as far downstream as Verdi, Nevada. A velocity of slightly more than 10 feet per second was possible farther downstream. A reasonable tractive force can be estimated for the movement of most of the observed boulders. It is concluded that a tractive force of from 20 to 30 pounds per square foot was sufficient to move the largest boulder, measuring 40 × 20 × 10 feet above ground surface, in a 40-to 80-foot-deep flood moving nearly 30 feet per second on a slope of 0.007.