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36 LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COLORADO FIGURE 27.-The Roaring River valley at river mile 1.14 (aerial); down-valley is to the right. From the left edge of the photo. the channel is scoured to bedrock .and very steep; 8 wide depositional reach is at a mountain meadow; a short. steep-scoured reach is defined by the slope failures on the left bank; and, finally. a flatter depositional reach extends off the right edge of photo. determined by surveying the height of the deposit above adj acent high-water marks; triangulation of sediment thickness was determined from the slope of the original ground surface beneath the sediments, which produced a maximum thickness of 44 ft in a very small area near the head of the fan. Average thickness of the alluvial fan is only 5.3 ft. A contour map of the sediment thickness of the Roaring River alluvial fan is shown in figure 36. Volume of sediment in the alluvial fan was calculated two ways; both ways produced approximately the same volume. First, the area between the contour lines shown in figure 36 was measured and multiplied by the average thickness of the bounding contours. Second, a Theissen- Polygon method using the 14 data points was used. The calculated sediment volume, assuming 1/3 porosity, is 364,600 yd', which is enough sediment to cover a foot- ball field to a height of 205 ft. If a specific gravity of 2.7 is assumed for the granite-gueiss sediments, the unit weight is 168.5 Ibs/ft3. This produced a sediment weight of 829,000 tons, which is approximately 1 y, times the sediment load deposited at the mouth of the Mississippi River in 1 day (Milliman and Meade, 1983). Characteristics of the surface sediments on the fuJaring River alluvial fan changed in a downstream direction. The size of the single largest particle in a 3-ft radius, every 50 ft down the maj or flow axis of the fan, is plotted in figure 37. The particle size changed from 7.5 ft at the head of the fan to 1 ft midway down the fan, where the largest concentrations of big boulders ended, to 0.002 ft at the toe of the fan, overlapping the flood plain of the Fall River. Winter winds during 1982-83 subsequently eroded some of the flood-plain sand and silt, which collected against U.S. Highway 34 road em- bankment (fig. 1) as dunes with heights of 2 to 3 ft. The single largest boulder known to have been transported onto the surface of the fuJaring River alluvial fan is 14X17.5X21 ft, and weighs an estimated 452 tons (fig. 38). The force of the boulder-charged flow over Horseshoe Falls battered and destroyed most of the vegetation along tile main flow path. Some trees still standing after the flood were struck so hard by trans- ported boulders that their tops were snapped off. The large volume of sediment deposited in the Roar- ing River alluvial fan dammed the Fall River in Horse- shoe Park upstream from the fan, forming a small lake (fig. 35). By July 1983, the lake had established an equilibrium between inflow of the Fall River upstream and outflow along a marginal channel that formed at