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<br />~ <br /> <br />GEOMORPHIC EFFECTS OF THE FLOOD <br /> <br />45 <br /> <br /> <br />FIGURE 42.-The Fall River channel about 0.3 mile below Cascade Lake dam, showing extensive channel scour. View <br />is upstream. <br /> <br />FALL RIVER: CASCADE LAKE DAM <br />TO ESTES PARK POWERPLANT <br /> <br />The distance from the Cascade Lake dam to the Estes <br />Park powerplant is about 0.9 mi, and the Fall River drops <br />about 400 ft; average slope is about 8 percent. The slope <br />of the Fall River steepens abruptly below the Cascade <br />Lake dam (fig. 2), because the Fall River flows over the <br />Pleistocene terminal moraines that dammed the river in <br />glacial times. Fall River follows three well-defined <br />separate channels that course through the morainal sedi- <br />ments. Immediately below the Cascade Lake dam, above <br />the Aspenglen Campground, the floodwaters scoured the <br />glacial sediments as much as 30 ft (fig. 42). The bed <br />material is extremely coarse, bouldery gravel Some of <br />the largest boulders believed to have moved were 5.8X6.4 <br />X8.4 ft; 3.8XlO.2X14.7 ft; 5.3X5.8X6.5 ft; and 4.2X4.8X8 <br />ft. Many trees in the flood path were either broken, under- <br />cut and toppled, or severely scarred and stripped of bark. <br /> <br />CATASTROPHIC FLOOD FEATURES <br /> <br />'I\vo types of sedimentary features were especially well <br />developed during this flood-boulder berms and step-pool <br />long profiles. On the west side of Roaring River alluvial <br /> <br />fan, and below Cascade Lake dam above the Aspenglen <br />Campground, boulder berms similar to those described <br />from other catastrophic floods in steep mountain chan- <br />nels (Scott and Gravlee, 1968, p. M13-M14) were. well- <br />developed (fig. 43A). These boulder berms are boulder <br />levees deposited by floodwaters in the two locations <br />where sufficient sediment was entrained in the flow to <br />temporarily produce an inertial granular flow (Pierson <br />and Costa, 1986). These conditions existed for only two <br />short reaches where (1) the Roaring River eroded through <br />the high lateral moraine at Horseshoe Falls, and (2) the <br />Fall River deeply eroded Pleistocene terminal moraine <br />sediments below Cascade Lake dam. <br />An inertial granular flow is a special type of sediment <br />gravity flow in which (1) large amounts of coarse debris <br />and organic matter are being transported down steep <br />channels, (2) the flow is noncohesive because of the <br />relatively small percentage of silt and clay, and (3) the <br />flow is highly turbulent. Flows of this type have been <br />described by Church and DeSloges (1984). <br />Particle-size analysis of a sample of matrix material <br />from the right boulder berm or debris-torrent levee at the <br />Roaring River alluvial fan is shown in figure 43B. The <br />median particle size is 0.118 in., and the matrix contains <br />only 0.5 percent clay. The berm ends with a steep front <br />