FLOOD06904 - Laserfiche WebLink

Home Browse Search

GEOMORPHIC EFFECTS OF THE FLOOD 33 ) . 'iI- ";,. - FIGURE 24.-Elkhorn Avenue, Estes Park, looking downstream from river mile 12.0 (aerial). Big Thompson River is along the right side of the photograph; Lake Estes is in the top righthand corner. Photo courtesy of Zenas Blevins, U.S. Bureau of Reclamation. scoured reaches, very large boulders in ground moraine were undercut and had simply fallen onto the channel floor. The criteria used to establish that a particular boulder did, in fact, move in the flood were: (1) Percus- sion marks on all sides of the boulder, not just on the upstream side; (2) balanced and wedged smaller rocks, and broken and buried vegetation under larger boulders; and (3) boulder clusters and berms in streamlined deposi- tional forms, with imbricated structure parallel to the flow direction (fig. 30). Some of the largest measured boulders known to have been moved in the Roaring River valley have dimensions of 5.5X8X12.5 ft, 7X12X13 ft, and 3X7X8 ft. Larger boulders were observed in the channel, but minimal or no evidence existed to indicate that they had been moved by the dam-failure flood. Much less conspicuous, but nevertheless ubiquitous. were sand deposits, Sands were deposited almost continuously, away from the main flood channel, within the edge of the surviving forest and vegetated areas on the valley sides below high-water marks. Some sand deposits were 4 ft thick and had steep wave fronts facing downstream (fig. 31). Sand deposits along the Roaring River are very coarse and massive, or have weak horizontal laminations (fig. 32). About 400 ft downstream from the Lawn Lake dam, sand splays are extensively developed on the right bank. High-water marks indicated the flood flow was 3.5 ft deep over the top of these splays. Along the edge of the splay next to the deeply eroded channel, backset beds, interpreted to be antidune structures, were preserved (fig. 33). These backset beds were diffuse, curved, and TABLE 7-Channel conditions. Roaring Riuer Distance downstream from Lawn Lake dam, Average slope, Channel in miles in percent condition Below Lawn Lake dlUll 0.00-0.24 7.5 scour .24- .68 6.9 deposition ,68-1.00 25 scour 1.00-1.16 9.2 deposition 1.16-1.49 23.5 scour 1.49-1.70 5 deposition 1. 70-1.95 13.3 scour 1.95-2.23 5.5 deposition 2.23-2.74 7.1 scour 2,74-3.62 5.7 deposition Horseshoe Falls 3.62-4.02 26 scour