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GEOMORPHIC EFFECTS OF THE FLOOD FIGURE 21.-The Fall River valley downstream from Aspenglen Campground (aerial). View is upvalley from about river mile 8.5. divided into three reaches: (1) The Roaring River from the Lawn Lake dam to Horseshoe Park, (2) the Fall River through Horseshoe Park to the Cascade Lake dam, and (3) the Fall River from the Cascade Lake dam to Lake Estes. ROARING RIVER: LAWN LAKE DAM TO HORSESHOE FALLS Geomorphic effects of the dam failure on the Roar. ing River were catastrophic. The data which follow were from field measurements and aerial photographs of the flood path, taken within 4 hours after the flood, with scales of 1:5,040 to 1:8,400. Prior to the flood, typical dimensions of the Roaring River were 10 to 16 ft wide and 1 to 2 ft deep. The channel was filled with numerous large glacial boulders (fig. 16A); in the less steep moun. tain meadows, the channel followed a sinuous course 31 between finer-grained channel banks of gravelly, coarse sand and silt. Following the flood, valley bottoms were severely eroded and rearrahged. The channel had scoured lateral- ly and vertically' into underlying ground moraine. Widths varied from 70 to 500 ft, and the channel was scoured from 5 ft to as much as 50 ft locally (fig. 26). The natural long profile of the Roaring R,ver was a series of steep channel reaches separated by more gen. tle mountain meadows. Following the flood, these dif- ferences became exaggerated. Scour occurred along 11,900 ft of channel, or 56 percent of the length of the Roaring River; deposition occurred along about 9,300 ft in the flatter parts of the valley floor (fig. 27). The average slope of the Roaring River is 10 percent; but locally, reaches are as steep as 26 percent and as low as 5 percent. The threshold slope separating the scoured reaches from the depositional reaches seemed to be about 7 to 9 percent (table 7). Scoured reaches are steep, and the newly eroded chan- nel is narrow and deep (fig. 28); also see the Roaring River valley cross section at river mile 0.55 (fig. 58) in the Supplemental Cross-Section Data section at the end of the report. In many places, scour removed all overly- ing glacial' material and exposed fresh bedrock in the channel bottom (fig. 27). The flood eroded laterally in. to glacial sediments, and left steep, overhanging banks, with numerous large boulders half-exposed and ready to fall into the channel below (fig. 26). Thousands of trees in the valley floor were uprooted as their footings were eroded. Little evidence was found of remaining rooted trees having been snapped off or shattered; most trees probably were lost from toppling or undermining and caving of eroding banks during the flood wave. Once incorporated into the floodflow, trees were exten- sively battered and scarred (fig. 17. Depositional areas were characterized by more gen- tle gradients at the breaks in slope below steep reaches. These areas of relatively wide mountain meadows generally had slopes less than 7 to 9 percent. Sediments deposited in the meadow areas were relatively thin, generally no more than 2 to 8 ft thick. Locally, deposits were measured to 10 ft thick (fig. 29). Deposited material in each meadow came from the scoured-valley sides and bottoms of the steep reach upstream. No large boulders or rocks traveled any farther than about 1,000 ft; most probably were moved less than 100 ft. Nearly all the boulders were fresh, with a minimal number of percussion marks, indicating that they did not travel far or very long in the flood. Some very large boulders (greater than 6.ft diame- ter) were moved in the floodflow. It was not always immediately evident that a particularly large rock was transported by the flood; in many places along deeply