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1. For a breach width of 25 feet, the peak discharge would have been 7,000 cubic feet per second less downstream from Lawn Lake dam to 1,300 cubic feet per second less at Estes Park. Maximum flood depths would have averaged 0.6 foot lower; the flood wave would have reached Estes Park at the same time. For this hypothetical case, Cascade lake dam still would have failed. 2. For a breach width of 200 feet, peak discharge would have been 22,600 cubic feet per second greater downstream from Lawn lake dam to 5,400 cubic feet per second greater at Estes Park. Maximum flood depth would have aver- aged 2.7 feet higher; the flood wave would have reached Estes Park 0.4 hour ear 1 i er. The mode 1 a 1 so i ndi cated that the outflow peak di scharge from the worst-case failure of lawn Lake dam could have been at least 56,000 cubic feet per second. If Cascade lake dam had not failed (or had not been present), peak discharges would have been reduced by 11,300 cubic feet per second immediately downstream from the dam to 500 cubi c feet per second 1 ess at Estes Park. Maximum flood depths would have averaged 0.6 foot lower, and the flood wave would have reached Lake Estes 0.3 hour later. INTRODUCTION Just before sunrise, at about 0530 MDT (Mountain Daylight Time), on the calm, clear morning of Thursday, July 15, 1982, the privately-owned Lawn Lake dam, a 26-ft-high earthen structure, located at an elevation of about 11,000 ft in Rocky Mountain National Park, breached due to a piping failure (Office of the State Engineer, 1983), releasing 674 acre-ft and an estimated peak discharge of 18,000 ft3/s of water down the Roaring River (fig. 1). "I started to hear a sound like an airplane. Also, there were loud booms. It got louder and louder. I thought it was breaking the sound barrier. I kept looking for a plane, but couldn't see one. I got suspicious and started to look upstream. I saw trees crashing over and a wall of water coming down. I started to run as fast as I could for high ground. There was a deafening roar. I fell and got up and kept running. I stood on high ground and watched it wipe out our campsite. It knocked everything in its path over; Steve didn't stand a chance." I With these words Steven Cashman described his harrowing experience with the flood that swept his camping companion to his death in the Roaring River. Other campers along the Roaring River estimated the wall of water to be 25 to 30 ft high, carrying with it large trees and boulders, 50 that the water looked 1 i ke a "wet, brown cloud" and sounded 1 i ke extreme ly loud continuous "thunder" or a "freight train." The flood, attenuated by the relatively flat Horseshoe Park (an old glacial lakebed), continued along the Fall River, where it caused Cascade lake dam (an approximately 12-ft-high, 12-acre-ft concrete gravity dam) to fail because of tipping over from overtopping. Peak discharge upstream from Cascade lake dam was 7,210 ft3/s; however, discharge increased to 16,100 ft3/s or more as a result of the Cascade Lake dam failure. The flood continued to 3