Abstract

The July 15, 1982, Lawn Lake flood in Rocky Mountain National Park, Colorado, was caused by the failure of a 79-yr-old earthen dam. Peak discharges of the flood far exceeded naturally occurring flows, and it caused severe channel disturbance along most of Roaring River and some parts of Fall River. This study documents the geomorphic response of a 5-km reach of Fall River in the 5 yr following the flood. In 1983, the first year after the Lawn Lake flood, snowmelt flows were well above average. These high flows together with very high sediment yields from Roaring River resulted in significant geomorphic changes on reaches of Fall River downstream. During the 1983 snowmelt runoff, ∼15.5 x 106 kg of bed-load sediment was eroded from the upper part of the study area. These loads were at least 1,000 times higher than before the Lawn Lake flood. Most of this sediment was then deposited in a highly sinuous reach of Fall River in the lower part of the study area. This reach had not been much affected by the Lawn Lake flood, but sedimentation during the period of high flow in 1983 completely filled in the channel, resulting in the formation of a continuous 2.3-km-long depositional zone. In 1984, sediment yield from Roaring River declined dramatically, and this trend continued for the next 3 yr. By 1987, the bed-load sediment yield in the upper reaches of Fall River was only about 0.4 x 106 kg/yr. The decline in sediment loads resulted in progressive erosion and recovery of the original channel of Fall River in the depositional zone reach. Recovery in the upstream part of the depositional zone was complete by 1985. Recovery in the downstream part of the depositional zone took longer because of the continued supply of sediment and because the sediment was mobile less of the time. As of 1987, about 80% of the material initially stored in the sedimentation zone had been eroded. Bed-load sediment yields at a sampling site 1 km downstream of the terminus of the depositional zone ranged from 5 x 106 to 11 x 106 kg/yr, but showed no significant decline over the 5-yr study period. The average rate of bed-load transport through this reach was at least 100 times greater than before the Lawn Lake flood, but few discernible channel changes resulted from the higher loads.

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