Effects of a landslide complex on sediment discharges and loads in the Muddy Creek drainage basin and deposition into Paonia Reservoir, west-central Colorado, 1986-87

Abstract

Total-sediment load upstream from and downstream from a landslide complex in the Muddy Creek drainage basin in Gunnison and Delta Counties was estimated by using stream-discharge and sediment data collected from June 1986 through July 1987. The relation of suspended-sediment discharge and bedload discharge to stream discharge was determined by using logarithmic regression analysis and analysis of covariance. The combined total-sediment load of East Muddy Creek, Spring Creek, and West Muddy Creek represents the total-sediment load of Muddy Creek upstream from the landslide complex and is assumed to be representative of the total-sediment load before movement of the landslide complex. The total-sediment load was estimated to be 101,000 tons during a 216-day period. The total-sediment load of Muddy Creek downstream from the landslide complex, but upstream from Paonia Reservoir, included sediment contributed by the landslide complex and was estimated to be 147,000 tons for the 216-day period. During a 483-day period, the total-sediment load of Muddy Creek upstream from Paonia Reservoir was estimated to be 293,000 tons. The totalsediment load of Muddy Creek downstream from Paonia Reservoir was estimated to be 14,900 and 51,100 tons during the 216-and 483-day periods; the trap efficiencies of the reservoir were estimated to be 90 and 83 percent. The total-sediment load of East Muddy Creek, Spring Creek, and West Muddy Creek during the 216-day period was estimated to be about 400 tons per day, and is assumed to be representative of the sediment load deposited into Paonia Reservoir prior to movement of the landslide complex. There was a loss of about 2,100 acre-feet of water-storage capacity in the reservoir prior to movement of the landslide complex. The land-surface elevation in a ponded reach of East Muddy Creek upstream from the central slide of the landslide complex increased an average of 7.5 feet between August 1985 and June 1986. The volume of material necessary to increase the elevation of the land surface by 7.5 feet is about 2 million cubic feet. Some of the increase in the elevation of the land surface is a result of deposition of sediment by East Muddy Creek and some may be from deformation of the land surface as a result of movement of the landslide complex. Four channel cross sections of Muddy Creek near the upstream reach of Paonia Reservoir were resurveyed to determine changes in the volumes of sediment since 1962, when Paonia Dam was completed. The maximum change in sediment volume in a cross section between May 1962 and September 1987 was more than 2,700 cubic feet. Much of the sediment deposited in the upper reaches of Paonia Reservoir is scoured from this area and redeposited farther downstream in the reservoir.