Effects of Climate Variations and Soil Conservation on Sedimentation of a West-Central Oklahoma Reservoir

The effectiveness of soil conservation practices at reducing watershed sediment yield and effect on reservoir sedimentation is generally difficult to quantify. Here, a sedimentation survey of the Fort Cobb Reservoir in west-central Oklahoma and sediment load measurements on contributing tributaries provide an opportunity to address this question. During the second half of the twentieth century, extensive soil conservation practices were implemented on the Fort Cobb Reservoir watershed. Sediment and flow observations were made on major tributaries in 1943–1950 (preconservation time period) and again in 2004–2008 (postconservation time period). These data were used to compare watershed sediment yield and reservoir sedimentation during pre- and postconservation periods. Suspended sediment-discharge rating curves were developed for each of the two time periods and used to estimate average annual watershed sediment yield. Initial sediment yield estimates for pre- and postconservation conditions were 254,900 Mg/year (3.306 Mg/year/ha) and 247,700 Mg/year (3.213 Mg/year/ha), respectively. The apparent lack of responsiveness of sediment yield to conservation efforts was explained by a shift in the mid-1980s toward wetter climatic conditions. The wetter climate led to increased soil erosion, sediment transport, and sediment yield that offset reductions achieved by conservation efforts. Approximately a 60–65% reduction in sediment yield would have been achieved by conservation efforts if the climate characteristics had been constant over time. The mutually offsetting effects of a wetter climate and conservation efforts on sediment yield resulted in unabated sedimentation of the Fort Cobb Reservoir. The interaction between wetter climate and effectiveness of conservation practices was the basis for projecting future sedimentation rates and reservoir life span, which are shown to vary over a wide range depending on assumptions of prevailing climate over the next decades.