Sedimentation in a small lake following conversion of bottomland forest to row cropping

Abstract

Areas of native bottomland hardwood forest in the lower Red River basin in Louisiana were cleared for row cropping around 1970. Erosion and sediment deposition in channels and lakes presumably increased, affecting hydrologic connectivity and lake ecosystems. Sediment deposited in a representative small lake after land conversion was determined in 2000 from the distribution of cesium (137Cs) with depth. Sediment consisted of low-density clay deposited above dense silt loam. Organic carbon (C) and 137Cs profiles had lower and upper concentration maxima. The lower maxima established the circa 1970 lakebed at ≤0.9 m (3 ft) above the morphological discontinuity and from 2.1 to 1.3 m (6.9 to 4.3 ft) below the 2000 lakebed at positions midlake and halfway to the bank, respectively. The upper maxima were likely due to progressive land conversion around the lake, coupled with increasing clay content of sediment upwards to the 2000 lakebed. A quiescent settling model was used to estimate the composition of runoff water necessary to produce the depth, texture, and density of sediment above the lower 137Cs maxima. The Annualized Agricultural Nonpoint Source water quality model was calibrated to match this composition, and sediment accumulation under alternative management scenarios was calculated. Compared to conventional-till (CT) soybeans (Glycine max L.) for 30 years, CT with cover crop (CC), no-till (NT), NT with CC management, and forest might have reduced sedimentation in the lake by about 20%, 40%, 60%, and 98%, slowing changes in light penetration, temperature, turbidity, and chemistry that affect the lake ecosystem.