Agriculture in the Upper Washita River Basin represents mixed crop-livestock systems of the Southern Plains. Research in the Little Washita River Experimental Watershed and the Fort Cobb Reservoir Experimental Watershed addresses interactive effects of variable climate, land use, and management on environmental quality. The Little Washita River watershed provides opportunities to explore impacts of flood retarding impoundments within a watershed. The Fort Cobb Reservoir watershed provides opportunities to study effects of agricultural conservation on a large eutrophic reservoir. Analysis of 1940 to 2005 data from the Fort Cobb Reservoir watershed showed that precipitation increased 33%, corresponding runoff increased 101%, and sediment yield increased 183% when comparing multi-year wet periods to multi-year dry periods. Depth to groundwater exhibited seasonal and interannual variation. A rapid geomorphic assessment indicated that unstable stream channels dominate the stream networks. Phosphorus concentration in streams was correlated to multiple attributes of the contributing areas, including contributing area, slope, stream density, and channel stability. Anticipated outcomes are improved understanding of environmental effects of conservation, new approaches to mitigation of water quality problems, and tools to support strategic placement of conservation practices on the landscape to achieve environmental goals.

New and innovative methods for measuring soil loss and its impact on productivity are needed to accurately assess the value of soil lost by erosion. Cesium‐137 (137Cs), a radionuclide from nuclear weapons tests which is strongly adsorbed to clay, can be used to trace sediment movement. Measurements of 137Cs concentrations in uneroded soils across the southern United States indicated that the 137Cs input was proportional to the average annual precipitation. Distribution of 137Cs within the profile was related to soil properties rather than to rainfall. Cesium‐137 measurements on a small native grass watershed in Oklahoma showed considerable spatial variability in the 137Cs concentrations, but the variability was random and was not correlated with changes in slope. Cesium‐137 concentrations measured on a 10‐m grid in an adjacent small watershed that had been cultivated for 8 yr showed differences with slope positions even though only 17.8 Mg/ha sediment had been removed from the watershed during an 8 yr measurement period. Also, 137Cs concentrations in the cultivated watershed were significantly lower than in the uncultivated watershed. These data show that 137Cs measurements can be used at low erosion rates if enough samples are analyzed. Further research is needed to determine the number of samples needed for different watersheds and to refine the method.

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Long‐term land use and reservoir sedimentation were quantified and linked in a small agricultural reservoir‐watershed system without having historical data. Land use was determined from a time sequence of aerial photographs, and reservoir sedimentation was determined from cores with 137Cs dating techniques. They were linked by relating sediment deposition to potential sediment production which was determined by the Universal Soil Loss Equation and by SCS estimates for gullied land. Sediment cores were collected from Tecumseh Lake, a 55‐ha reservoir with a 1,189‐ha agricultural watershed, constructed in 1934 in central Oklahoma. Reservoir sediment deposition decreased from an average of 5,933 Mg/yr from 1934 to 1954, to 3,179 Mg/yr from 1954 to 1962, and finally to 1,017 Mg/yr from 1962 to 1987. Potential sediment production decreased from an average of 29,892 to 11,122 and then to 3,589 Mg/yr for the same time periods as above, respectively. Reductions in deposition and sediment production corresponded to reductions in cultivated and abandoned cropland which became perennial pasture. Together, cultivated and abandoned cropland accounted for 59 percent of the watershed in 1937, 24 percent in 1954, and 10 percent in 1962. Roadway erosion, stream bank erosion, stored stream channel sediment, and long‐term precipitation were considered, but none seemed to play a significant role in changing sediment deposition rates. Instead, the dominant factor was the conversion of fields to perennial pastures. The effect of conservation measures on reservoir sedimentation can now be quantified for many reservoirs where historical data is not available.

The Buffalo River is a tributary to the Mississippi River in west‐central Wisconsin that drains a watershed dominated by agricultural land uses. Since 1935, backwater from Lock and Dam 4 on the Mississippi River has inundated the mouth of the Buffalo's valley. Resurveys of a transect first surveyed across the lake in 1935 and cesium‐137 dating of backwater sediments reveal that sedimentation rates at the Buffalo's mouth have remained unchanged since the mid‐1940s. Study results indicate that sediment yields from the watershed have persisted at relatively high levels over a period of several decades despite pronounced trends toward less cultivated land and major efforts to control soil erosion from agricultural land. The maintenance of sediment yields is probably due to increased channel conveyance capacities resulting from incision along some tributary streams since the early 1950s. Post‐1950 incision extended the network of historical incised tributary channels, enhancing the efficient delivery of sediment from upland sources to downstream sites.