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The current generation of basin scale models perform well in upland areas, but relatively poorly in-stream. The source of this weakness is either the use of a unrealistic model for a particular stream system or the lack of data to parameterize the in-stream model. Limited resources cripple our efforts to setup highly parameterized in-stream models. To overcome these issues, we applied a very simple in-stream phosphorous model to SWAT predicted loads from the uplands areas of the Illinois River Basin. The Illinois River drains one million acres of eastern Oklahoma and western Arkansas. The Illinois River is one of Oklahomas most valued scenic rivers and is subject to a proposed 0.037 mg/l total phosphorus limit. The poultry industry is heavily involved in the eastern portion of the basin, which also contains a number of large point sources. A custom in-stream phosphorus model was developed to route and transform SWAT predicted phosphorus loads from upland areas through the Illinois River and its tributaries. The in-stream phosphorus model is very simple with three pools being tracked in each stream segment and five basin-wide input parameters. The generalized in-stream model is shown in Figure 1. The phosphorous model handles both soluble and particulate forms. Conversion of soluble to particulate forms is estimated using a first order decay based on reach length, stream flow, and a decay coefficient. This decay coefficient is modified depending on the ratio of point source to nonpoint source phosphorus load present in each segment. Deposition or reentrainment of particulate phosphorus to or from the benthos is based on the stream flow/drainage area ratio, and three coefficients governing the direction and magnitude of phosphorus movement. During average and low flows particulate phosphorus is deposited with sediments, during high flows phosphorus is scoured from the streambed. A brute force optimization program was written to calibrate this in-stream model to observed water quality measurement throughout the basin.