The Kennet model is a new model of in-stream phosphorus (P) and macrophyte dynamics. Based on mass balance equations, the model represents the interactions between P and the suspended and bed sediments, the uptake of P by epiphytes and macrophytes, and the exchange of P between the water column and the pore water. The model simulates the total phosphorus (TP) and the soluble reactive phosphorus (SRP) concentrations observed in a reach of the River Kennet. Furthermore, the model simulates the generalized macrophyte growth patterns and total biomass observed in rivers throughout southern England. A general sensitivity analysis, based on Monte Carlo simulations and parameter values derived from the literature, identifies the key parameters controlling the model behavior when simulating macrophyte growth. The most important parameters are those that directly control macrophyte growth, those that define the epiphyte growth, and those that relate to the storage of P in the streambed. ¿ 2001 American Geophysical Union