Algal growth and the water quality effects were studied and modeled in the lower reaches of the Colusa Basin Drain as a stream system. Statistical Techniques such as the correlation matrix and the stepwise multiple regression are performed on such variables as algal biomass, ammonia, nitrate, orthophosphate, temperature, and the solar radiation intensity in deriving an algae statistical model. Orthophosphate is shown as the limiting nutrient controlling the growth kinetics of algae. .A one-dimensional dynamic mathematical and computer model is formulated to simulate suspended algae and water quality parameters. The mathematical solution is obtained by a finite-difference implicit method. The model is calibrated and confirmed with the data collected from the lower 30 km reaches of the Colusa Basin Drain. The simulation results for both the statistical and the dynamic models are in good agreement with measured values. The model can be used to evaluate water quality in stream systems.