This chapter presents a review of state-of-the-art freshwater modeling and provides discussion of nutrient and toxic chemical water quality models. The main thrust behind the development of water quality models has been the need to predict the results of man-made influences on our water resources, ranging from rainwater to seawater. The bulk of the water quality models which have been developed over the past few decades have been directed toward the problem of eutrophication which has been caused by the significant increase in the loadings of nutrients, particularly phosphorus, to both large and small lakes. Numerous studies have determined that the concentration of total phosphorus in lakes can be used as an indicator of lake trophic status algal population densities as measured by chlorophyll a concentrations and water clarity. Lake Erie has been the focus of several water quality modeling studies for the last two decades.