Major influences and findings for water quality and biology in central Illinois, including the Illinois River from Ottawa, Illinois to Valley City, Illinois, are described and illustrated. Samples were collected to determine nitrate, phosphorus, pesticides, volatile organic carbon compounds, and radon-222 in streams and ground water. Agricultural and other land-use practices are discussed in relation to their effects on water quality and aquatic life and habitat. Implications of arsenic in ground water are examined. The interactions between nutrients and stream algal populations are discussed. Results are compared with other studies across the country.