Large temperate rivers around the world have been historically affected by human activities and face an uncertain future in which water quality may be further threatened by climate change, urban development, agriculture, exotic species, recreation, and transportation uses (Meybeck and Helmer 1989; Zhang et al. 1999). In addition, large rivers can transfer water quality pollutants from land to sea, such as agricultural nitrogen in the Mississippi River causing hypoxia in the Gulf of Mexico (Burkart and James 1999).