Abstract: The findings of a problem-oriented, river-system investigation of the water quality and streamflow characteristics of the Raritan River, N.J. are described. Information on streamflow duration, time-of-travel measurements, and analyses of chemical, biochemical and physical-water quality are summarized and used to define relations existing between water quality, streamflow, geology, and environmental development in the basin's hydrologic system. Stream quality, particularly in the lower urban areas of the basin, is shown to be deteriorating with time at most of the sampling sites reported. For example, average dissolved-oxygen concentration is reported to be undersaturated at all sampling sites and is decreasing with time at most sites. Biochemical-oxygen demand is increasing at most sites, as are the dissolved-solids content.