Relations between water-quality and flow characteristics and the relative importance of constant (point sources and ground-water discharge) and intermittent (nonpoint storm runoff) sources were determined for eight water-quality stations located on the Flat Brook and the Delaware, Musconetcong, Whippany, and Saddle Rivers. Water-quality and streamflow data were categorized on the basis of streamflow at the time of sample collection. Differences in concentrations and yields of selected water-quality constituents, including nutrients and bacteria, (1) among the stations during eight streamflow conditions and (2) at each station (a) between base flow and stormflow; (b) among before, during, and after a storm; and (c) among low, medium, and high flows were determined and related to the predominant type(s) of land development in the areas contributing drainage. At the station on the Delaware River, yields of fecal-coliform bacteria were affected more by contributions from storm runoff than by contributions from point sources and ground-water discharges; yields during a storm [7.0 x 108 (MPN/d)mi2 (most probable number per day per square mile)] were greater than yields during base flow (3.7 x 108 (MPN/d)mi2). Yields of nitrate plus nitrite, alkalinity, and chloride were affected more by contributions from point sources and ground-water discharges than by contributions from storm runoff; yields of these constituents were not significantly different during base flows and stormflows. At the Flat Brook and Whippany River stations, yields of most water-quality constituents were affected more by contributions from storm runoff than by contributions from point sources and ground-water discharge. For example, yields of nitrate plus nitrite were greater during stormflow (1.20 (lb/d)/mi2 (pounds per day per square mile) and 15.88 (lb/d)/mi2, respectively) than during base flow (0.26 (lb/d)/mi2 and 8.20 (lb/d)/mi2, respectively). At the Musconetcong River station, yields of total nitrogen, alkalinity, and chloride were affected more by contributions from storm runoff than by contributions from point sources and ground-water discharge. At three of the four water-quality stations on the Musconetcong River, yields of total phosphorus and bacteria were affected less by contributions from storm runoff than by contributions from point sources and ground-water discharge. At the Saddle River station, yields of alkalinity and chloride were affected more by contributions from storm runoff than by contributions from point sources and ground-water discharge; for example, yields of chloride during stormflows (707 (lb/d)/mi2) were greater than during base flows (401 (lb/d)/mi2). Yields of total phosphorus were affected less by contributions from storm runoff than by contributions from point sources and ground-water discharge; yields during base flows (4.00 (lb/d)/mi2) and stormflows (4.67 (lb/d)/mi2) were similar. Concentrations and yields of total phosphorus and nitrogen, and nitrate plus nitrite were strongly related to the amount of development in each drainage basin. At stations on the Saddle and Whippany Rivers, which drain areas with substantial development, concentrations and yields for all streamflow categories were higher than at stations on the Flat Brook and Delaware River, which drain areas with little development. Results of the Tukey test indicate that there are significant differences in total phosphorus concentrations at Saddle River and Flat Brook during base flows (0.77 mg/L (milligrams per liter) and 0.02 mg/L, respectively) and stormflows (0.42 mg/L and 0.02 mg/L, respectively).