Water samples were collected during 2002-04 at monitoring stations on 14 streams either within or entering the Delaware Water Gap National Recreation Area. The samples were collected from April through December of each year, mostly under low (base-flow) conditions, and were analyzed for major ions and nutrients (nitrogen and phosphorus). Results of the analyses, in concert with land-use information in the drainage basins associated with the samples, were used to define water-quality characteristics; to identify relations among water quality, streamflow, and season; and to establish a baseline and develop a method that could be used to detect future changes in water quality. For a given water-quality characteristic, median values commonly varied among the 14 water-quality monitoring stations. For example, the median concentration of total phosphorus at the station on Sand Hill Creek (0.033 milligrams per liter as P) was four times the corresponding median concentration at the station on Vancampens Brook (0.008 milligrams per liter as P). Results of correlations between median values of water-quality characteristics and land-use characteristics of the drainage basins indicate that agricultural practices and the presence of wetlands could be important factors affecting the concentrations of total nitrogen and total phosphorus in these streams. Results of analyses of samples from the nine stations without permitted wastewater facilities in their basins indicate that medians of both total phosphorus and total nitrogen increased with an increase in the area of agricultural land in the drainage basins; the levels of significance are 0.01 for total phosphorus and 0.01 for total nitrogen. When only the seven stations without permitted wastewater facilities and with less than 5 percent of the basin in agricultural land are considered, median concentrations of total phosphorus and total nitrogen increased with an increase in the area of wetlands in the basins; the levels of significance are 0.003 for total phosphorus and 0.03 for total nitrogen. Linear equations between values of each water-quality characteristic at a station, streamflow, and season were developed by use of Tobit regression. The variations of water quality with streamflow and with season were identified from these equations. Concentrations of total phosphorus, total nitrogen, and attenuation turbidity increased with increasing streamflow at more stations than concentrations decreased with increasing streamflow. Concentrations of dissolved orthophosphate phosphorus, dissolved nitrate plus nitrite, dissolved ammonia, and major ions decreased with increasing streamflow at more water-quality stations than concentrations increased with increasing streamflow. Most water-quality characteristics varied with season at most stations due to reasons other than the seasonal variation in streamflow. Concentrations of total phosphorus and total nitrogen during the summer (July-September) often exceeded concentrations during the spring (April-June) and fall (October-December). As one example, concentrations of total nitrogen at the monitoring station on Big Flat Brook are between 0.1 and 0.2 milligrams per liter as N in the spring and fall, but increase to between 0.2 and 0.3 milligrams per liter as N during the summer. A method based on the linear equations relating water quality to streamflow and season was developed to detect differences in water quality between current (2002-04) and future conditions. Changes in water quality would be identified by detecting differences between the intercept of the equation with current water quality and the intercept of the corresponding equation with future water quality. The intercept represents an estimate of the water quality at a station with a streamflow of 1 cubic foot per second during a season in which the seasonal variation of water quality is minimal. The method to detect future changes in water quality allows for an