The study area lies west of Tacoma, Wash., and includes the Gig Harbor peninsula (W square miles) and 15 square miles of adjacent areas, all of which are experiencing rapid population growth.Average inflow to the hydrologic system of the study area is 254 cubic feet per second (ft^/s). Outflow consists of evapotranspiration (88 ft^/s), surface-water outflow (132 ft^/s), and subsurface outflow (34 ft^/s). Recharge to the ground-water reservoir from precipitation is 93 ft-Vs.The study area is a remnant of a glacial-drift plain nearly surrounded by marine embayments. Three main water-bearing units exist; the Colvos Sand Member of the Vashon Drift, the Salmon Springs(?) Drift, and the pre-Salmon Springs(?) deposits.As of 1978, water use was negligible. Average rate of use was 2.4 ft^/s, all from ground-water sources. Much of this water (1.4 ft-Vs) was discharged to septic systems.Mean annual flows in streams show a close relationship to basin size, ranging from 1.4 to 2.0 ft-Vs per square mile of basin. Low flows are very consistent; 7-day low flows for 50-and 1-percent probabilities differ by 25 percent or less at each site.The only significant lake in the area, Crescent Lake, is probably at a later stage of eutrophication than average lakes in the surrounding area. Crescent Lake may have experienced increased eutrophication from 1970 to 1978.Dissolved-solids concentrations in the fresh ground-water reservoir increase with depth, from about 80 to 95 mg/L (milligrams per liter) and average about 290 mg/L in wells tapping the freshwater-saltwater zone of diffusion. The use of water has apparently had little effect on its quality, although nitrate-plus-nitrite concentrations were slightly greater in shallow ground water than in deep ground water.Ground-water quality is generally within the acceptable limits of the Federal Safe Drinking Water Act. The maximum contaminant levels for fluoride, turbidity, coliform bacteria, cadmium, lead, mercury, and selenium have been exceeded in a few samples. Recommended limits have been exceeded for iron, manganese, chloride, dissolved solids, pH, and color.The ground-water reservoir can be developed to a greater extent. A system of properly spaced wells would probably yield at least 11 ft-^/s, about five times the 1978 rate of ground-water use.A maritime climate prevails, with cool, dry summers, and mild, wet winters. Winds are generally from the southwest in fall and winter and from the northwest in spring and summer. Afternoon temperatures are typically in the 70's (°F) in summer and in the 30's (°F) in winter. Temperatures exceed 83°F on about 10 days per year, and freezing temperatures occur during about 45 nights per year. Relative humidity ranges from 50 to 85 percent in summer and from 75 to 90 percent in winter.