The Swinomish Indian Reservation occupies 17 mi²on Fidalgo Island, northwestern Washington. Six square miles are underlain by mudflats or low-lying alluvial deposits, and are not part of the study area. An appraisal of the water resources of the reservation was made because the Swinomish Tribal Community expects rapid economic and population growth in the near future.

Average inflow to the hydrologic system of the reservation is 24 ft³/s (cubic feet per second). Outflow consists of evapotranspiration (15 ft³/s), subsurface outflow (5 ft³/s), and surface-water outflow (4 ft³/s). Recharge to the ground-water reservoir is 8 ft³/s.

Most of the study area is a remnant of a glacial drift plain underlain by three types of unconsolidated deposits. The uppermost unit consists primarily of till, the intermediate unit is predominantly sand and gravel, and the lowermost unit is nearly all clay and silt. The total storage capacity is about 6.3 billion cubic feet of water.

During 1976 human interaction with the hydrologic system was negligible, with an average rate of water use of 0.19 ft³/s. Seventy percent of this was pumped from the ground-water reservoir and the rest was imported.

Water levels in six wells extending below mean sea level were found to fluctuate as much as 2.8 feet in response to tidal fluctuations, representing maximum tidal efficiencies of 42 percent. Below sea level is a freshwater-saltwater zone of diffusion at least 150 feet thick.

Dissolved-solids concentrations are estimatd to be 10-20 mg/L (milligrams per liter) in precipitation, increase to 15-35 mg/L due to evapotranspiration, reach 45 mg/L in direct runoff, and increase to about 160 mg/L in shallow ground water and about 245 mg/L in deep ground water. In the zone of saltwater diffusion concentrations up to 1,570 mg/L were measured. Human interaction with the hydrologic system has had little apparent effect on water quality.

Ground-water quality is generally within the acceptable limits of the Federal Safe Drinking Water Act. The maximum contaminant levels for turbidity, arsenic, and coliform bacteria have been exceeded in a few samples.

Recommended limits have been exceeded for iron, manganese, chloride, dissolved solids, pH, and color. Most of the large concentrations of these constituents were in water from the zone of saltwater diffusion.

The ground-water reservoir can be developed to a greater degree. If 20 percent of the 8 ft³/s of ground-water recharge can be intercepted, a net rate of ground-water withdrawal of 1.6 ft³/s can be attained. Aquiculture development is possible on the two largest streams in the reservation in the form of incubation stations handling 600,000 eggs each.