Water-resources of western Douglas County, Oregon

Abstract

In western Douglas County, Quaternary coastal dune sands and marine terrace deposits may have the best potential for ground-water development. Yields of 200 gallons per minute have been reported from wells completed in Quaternary fluvial deposits along the lower Umpqua River. The entire area is underlain by Tertiary marine sedimentary rocks that yield quantities of water barely adequate for domestic use. On the basis of wells sampled and the constituents analyzed, ground-water quality was generally good, but the recommended maximum concentration of 300 micrograms per liter for iron in drinking water was exceeded in about one-third of the samples. Water from seven wells was analyzed for toxic trace metals and had concentrations less than the recommended criteria levels as determined by the U.S. Environmental Protection Agency. The project area has a temperate marine climate, with an annual rainfall ranging from 70 to 100 inches, of which 80 percent falls from October to March. Average annual runoff from eight streams in western Douglas County was estimated to range from 2.4 cubic feet per second per square mile for Elk Creek to 6.8 cubic feet per second per square mile for Scholfield Creek. The estimated 7-day, 20-year low flow ranges from O.OI cubic foot per second per square mile for Weatherly Creek to 3.6 cubic feet per second per square mile for the Smith River. The dissolved-solids concentration in the Umpqua River is low and stable, with little seasonal and yearly variation. Likewise, the eight small streams in the project area have low dissolved-solids concentrations but have noticeably higher nitrite plus nitrate nitrogen concentrations than those of the Umpqua River. Also, during low flows of several small streams, dissolved-iron concentrations increased significantly. All the lakes in the project area have dissolved-solids concentrations of less than 100 milligrams per liter and, except for Loon Lake, have limited phosphorus available for algal production. Tahkenitch and Elbow Lakes are considered to be the most active in terms of biological productivity. A survey in the Umpqua estuary during a high tide and low streamflow condition indicated that saltwater intrusion extends to river mile 20 in the Umpqua River, which limits the use of the river water downstream from that point. INTRODUCTION The rapid economic development and population expansion in western Douglas County, together with heavy tourism during most of the year, have necessitated the evaluation of the quantity, quality, and distribution of the water resources of the area. Although flow of smaller streams may be large during winter, it is small in summer when demand is greatest. The purpose of this study is to assess the water resources of western Douglas County, including an evaluation of quantity, quality, and distribution of ground and surface waters in the study area. This investigation is part of a continuing cooperative program between Douglas County and the U.S. Geological Survey to evaluate the water resources in Douglas County. U.S. Forest Service and Bureau of Land Management personnel cooperated with and assisted the authors in the collection of field data. The helpful cooperation of these people, and especially of the well owners who permitted access to their wells to collect ground-water data, is gratefully acknowledged. PREVIOUS INVESTIGATIONS Rinella and others, (1980) evaluated the water resources in the Reedsport area. Many of the data collected for that study are included in this report. Four U.S. Geological Survey reports from earlier investigations in Douglas County deal with areal evaluations of ground-water availability and quality. The areas are: (1) Drain-YoncaI la (Robison and Col I ins, 1977), (2) Sutherlin (Robison, 1974), (3) Winston (Robison and Col I ins, 1978), and (4) Myrtle Creek-Glendale (Frank, 1979). Sediment yields in the Umpqua River basin were reported by Onions (1969) and Curtiss (1975). Chemical quality of streams in the Umpqua River basin was described in a report by Curtiss (1969) and the lakes of Douglas County were inventoried and reported by Rinella (1979). Hampton (1963) studied the ground water of the coastal dunes near Florence, 25 mi north of Reedsport. Ground-water resources of the coastal dunes north of Coos Bay, about 25 mi south of Reedsport, were studied by Brown and Newcomb (1963), and later by Robison (1973). GEOGRAPHIC FEATURES The western Douglas County area (fig. 1) covers about 700 mi 2 in southwestern Oregon. The area is drained by the Smith River, the lower Umpqua River and its tributaries, and a few small streams that drain directly into the Pacific Ocean. The area consists mostly of the rugged, heavily forested Coast Range, but includes a narrow strip of coastal beach and sand-dune land. The principal city along the Douglas County coast is Reedsport (population 4,970). Smaller coastal communities are Gardiner to the north and Winchester Bay to the south. Elkton (population 180) is the principal inland community in the study area. 124°15' 1 24°00' 123°30'