Siltcoos Lake (silt’-koos), at 3,164 acres, is the largest lake on the Oregon Coast of the United States. Fed by runoff from a basin of about 68 square miles in Douglas and Lane counties, it is located about 7 miles south of Florence and 0.5 miles east of U.S. Route 101. Its name comes from a Lower Umpqua (Siuslaw) placename, a variant of which is Tsiltcoos.
The lake, with a mean elevation of 8 feet above sea level, empties into the Siltcoos River, which meanders generally west for about 3 miles to its mouth on the Pacific Ocean. A dam about a half-mile downriver regulates the lake’s outflow. Siltcoos Lake is a submerged remnant of a Siltcoos River delta that existed before the most recent ice age. The lake formed after melting glaciers caused a rise in sea level that drowned the lower reaches of Oregon’s coastal rivers. Sediments from the sluggish rivers formed sand dunes, behind which the ancestral mouths of rivers like the Siltcoos became lakes. Nearby Woahink Lake is similar remnant. Fed mainly by four small freshwater tributaries—Woahink, Fiddle, Maple, and Lane creeks—the lake is very shallow. About 80 percent of it is less than 15 feet deep, and 32 percent of it consists of shoal areas. The shallowness contributes to the lake’s eutrophic nature, related to extensive populations of Elodea, Myriophyllum, and other aquatic plants. A wide variety of warm-water fish thrive in the lake, where nutrients are plentiful, but these nutrients have also led historically to troublesome algal blooms. Recreation Siltcoos Lake is a popular destination for fishing and is home to many warm-water species as well as transient sea-going fish such as salmon and steelhead. Anglers try for bass weighing up to 9 pounds, as well as perch, catfish, crappies, bluegill, and cutthroat trout. Camping is available at several spots within a few miles of the lake. These include Tyee Campground along the Siltcoos River; Waxmyrtle and Lagoon campgrounds, accessible from Route 101 south of the lake, and Driftwood II Campground, west of Lagoon Campground. Honeyman State Park, with hundreds of campsites, is about 2 miles north along Route 101.