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North Coast Lakes

Lakes, Ponds and Reservoirs

Although there are many lakes along the Oregon Coast, there are very few in the Coast Range. This is because high precipitation over the last two million years, combined with tectonic uplift of the mountains, has produced very steep-walled, narrow valleys throughout the range. Glacial and volcanic processes, responsible for most of the Cascade Mountain lakes, were virtually non-existent in the Coast Range. However, landslides in the rugged topography are quite common, and the few natural lakes that do exist were formed where massive slides blocked a river valley, impounding water behind it. The only two large natural lakes in the Coast Range, Triangle Lake and Loon Lake, were formed in this manner.

Baldwin (1976) described the formation of Triangle Lake. It was blocked by a tilted mass of sandstone that evidently came from high on the north slope of the valley drained by Lake Creek. The creek found its outlet against the south wall where it is incised in bedrock rather than in the slide material. This condition will insure the long life of the alluvial valley, but not necessarily of Triangle Lake, which is only a remnant of a long lake that used to extend several miles upstream. Most of the old lake has been filled in by sediment and the size of the lake is also slowly reduced as erosion of the outlet reduces the water level.

At the extreme southern tip of the North Coast Basin, there are three significant coastal lakes: Woahink, Siltcoos, and Tahkenitch.

Woahink Lake is located south of Florence and about three miles inland from the Pacific Ocean. Its southern end lies adjacent to US Highway 101. The lake basin owes its origin to the fluctuations in sea level, alternating periods of submergence and emergence of the coastal zone (Cooper 1958). During periods of submergence, the mouths of coastal streams were inundated by higher water. Many small streams lacked the erosive ability to prevent the obstruction of their mouths by sediment impoundments. Woahink Lake, lying on a marine terrace, was formed in this manner. The stream system that was inundated to form the lake basin was probably a tributary of the ancestral Siltcoos River.

The water surface of Woahink Lake is 38 feet above mean sea level, and the bottom at its deepest point is 74 feet deep, or about 36 feet below sea level, the lowest of any of the sand-dune dammed lakes on the Oregon coast.

The lake empties southward into adjacent Siltcoos Lake through the Woahink Creek outlet. The total drainage basin is only 7.4 miles in area, 16.6 percent of it covered by the lake itself. Thus the hydrologic retention time is relatively long. The small drainage basin is, for the most part, covered with a coniferous forest and receives approximately 80 inches of precipitation annually.

Adjoining the lake on the west side is a series of dune complexes, including a large active dune to the southwest which is very apparent along the west side of the highway. This area is within the Oregon Dunes National Recreation Area. The eastern shoreline abuts the foothills of the Coast Range. Most of the basin consists of privately owned land and about 85 percent of the shoreline is also in private ownership. A number of lakefront residences, including both summer cabins and permanent dwellings, are located on the shoreline, most of them on the eastern side which is interlaced with a network of local roads.

The remainder of the shoreline, about 15 percent of the length along the northwest side, is within the Jessie C. Honeyman State Park. This is a 522 acre park which is an excellent nature sanctuary despite the large amount of development for recreational purposes. Dense growths of shrubs, three small lakes, the beach, and the forest provide food and habitat for a great variety of birds and mammals. A nationally endangered species of pitcher plant (Darlingtonia californica) grows in the bog area within the park, and mixes with other typical bog species, such as sedges and water lilies. The bogs water supply is maintained by several small creeks entering from the north and by a naturally high water table. The Nature Conservancy has identified the bog as a critical natural area in need of protection. Darlingtonia is now protected only in the Darlingtonia Botanical Wayside, and the pitcher plant will not last long as a species if this protected site remains as small as it is. A larger bog is needed to provide sufficient buffer for the population.

The above average mean depth of Woahink Lake (32.6 feet) makes it somewhat less susceptible to adverse cultural impacts than many other coastal lakes in Oregon (Larson 1970). Nevertheless, its current oligotrophic status and popularity with many users make it particularly vulnerable to cultural eutrophication. Also, several land development projects have denuded large areas that adjoin the lake, generating exposed and unconsolidated material, much of which has eroded and washed into the water. Continued development will aggravate this problem of bank erosion. The control of future impacts on the quality of water in Woahink Lake is important for many reasons, not the least of which is that water for years has been pumped without treatment from the lake for drinking and domestic purposes. Any degradation in water quality would impair its use for this purpose, and would also impair enjoyment of the lake for recreation.

Siltcoos Lake, with a surface area of 3164 acres, is the largest lake on the Oregon coast. Not much is known about the origin of the name except that it is said to be the name of a local indigenous chief. It was formerly spelled Tsiltcoos. Like other lakes in the area it was formed by fluctuating sea levels during and after the most recent ice age. Siltcoos Lake is very shallow; the water surface is about eight feet above mean sea level and the deepest point in the lake basin is about 14 feet below sea level.

Tahkenitch Lake has a surface area of 1674 acres. The name is from an Indian word that is said to mean "many arms". Because of the lakes highly dendritic shape it has a shoreline of over 25 miles. The mean depth is 10.9 feet, with a maximum depth of 23 feet.


Johnson, Daniel M. et al. Atlas of Oregon Lakes. Oregon State University Press, Corvallis, OR. 1985.

Baldwin, E.M. Geologic Map of the Lower Siuslaw River Area, Oregon. U.S. Geological Survey, Oil and Gas Investigations Map OM 186. 1956.

Cooper, W. S. Coastal sand dunes of Oregon and Washington. N.Y. Geol. Soc. Am. Memoir 72. New York, NY. 169 pp. 1958.

Compiled by John Ame, Science Writer (2007)