The Siuslaw basin

The Siuslaw basin drains 780 square miles along the central Oregon Coast. The headwaters are near Cottage Grove south of Eugene and the mouth is in the city of Florence. The bedrock geology is almost entirely Tyee Sandstone. The precipitation pattern is unusual in that the headwaters of the mainstem Siuslaw river receive approximately ½ (50 inches) that of headwater tributaries on the west side of the Oregon Coast Range (100+ inches). The salmonids found in the basin include: Coho salmon, Chinook salmon, chum salmon, steelhead, and cutthroat trout. The Siuslaw was historically one of the most productive per unit area coho basins in the western United States. The major basin factors responsible for the decline in the freshwater carrying capacity of the system include: timber harvest and road building on steep unstable slopes, diking and draining of wetlands, transportation arteries along and in riparian zones, and riparian timber harvest.

The Siuslaw and its major tributary Lake Creek drain 780 square miles primarily in the Oregon Coast Range (Siuslaw Watershed Council 2002). The headwaters of the Siuslaw transect the Coast Range Mountains and heads in the Willamette Valley south of Eugene (Armantrout 1997). In the headwaters of the mainstem Siuslaw the predominant surface geology is the Fisher and Eugene Formation of sandstone and silt (Walker and MacLeod 1991). The rest of the basin is overwhelmingly Tyee sandstone, which is the dominant surface geology of the central Oregon coast.

The precipitation pattern is unusual. The area with the highest precipitation, over 100 inches annually, is the summit of the Coast Range in the western half of the basin. The headwaters of Lake Creek and the Siuslaw mainstem both head in the Willamette Valley in the rain shadow of the Coast Range (Armantrout 1997). These areas receive less than ½ of the precipitation about 45 inches found at the summit of the Coast Range. Rain-on-snow events occur and sometimes are factors in large floods.

The predominant salmonids in the basin include: Chinook, coho, and chum salmon, and steelhead and cutthroat trout (Siuslaw Watershed Council 2002). The Chinook population is one of the healthiest stocks on the coast. It is approximately 80% of the 1900s population level. On the other hand, the rest of the salmonids are depressed (Siuslaw Watershed Council 2002). Coho salmon probably averaged over 150,000 fish annually with a large run being about 500,000 fish. During the 1990s the average coho run in the Siuslaw was 4,000-5,000 fish. The Siuslaw river is on the southern boundary of the distribution for chum salmon. The abundance of chum salmon varies greatly from year to year. Steelhead numbers are depressed considerably from their abundance in the 1920s. Resident cutthroat trout are distributed throughout the basin and their population strength is not known but it is not assumed to be depressed. On the other hand sea-run cutthroat are severely depressed. Only a few individuals are annually caught in the river. Historically, the Siuslaw was internationally renowned for its sea-run cutthroat fishery.

The major basin factors responsible for the decline in the freshwater carrying capacity of the system include: timber harvest and road building on steep unstable slopes, diking and draining of wetlands, transportation arteries along and in riparian zones, riparian timber harvest, and stream cleaning (Siuslaw Watershed Council 2002). . Riparian timber harvest and the diking and draining of wetlands began with European settlement in the 1850s. The Siuslaw settlement history is unusual in that the headwaters in the Willamette Valley were settled first. European settlement of the mouth of the river did not occur until the late 1870s. Timber harvest and road building on steep slopes and headwalls began in earnest in the 1950s. It is claimed that the timber harvest levels from the Mapleton Ranger District from 1950 to 1990 exceeded the timber harvest levels of any area of equal size in the world including the tropical rainforest. The building of roads and rail lines began in the 1880s. These activities resulted in increased large debris flows. Also, stream cleaning in the 1960s to 1980s simplified the stream habitats and reduced the ability of the stream channels to stop and hold sediment and organic matter during large floods.

Literature Cited:

Armantrout, N.B. 1997. Watershed Analysis and Restoration in the Siuslaw River, Oregon, USA. Department of the Interior. Bureau of Land Management.

Siuslaw Watershed Council. 2002. A Watershed Assessment for the Siuslaw Basin.

Walker, G.W. and N.S. MacLeod. 1991. Geologic Map of Oregon. U.S. Dept. of Interior. Geologic Survey.