The South Santiam subbasin drains approximately 1,040 square miles. With steep, mountainous terrain in the east and a low floodplain to the west, the watershed is characterized by much variation in elevation, ecoregions, and land use practices. The mainstem river is 65 miles long. Including all tributaries, the watershed has a total of 1,013 stream miles coursing through it. Elevations in the basin range from 220 feet to 5,721 feet.

The watershed supports three communities (Sweet Home, Lebanon, and Scio), and is located in Linn County, Oregon. Approximately 68% of the basin is privately owned, and 32% in public lands. Land use in the watershed is 85% forest practices, 11% agriculture, and 4% urban/residential.

Two reservoirs, Foster and Green Peter, impound water on the South Santiam. Foster Reservoir is approximately 5.6 km (3.5 mi) long. Primary use of the reservoir is recreation in the summer and flood control in the winter and spring.

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