Thirty-five percent of the North Santiam River Basin is composed of older volcanics and tuffaceous sedimentary rocks and approximately 30 percent is younger basalts. The older, weathered basalts and volcaniclastic rocks may be responsible for the colloidal suspensions and resulting high turbiditities in Detroit Lake. Studies have identified assemblages of colloidal minerals in both the water column, and in alluvial fan and delta areas of Western Cascade streams and reservoirs. The upper basin is composed principally of High Cascade volcanics of the Pliocene age, with timber harvesting as the primary land use activity. The lower basin, below Mehama, is a large alluvial plain, except where volcanic and marine sedimentary rocks of the Eocene and Pliocene age are exposed in the foothills.
Agriculture is the primary land use activity in the lower basin.
Erosion potential from the banks is high, particularly where mature vegetation is not present, causing landslides and mass wasting during extreme precipitation events.