Geology
The Russian River is predominantly underlain by the Franciscan formation, a mélange of Jurassic-Cretaceous age, formed at the bottom of the Pacific Ocean over 100 million years ago. Franciscan sediments consist of a jumbled mass of muddy sandstones and cherts interlayered with basalt lava flows-crumpled sea floor sediments that form the bulk of the Coast Range. The Franciscan lithology is very unstable and landslides are common throughout most mountain regions within the basin. Elevations within the basin range from sea level at the mouth to 4,344 feet at the summit of Mt. Saint Helena in the Mayacamas Mountains to the east. Historic lava flow associated with Sonoma Mountain may have contributed to the isolation of the Russian River from the Petaluma and Sonoma Rivers (Hopkirk 1974). The river passes through a series of broad alluvial valleys and narrow bedrock constrictions along its course. Alluvial regions bordering the mainstem include the Ukiah and Hopland valleys in Mendocino County, and Alexander Valley and the Santa Rosa Plain in Sonoma County. The area within the basin consists of 85% hills and mountains and a mere 15% alluvial valleys (SEC 1996). Present drainage patterns in the Russian River region are similar to drainage patterns for the North Coast Ranges and are the result of Pleistocene down-faulting (Hopkirk 1974). Faulting in the North Coast Ranges follows northwest to southeast orientation, generally, and thus many streams (including the upper run of the Russian River) follow this orientation. With the onset of the Wisconsin glacial epoch, sea level changes combined with down-warping along the coast contributed to flow pattern changes as southeasterly flowing rivers of the area were redirected westward (Hopkirk 1974). Eventually the headwaters of the upper Russian River became the headwaters of the Eel, Navarro and Gualala river systems.
Perhaps the most striking character of the Russian River drainage is the sharp turn to the west that the mainstem takes near its confluence with Mark West Creek, where “After following for fifty miles its regular southeasterly course to Santa Rosa Valley, it turns away from this flat and uninterrupted alluvial plain which opens directly to San Francisco Bay, and flows westward to the ocean through twenty miles of rugged canyon, winding through a highland that varies from eight hundred to twelve hundred feet in elevation (Holway 1913).” Holway, in his 1913 paper, hypothesizes that a likely explanation for this is “that the transverse portion of the river from the open valley through the highland was antecedent to, and persisted through, the uplift which made the highland.” Historically, the waters of Clear Lake drained through two outflowing streams. Westward flows passed through Cold Creek into the Russian River, while Cache Creek drained the Eastern side of the Clear Lake Basin with flows eventually joining the Sacramento River. Flows from Cache Creek were eventually cut off by lava flows and water from Cache Creek joined with that from Cold Creek to flow into the Russian River (Hopkirk 1974). It is believed that within the past few centuries, however, a large landslide plugged the western Clear Lake outflow, isolating the lake from the Russian River basin (Alt 1975) and reestablished flows into Cache Creek through a sag in the lava flow near the mouth of Cache Creek. Present geology provides for the continued drainage of Clear Lake through its Eastern outlet. Historic flows from Clear Lake into both the Russian River and the Sacramento system explain why the fish assemblage in the Russian River today is so similar to that of the Sacramento system.
