Physical Setting of Willow Creek (Prunuske Chatham, Inc.)
Willow Creek flows out of an 8.7 square mile watershed into the Russian River approximately 2.3 miles upstream of its mouth at the coast. The stream flows in a northwest trending valley between two steep ridges of the Coast Range. Watershed elevations range from zero feet at the confluence to 1,481 feet at Koerber Peak. Precipitous canyons enclosing very steep, ephemeral channels characterize the upper section of the watershed. These canyons transition into broad ridges with steep slopes forming the valley walls. In the mid and lower section of the watershed a wide, flat alluvial valley through which Willow Creek flows separates the broad ridges and valley slopes.
Willow Creek, on the western edge of the Coast Range, is underlain by the Franciscan mélange and Great Valley complex. Both of these formations are composed of folded, sheared and metamorphosed near-shore sediments. The Great Valley complex is similar to that of the Franciscan in age and composition, except that it was not folded and metamorphosed to the same degree (Alt and Hyndman 1975). Common rock types in the Franciscan complex include greywacke, chert, greenstone, gabbro, serpentinite, rodingite, limestone, eclogite, and exotic blocks of other compositions (DPR 2004, citing Daly 1980). Steep slopes, shallow soils, and high erosion rates from slumping, landsliding, and slope wash are typical of the Franciscan portions of the Coast Range.
Concentrated landuse by European immigrants began as early as the 1830s with the Russian settlement in lower Willow Creek. The early settlers cleared the valley of riparian forest upstream of the second bridge area to develop agricultural fields, attempted to drain the wetlands, and diverted the spring-fed tributary flows into the valley. Intensive grazing as well as logging occurred on the ridges and tributaries. Between 1850 and 1900 the majority of the watershed was logged and lumber was hauled out by means of steam donkeys and a railroad that ran along the creek. Upper ridges with out forests were continuously grazed by cattle and sheep up until recently when property became State land.
By the 1940s the watershed appears to have recovered from early logging and intensive landuse. The watershed was in fairly good condition (from aerial photo analysis) with dense stands of second growth forest and little evidence of channel erosion or deposition (Trihey 1995). In the late 1950s through the 1960s the condition of the watershed declined precipitously with logging of much of remaining old growth and secondary growth from the lower redwood grove, within the inner gorge, all the way to the watershed divide. Large tracts were clearcut and tractor-yarded and small channels were used as skid trails (Trihey 1995). The clearcutting and tractor yarding on the steep, landslide-prone slopes combined with the removal of large woody debris from the small tributary channels led to extensive slope failures, gully erosion, and channel sedimentation in the upper watershed in the 1960s through the 1980s (Trihey 1995). Logging roads and the County road developed significant rill and gully systems during this period. The major storm events in 1982, 1995, and 1996 initiated many deep- and shallow-seated landslides in the inner gorge and transported large amounts of sediment to the main channel. Although the upper watershed is recovering, the impact of these landuse induced erosion and deposition processes can still be seen in continued gully development, road induced erosion, and large volumes of unconsolidated sediment moving through the system and in to lower Willow Creek.