The upper reaches of the Russian River system originate in the rugged, mountainous terrain of the Inner North Coast Ranges, which include the coastal ranges to the west of Hwy 101 and the Mayacmas Mountains lying to the East. Being further from the coast, and experiencing a rainshadow effect, the ranges to the East are dryer and support large areas of chaparral dominated by Chamise (Adenostemma fasciculatum), Manzanita (Arctostaphylos spp.), and Scrub Oak (Quercus spp.).
Patches of serpentine are common and are characterized by conspicuous vegetation and plant communities, including many rare, endemic species that are completely different from those that surround them. Oak woodland and savanna occur at lower elevations on sites with deeper, more fertile soils and support the highest hardwood diversity in the state including common oak species such as Blue Oak (Quercus douglassii), Valley Oak (Q. lobata), Interior Live Oak (Q. wislizeni), as well as California Buckeye (Aesculus californicus), Bay Laurel (Umbellularia californica), and Pacific Madrone (Arbutus menziesii).
Exotic annual grasses and forbs dominate areas of grassland, although, depending on soil moisture availability and disturbance history, some sites contain strands of native perennial bunchgrasses such as California Oat Grass (Danthonia californica), California Fescue (Festuca californica), and Blue Wild Rye (Elymus glaucus).
The coastal ranges within the Russian River watershed to the West of Hwy 101 are influenced by summer fog, and therefore retain a bit more moisture than the lands to the east. The botanical makeup consists of a mosaic of North coast coniferous, upland broadleaf forest, and oak woodland vegetation types. Forested areas support a diverse mix of native hardwoods and conifers and except where recent clearing has occurred, the vegetation is structurally complex with well-developed tree, shrub and herbaceous layers. Douglas Fir (Pseudotsuga menziesii), Tanoak (Notholithocarpus densiflorus), Black Oak (Quercus kelloggii), and Pacific Madrone (A. menziesii) often dominate north and east-facing sites, whereas mixed hardwood – coniferous forests and woodlands occupy drier locations and include a variety of oak species, California Buckeye (Aesculus californicus), Bay (Umbellularia californica), and Canyon Live Oak (Q. chrysolepis). Common understory shrubs include several species of manzanita (Arctostaphylos spp.), Poison Oak (Toxicodendron diversilobum), and Coyote brush (Bacccharis pilularis). Riparian corridors are often lined with a diverse suite of wetland species including species such as willow (Salix spp.), Fremont cottonwood (Populus fremontii), alder (Alnus spp.), and big leaf maple (Acer macrophyllum).
Lower River to the Pacific
The lowest reaches of the Russian River system are located within the coastal fog belt of the outer North Coast Ranges and experience cool, moist, maritime conditions in contrast to the more continental climate of the inland valleys. A north coast coniferous vegetation type characterizes this region with tree species such as Redwood (Sequoia sempervirens), Douglas Fir (Pseudotsuga menziesii), and Tanoak (Notholithocarpus densiflorus). Patches of upland Coastal Prairie occur on south-facing slopes and ridges, remnants of a once vast and diverse grassland ecosystem that stretched along the northern California Coast.