Valley Habitats

Valley Oak Savanna

Photo courtesy of the Marin History Museum; Brady Collection (Aero Photographers, Inc.).

Valley oak savanna is one of the signature habitats of alluvial valleys of the San Francisco Bay region. This habitat historically consisted of large scattered valley oaks (Quercus lobata) surrounded by expanses of native grasses and forbs (small herbaceous plants). Many of the native grasses have now been replaced by non-native species, but the greater threat to the integrity of this habitat has been urbanization and the associated loss of the valley oak trees.

Valley oak savanna was historically a much more prominent habitat type in the valleys of the Miller Creek Watershed. The grand valley oaks standing among fields of native grasses and wildflowers would have been emblematic of the watershed. The historical extent of valley oak savanna is estimated based on historical aerial photos, historical maps as well as historical ecology studies of similar watersheds in the region. Individual trees were mapped from the historical data, and their identification was verified by examining surviving specimens in the field. All evidence indicates that valley oak savanna extended over almost the entire valley floor.

Valley wetlands

Photo courtesy of the Marin History Museum; Brady Collection (Aero Photographers, Inc.).

Valley wetlands is a broad category. It includes seasonally wet grasslands, seasonal and perennial freshwater marshes, as well as vernal pools and swales. While patches of this habitat were not mapped at this scale, these habitats probably existed in the lower reaches of the valley between the area now occupied by Highway 101 and the historical upland margin of the tidal marsh. Oblique aerial photos from the Brady Collection (1955) clearly show vernal pools or swales and a prominent seasonal wetland (or perhaps a large vernal pool) on the Silveira properties.