Upper Reach
The upper one half mile (800 m) of San Bruno Creek is the most rugged and natural reach as the stream winds through steep canyons of about 45 degrees in slope. The underlying geological formation of this upper catchment basin is Pleistocene Colma Formation, which continues eastward in the basin under most of the San Francisco Bay Flood Plain. Principal flows of the creek are within the winter months of November to March, the only season of meaningful rainfall in the Bay Area. Groundwater in the middle to upper basin varies between about 70 to 150 feet (21 to 46 m), and flows generally eastward toward the Bay. This wooded upper reach watershed consists of a mixed oak woodland with coast live oak, Quercus agrifolia, as a dominant species. Other larger plants are the California Bay Laurel and the understory species Toyon. The understory also includes the following wildflowers: evening primrose (Renothera ovata), wild radish (Raphanus satira), wild cucumber (Marah fabaceus), scotch broom (Cytisus scoparius), miners lettuce (Montia perfoliata), fillaree (Erodium cicutarium), vetch (Vicia americana), sweet clover (Melilotus indius), lupine (Lupinus sp.), and California poppy (Eschscholzia californica).
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