The Mountain's Topography and Climate
San Bruno Mountain’s topography, climate and geology come together to provide a habitat that supports an array of rare, endangered and unique species of plants and animals. It has been cited as one of the most important and threatened biodiversity sites in the world (Edward O. Wilson, The Diversity of Life, 1999.) Elizabeth McClintock (1912-2004), a botanist at the California Academy of Sciences, considered the San Bruno Mountains a “botanical treasure.” In her publication, A Flora of the San Bruno Mountains (1990, California Native Plant Society), she recorded the diversity and rich heritage of the Mountain’s plant life that resulted from its topography, climate and geography.
The San Bruno Mountains stretch in a northwest direction across the San Francisco Peninsula at the northern end of San Mateo County. The San Bruno Mountains are separated from the Santa Cruz Mountains by the wide Merced Valley on its southern and western edges (now populated by Daly City, Colma and South San Francisco.)
The San Bruno Mountains are made up of 2 long ridges connected at the northwestern Saddle Area. The ridges are separated for most of their lengths by Guadalupe Valley (where the city of Brisbane and the Crocker Industrial Park are located.) The southeastern and highest ridge, with a 1,314 ft peak at the Summit, is usually referred to as San Bruno Mountain and extends 4 miles from the San Francisco Bay at the eastern Sierra Point to the edge of Daly City in the west. The northeastern ridge is known as Guadalupe Hills (Crocker Hills) and extends from Crocker Avenue on the west to Bayshore Boulevard on the east. The northeast ridge has been ground zero for recent battles to curb continued development pressure since it is known to contain prime habitat for the endangered Callippe Silverspot butterfly.
San Bruno Mountain rises steeply at its southern edge from the Merced Valley that is characterized by mostly flat lands and low lying hills. Slopes of 50 percent or greater dominate the Mountain, the steepest areas located around the Mountain’s edges and in the many ravines. The San Bruno Mountains are drained by three main watersheds that eventually empty into San Francisco Bay. Colma Creek Watershed is the largest, with its headwaters in the Saddle Area. Colma Creek is also fed by the April Brook tributary that drains the slopes and ravines west of the Summit. Guadalupe Creek, the main tributary of the Guadalupe Valley Watershed, has its headwaters in Wax Myrtle, Dairy and Romanzoffia Ravines. The Paradise Valley Watershed drains the southern slope of the Mountain.
The varied topography of San Bruno Mountain, with its exposure to either the Pacific Ocean or San Francisco Bay, creates microclimates with dramatically different weather conditions at various locations. In general, the cool and mild climate experienced along the coast of the San Francisco Peninsula is also found on the Mountain. The weather is influenced by the presence or absence of the offshore Pacific High. The Pacific High, as it moves further north in the summer or further south in the winter, is responsible for the subsequent presence or absence of the Bay Area’s infamous marine layer and coastal fog. As the Pacific High moves further north in the summer, cooler winds can move inland from the Pacific. This moist cold air forms a thick fog layer along the coast. Even though the summer is usually without rain, the fog drip alone can provide significant moisture for plants and animals. San Bruno Mountain is also exposed to strong offshore winds that result from the difference in temperature between the Pacific Ocean and the interior valleys. The winds assault exposed peaks and are forced around the Mountain through the San Bruno Gap to the south and the Alemany Gap to the north. Plants have evolved and adapted to these persistent winds by becoming smaller in stature and closer to the ground for protection. Varying soil depths, and the resulting availability of moisture, also determine plant size, with smaller plants on thin soils and larger plants on deeper soils.