sentences:  The upper 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.