Fog drip may play a key role in the precipitation in the upper watershed. On Cahill Ridge, just west of San Mateo Creek and east of Pilarcitos Creek, at an altitude of 1,000 feet, Oberlander measured fog drip beneath tanoak ("Lithocarpus densiflorus"), coast redwood and three Douglas fir trees, the latter 125 feet tall. He found that the trees most exposed produced the most precipitation and in five weeks of measurement (July 20-August 28, 1951) fog drip below the tanoak produced 59 inches of precipitation, more than the total annual precipitation on nearby grasslands and chaparral. The Douglas fir produced 7-17 inches of fog drip and appeared to provide unique conditions supporting the orchids giant helleborine ("Epipactis gigantea") and phantom orchid ("Cephalanthera austiniae"), since these plants were found exclusively in these moist ridge tops.