Yulupa Creek - Geology

Geology

Many of Yulupa Creek's high headwater tributaries are dry in the summer, since rainfall is highly seasonal, with most of the approximately 30 in (76 cm) of annual precipitation occurring between the months of October and April: however, Ledson Marsh retains some smaller pools of water throughout most of the year. Ledson Marsh has a southern outlet to the north fork of Yulupa Creek.

The Yulupa Creek watershed was below the Pacific Ocean floor as recently as the Miocene era, around which time massive uplift and volcanic action formed the massif which comprises the headwaters higher elevations of the watershed of today. Elevations within the drainage basin of Yulupa Creek range from about 430 to 1800 feet (130 to 550 meters) above sea level. Sandstone is the dominant rock type, as a remnant of the ancient sea floor. Slopes within the headwaters area commonly range from 15 to 30 percent, and lower reaches have gradients of two to five percent. One of the major soil associations within the upper elevation riparian zone is Goulding cobbly clay loam, which contains roughly 25 percent cobblestones, as well as some basaltic exposures, betraying the volcanic past of the Sonoma Mountains formation. Typical soil depths in the headwater area range from 14 to 20 inches (35 to 50 centimeters). Much of the soil type in the Yulupa Creek riparian zone consists of Laniger loam, with rhyolite outcrops, another relic of the igneous history.