The San Lorenzo River (Spanish: Río de San Lorenzo) is a 29.3-mile-long (47.2 km) river in the U.S. state of California. The name San Lorenzo derives from the Spanish language for "Saint Lawrence" due to its reported sighting on that saint's feast day by Spanish explorers. Its headwaters originate in Castle Rock State Park in the Santa Cruz Mountains and flow south by southeast through the San Lorenzo Valley before passing through Santa Cruz and emptying into Monterey Bay and the Pacific Ocean.
Watershed and course The San Lorenzo begins as an intermittent stream around the precipitous slopes of Castle Rock Ridge, a major divide separating drainages of the San Francisco Bay area and the Pacific Ocean, in the Santa Cruz Mountains. Its headwaters is about 1 mile upstream of Castle Rock State Park on private property just south of the Saratoga Gap. Once inside the park's boundary, the perennial river flows approximately 1.5 miles before leaving the unit. Within state park boundaries, Craig Springs Creek, Tin Can Creek, and several small unnamed tributaries empty into the river. With its headwaters at an elevation of approximately 2,900 feet (880 m) above sea level, the San Lorenzo River drops 2,000 feet (610 m) in the first 3 miles. Starting just south of the Saratoga Toll Road Trail, the river is roughly paralleled by State Route 9 until it enters downtown Boulder Creek. The San Lorenzo reaches 500 feet (150 m) about 2 miles (3.2 km) above Boulder Creek and about 16 miles (26 km) from the mouth of the river. At this elevation, it is joined by Kings Creek from the left in the neighborhood of Redwood Grove, and Spring Creek from the right shortly thereafter. Small, steep tributaries feed the river from the west at Ben Lomond Mountain, while wider, more gently sloping tributaries feed the river from the east and northwest. The San Lorenzo River watershed drains 138 square miles (360 km2). The Branciforte Creek watershed is a major sub-basin of the San Lorenzo catchment-basin. The Newell Creek tributary was dammed to create Loch Lomond, a reservoir which supplies drinking water to Santa Cruz, California.
Discharge
The upper San Lorenzo generally receives more rainfall during the winter and early spring compared to the lower basin, discharging a higher than usual flow into the river mouth in Santa Cruz. Tributaries in the lower San Lorenzo are prone to atmospheric-river-caused floods, with these storms contributing to large volumes of run-off.
Geology As recently as the Paleogene period about 65 million years ago, much of California was still part of the Pacific Ocean. The San Lorenzo Valley was at the bottom of a shallow, tropical sea. Tectonic forces and faulting processes pushed up this flat ocean bottom into an ancient mountain range between 50 and 35 million years ago before sinking back into the shallow sea where it was covered with more sediments. About 30 to 25 million years ago, the Pacific Plate and North American Plate changed course and began moving sideways, creating a fault block called the Salinian Block. A second ancient mountain range began to rise during the Miocene, between 15 and 5 million years ago, which eroded and dropped back into the ocean, compressing sand and creating the Santa Margarita sandstone formation. Sediment deposits were laid during this event and epoch, about 10 million years ago, to create the three main rock types found within the San Lorenzo River basin: silt and clay became the layer of Monterey shale; the Santa Margarita sandstone; and as the sea deepened, the deposits became mud, forming the Santa Cruz mudstone layer. Tectonic forces from the convergence of the Pacific Plate with the North American Plate uplifted the land for a third time along the San Andreas Fault, pushing up the Santa Cruz Mountains between 3 and 4 million years ago. This tectonic compression caused uplift exposing older, predominately marine, Cenozoic sedimentary rocks to weathering, surface erosion, mass wasting, and landslides. The stream valleys and drainage networks of the San Lorenzo River watershed formed through the surface erosion and mass wasting of these sedimentary rocks during the end of the last Ice Age. Geologic erosion from these natural processes line most of the watershed's stream valleys. The Ben Lomond Fault is a 11-mile-long (18 km) subsidiary bedrock fracture within the San Andreas fault system, which had significant east–west vertical movement. The fault skirts the east and north sides of Ben Lomond Mountain, and is the fracture along which the Ben Lomond Mountain block was tilted up. Uplift of the southwestern block, which began about 40 million years ago, caused a vertical separation of approximately 600 feet (180 m) within the past 12 million years. Deformed rocks of late Cenozoic age indicate a continuation of faulting until about 85,000 years ago. As the Santa Cruz Mountains continued to rise, the San Lorenzo took advantage of rock that was fractured and weakened by faulting processes and began to cut the deeply incised San Lorenzo Valley. The river runs in or near the western edge, and sub-parallel, to the fault. Underlying rock strata in the San Lorenzo River basin include Lompico Sandstone and Vaqueros Sandstone.
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