[9] 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 Newell Creek tributary was dammed to create Loch Lomond, a reservoir which supplies drinking water to Santa Cruz, California.

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.

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.

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.

[14] 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.

[19] The Achistaca, an Awaswas Ohlone people, were located near present-day Boulder Creek and Riverside Grove of the upper San Lorenzo River area.

Beginning in the 18th century, contact with Europeans brought significant changes to the lifestyles of Native Americans in the San Lorenzo River basin.

The first European land exploration of Alta California, the Spanish Portolà expedition, gave the river its name when it passed through the area on its way north, camping near the west bank on October 17, 1769.

[8] Large western sycamore, black cottonwood, white alder, and California box-elder trees dominate the canopy and offer cooling shade for river inhabitants.

[27] Salmon and other fish populations declined dramatically after the establishment of water diversions, channelization, and dams that affect migration and rearing conditions.

Historically, coho were found in approximately 50 coastal drainages in San Mateo and Santa Cruz counties, but by the 1960s spawning runs were limited to 11 stream systems.

[29] In recent decades, continued growth has raised demand for water from the entire watershed system bringing attention to the need for further preservation of stream flows in order to protect young salmon, steelhead, and lamprey.

[32] Reduction in flow caused by dams, diversions (Felton and Tait), and evaporation losses from Loch Lomond Reservoir has had ecological consequences in the San Lorenzo River watershed and overall Santa Margarita Basin.

Historically, the delta with its large freshwater outflow and salt marshes provided an important breeding ground for aquatic species in the Bay.

[34] Groundwater use impacts on surface water baseflow supporting anadromous fisheries for coho salmon and steelhead, particularly in Bean and Zayante Creeks, is another issue.

[35] No flooding from the San Lorenzo River was reported when a flow event of nearly equal magnitude occurred in January 1982, but the water levels were much higher than one would expect, based on the original design.

The model was also used to predict transport of nitrates and other water pollution, and would be eventually used to study a variety of different land management and flood control strategies.

The plan called for creation of a continuous forest to be established on the upper banks of Valley Oak, Buckeye, Willow and other trees with seasonal foliage.