The San Lorenzo River is the defining geographic feature of the San Lorenzo Valley, flowing south through Ben Lomond on its way to the Pacific at Santa Cruz. In summer, locals and visitors use the river for swimming — there are good swimming holes in and near Ben Lomond, accessible from various pullouts along Highway 9 and the side roads that follow the river.
The river corridor supports riparian habitat including willows, alders, and coast redwoods, and the water runs clear and cold enough for steelhead trout. Winter storms can raise the river dramatically and flood the valley floor; in dry years it slows to a trickle in places. Spring and early summer offer the best combination of water volume and warm weather for swimming.
Access points include Ben Lomond Park and various informal spots along the highway. Follow local signage and check conditions before swimming; the river can be dangerous after storms.