Nicasio is a census designated place in Marin County, California. It is located 8 miles west-southwest of Novato, at an elevation of 194 feet.The Nicasio region is a hydrologic zone containing the four main drainages of Nicasio Creek up to the ridgelines defining their basin. This includes: the south fork of Nicasio Creek extending from Moon Hill, along the ridge separating Nicasio from San Geronimo and Samuel P. Taylor State Park, and extending northwest towards the northern end of Platform Bridge Road; the east fork of Nicasio Creek extending from Loma Alta and the ridge on the north side of Sir Francis Drake Boulevard, extending up through Big Rock and up towards Big-Rock Ridge; the Halleck Creek drainage, up to the ridges separating Nicasio from Novato; the northern drainages extending along the ridgelines of Rocky Ridge, Hicks Mountain and Black Mountain.A major landmark in the area is the man-made Nicasio Reservoir.