San Geronimo Creek is a stream in Marin County, California

Marin County, California

Marin County is a county located in the North San Francisco Bay Area of the U.S. state of California, across the Golden Gate Bridge from San Francisco. As of 2010, the population was 252,409. The county seat is San Rafael and the largest employer is the county government. Marin County is well...

, United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, which feeds into Lagunitas Creek

Lagunitas Creek

Lagunitas Creek is a stream in Marin County, California and is a major supply of drinking water to the area. The 20-mile long creek's headwaters are in the Coast Range and it flows into the Pacific Ocean, draining 103 square miles .The creek's source is the northern slope of Mount Tamalpais, a few...

 below Kent Lake

Kent Lake

Kent Lake is a reservoir in western Marin County, California formed by the construction of Peters Dam across Lagunitas Creek. Built in 1954, the lake is the most recent lake of the Marin Municipal Water District, as well as the largest...

.

Course

The creek rises in the hills west of the town of Lucas Valley

Lucas Valley-Marinwood, California

Lucas Valley-Marinwood is a census-designated place in Marin County, California, United States. As of the 2010 census, the population was 6,094.-History:...

. It descends northward into the San Geronimo Valley

San Geronimo Valley

San Geronimo Valley is a valley in Marin County, California, composed of four unincorporated towns: Woodacre, San Geronimo, Forest Knolls, and Lagunitas. Residents of San Geronimo Valley and surrounding areas refer to it as "The Valley." It is a fairly close-knit community, with a rather liberal...

 and the town of Woodacre

Woodacre, California

Woodacre is an unincorporated town and census-designated place located in the San Geronimo Valley in Marin County, California. Woodacre is located south-southwest of Novato, and 8 miles northwest of San Rafael, the nearest incorporated town is Fairfax, 4 miles east-southeast. Woodacre has the...

, then turns westward and parallels Sir Francis Drake Boulevard

Sir Francis Drake Boulevard

Sir Francis Drake Boulevard is an east–west arterial road in Marin County, California, running from just west of the Richmond-San Rafael Bridge to the trailhead for Point Reyes Lighthouse at the end of the Point Reyes Peninsula.It is named for the English explorer Sir Francis Drake, whose...

 through Roy's Redwoods, the towns of San Geronimo

San Geronimo, California

San Geronimo is a census-designated place located in the San Geronimo Valley in Marin County, California in the United States. San Geronimo is located southwest of downtown Novato, at an elevation of 292 feet...

, Forest Knolls, and Lagunitas

Lagunitas-Forest Knolls, California

Lagunitas-Forest Knolls is a census-designated place, composed of two unincorporated areas in the western half of the San Geronimo Valley in Marin County, California, United States. The population was 1,819 at the 2010 census....

. It then turns southwestward and empties into Lagunitas Creek on the eastern edge of the Samuel P. Taylor State Park

Samuel P. Taylor State Park

Samuel P. Taylor State Park is a state park located in Marin County, California. It contains approximately of redwood and grassland. The park contains about of old-growth forest, some of which can be seen along the Pioneer Tree Trail.-History:...

.

Ecology

San Geronimo Creek is within the Lagunitas Creek Watershed, which is home to the largest-remaining wild run of coho salmon (Oncorhynchus kisutch) in Central California. These coho are part of the "Central California Coast Evolutionarily Significant Unit," or CCC ESU, and are listed as "endangered" at both the state and federal level.

Efforts are also being made to protect and restore streams and habitat in the San Geronimo Valley, where upwards of 40% of the Lagunitas salmon spawn each year and where as much as 1/3 of the juvenile salmon (or fry) spend their entire freshwater lives.

Spawning salmon and trout may be viewed at Rory's Pools, located about 5 miles (8 km) west of Fairfax

Fairfax, California

Fairfax is an incorporated town in Marin County, California, United States. Fairfax is located west-northwest of San Rafael, at an elevation of 115 feet...

. There are jump pools and a fish ladder.

Bridges

At least seven bridges span San Geronimo Creek:

• at Meadow Way, 0.5 mile (0.80467 km) south of Sir Francis Drake Boulevard, a concrete continuous slab 70 feet (21.3 m) long, built in 1965.
• at Lagunitas Road, 0.03 mile (0.0482802 km) south of Sir Francis Drake Boulevard, a prestressed concrete tee beam 67 feet (20.4 m) long, built in 1964.
• Mountain View Road at Sir Francis Drake Boulevard, a steel bridge 51 feet (15.5 m) long, built in 1962 and reconstructed in 1974.
• at Montezuma Road 0.03 mile (0.0482802 km) south of Sir Francis Drake Boulevard, a concrete culvert 33 feet (10.1 m) long, built in 1938.
• at Sir Francis Drake Boulevard 7.6 miles (12.2 km) east of State Route 1
  California State Route 1
  State Route 1 , more often called Highway 1, is a state highway that runs along much of the Pacific coast of the U.S. state of California. It is famous for running along some of the most beautiful coastlines in the world, leading to its designation as an All-American Road.Highway 1 does not run...
  
  , a concrete tee beam 32 feet (9.8 m) long, built in 1929 and reconstructed in 1993.
• at Creamery Road, 0.04 mile (0.0643736 km) south of Sir Francis Drake Boulevard, a concrete Tee Beam 23 feet (7 m) long, built in 1948.
• at San Geronimo Valley Drive, 0.04 mile (0.0643736 km) east of Nicasio Valley Road, a concrete tee beam 22 feet (6.7 m) long, built in 1929.