Miller Creek watershed is unique in that it has relatively intact riparian vegetation along the creek. The watershed has 30 miles of channels.
The Miller Creek watershed supports a variety of habitat types:
- Open ridge lands and grazing lands in the upper watershed with annual grasslands interspersed with oak-bay woodland and oak savanna with patches of chaparral
- Well-developed riparian plant communities from Miller Creek Middle School upstream towards Mt. Shasta Drive
- Residential and limited commercial development along the narrow valley floor
- Some of the largest remaining tidally-influenced marsh habitats in the bay region that support abundant waterfowl populations
The Marin Countywide Plan identifies Miller Creek as an important habitat connector linking natural areas through Miller Creek and Marinwood to the Bay.
East of Highway 101, the lower reaches of the watershed support saltwater marsh and brackish-water marsh, both subject to tidal action. Freshwater seasonal wetlands occur in areas that were once historical baylands. These areas were diked off to provide agricultural land and now support oat hay production.
The St. Vincent’s School for Boys and Silveira Ranch properties provide critical habitat within the lower Miller Creek watershed. The site supports oak woodlands, valley oak savanna, tidal and seasonal wetlands, historic diked tidelands, seeps and swales, the Miller Creek riparian corridor, and grassland habitats.
Pacheco Ridge at the upper elevations of site supports intact native oak woodlands, an important habitat resource and community separator. The central location of the site provides habitat connectivity between the Gallinas Creek watershed to the south, San Pablo Bay to the east, and the Hamilton tidal marshes to the north.
The watershed supports a number of special-status plants and animals. Noteworthy special status species include:
- Salt marsh harvest mouse
- San Pablo song sparrow
- Ridgway's rail
- California black rail
- Steelhead salmon
The Miller Creek watershed is also known to support seven native fish species, including:
- Steelhead salmon
- California roach
- Threepine stickleback
- Staghorn sculpin
- Prickly sculpin
- Riffle sculpin
- Sacramento sucker
Nonnative common carp were introduced and persist in the watershed.
The Las Gallinas Valley Sanitary District reclamation ponds support over 200 bird species. There are active heron and egret nest colonies on shrub-covered islands at the Las Gallinas Valley Sanitary District treatment plant at the east end of Smith Ranch Road. Black-crowned night-heron, snowy egret, and great egret have been observed nesting on the islands.
In 1960, Marinwood, Miller Creek, and associated communities organized a community services district responsible for fire protection, parks and recreation, street lighting and open space. The Marinwood Community Services District now owns 812 acres of open space in the watershed including part of the ridge between the Miller Creek watershed and Novato.
The developed area of the watershed fills the valley, and 13% of the watershed is within the City of San Rafael.
Large portions of the ridgeline are managed by the Marin County Open Space District and the balance held by private ranches.
Changes to the watershed
Historic grazing practices and recent channel incision has caused destabilization of tributary channels. Bank erosion in mainstem Miller Creek is widespread. This erosion is characterized by vertical banks with little to no riparian vegetation. Often this bank erosion jeopardizes private property and structures.
The creek has gone through two recent cycles of incision, or downcutting of its creek bed and banks since the arrival of Europeans:
- Mid 19th century creek vegetation clearing and grazing practices initiated the first cycle of channel downcutting.
- Installation of the Grady Bridge in 1941 started a second cycle in the 1940s.
Erosion of the the mainstem Miller Creek is happening due to several factors:
- Increased storm drain runoff from the creeks being placed into pipes during valley housing development
- Large volumes of sediment produced by the upper watershed reduces pool depths and degrades habitat for fish and other species in the lower reaches
Downstream of Highway 101 and the SMART railroad bridge, the channel was rerouted and channelized in the 1920s. The creek was routed to the south, extended, and placed into a narrow, leveed channel with two 90 degree bends before reaching San Pablo Bay.
There is local interest in realigning the creek east of the SMART railroad bridge to provide a more natural, direct connection to San Pablo Bay. This could be included in the restoration work at McInnis Park.
- Land & Water Management for Upper Miller Creek and Environs
- Post project evaluation, Miller Creek, California: assessment of stream bed morphology, and recommendations for future study
- Miller Creek Restoration Feasability Study Calfed Proposal 1997 category III
- St. Vincent's/Silveira Advisory Task Force Recommendations
- Channel Stabilization and Restoration Design for Two Sites on Miller Creek Marinwood CSD Reach, San Rafael, CA
- Summary of channel assessment and design for Miller Creek Lassen-Shasta Reach (aka Darwin Reach)
- Baylands Ecosystem Habitat Goals: A Report of Habitat Recommendations
- Robert A. Leidy, Ecology, Assemblage Structure, Distribution & Status of Fishes in Streams Tributary to the San Francisco Estuary
- California Department of Fish and Game (CDFG). 2008. California Natural Diversity Database. California Department of Fish and Game, Sacramento, CA.
- Annotated Atlas and Implications for the Conservation of Heron and Egret Nesting Colonies in the San Francisco Bay Area