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Until September 1999, Roy's Dam impaired spawning migration of adult coho salmon and steelhead during winter stream flows. The returning coho adults at this location represent one of the last remaining runs within the Central California Evolutionary Significant Unit (ESU) which was listed as threatened in 1996 under the endangered species act. The Lagunitas Watershed, which encompasses San Geronimo Creek, is proposed as critical habitat for this ESU. This watershed drains into Tomales Bay, which is part of the Gulf of the Farallones National Marine Sanctuary.

NOAA and FishAmerica partnered with Trout Unlimited, the Marin County Department of Public Works, Mark Warner of Performance Excavators, Save the Valley (members of Watershed Preservation Network), Trout Unlimited, Marin Conservation League, Tomales Bay Association, SPAWN, and numerous other local parties to modify the defunct dam, which involved partial removal and alteration of the 10 foot high dam and the building of grade control weirs to facilitate unrestricted upstream migration of coho and steelhead.

The dam was first modified in 1997 by removing two feet of the crest height. In 1998, further work was accomplished that included erosion control, streambank stabilization, and stream cleanup to remove debris. To complete the dam modification, the existing dam face and spillway was excavated, new foundation supports were placed, and fish weirs were constructed to create a gradually descending set of pools from the current elevation of the dam crest to the tailwater below the spillway. Three weirs consisting of large boulders and reinforced concrete were placed at 10 to 15 foot intervals and span the width of the stream.

Total project's valued at $300 thousand when local contributions, volunteer labor and in kind goods and services are considered.

Streambed monitoring locations have been in place since 1997 to observe any degradation in the stream channel and associated substrate conditions. The monitoring will continue to determine the overall stability of the stream channel following the work at the project site. Visual observations and documentation of fish passage success and failures have been made at the project site since 1996, and will continue to determine the effectiveness of the modifications to the dam and the building of step pools. Volunteer organizations will continue to monitor the Roy's Dam Project for effectiveness in the coming years.


Watershed Preservation Network ~ PO Box 8, San Geronimo, CA 94963 ~ Email: