Final Supplemental Watershed Plan No. 1- Environmental Assessment for the Central Sonoma Watershed- Matanzas Creek Dam Rehabilitation Project

Final Supplemental Watershed Plan No. 1- Environmental Assessment for the Central Sonoma Watershed- Matanzas Creek Dam Rehabilitation Project

The document is the Findings of No Significant Impact (FONSI) for the Final Supplemental Watershed Plan No. 1- Environmental Assessment for the Central Sonoma Watershed- Matanzas Creek Dam Rehabilitation Project. The document describes how the US Department of Agriculture Natural Resource Conservation Service (NRCS) came to its conclusion that the project will not have a significant impact and that they approved the preferred alternative (alternative 3). The Preferred Alternative consists of full rehabilitation of the Matanzas Creek Dam. Rehabilitation includes raising the dam crest by 7.4 feet, raising the auxiliary spillway crest by 11.4 feet, grading the spillway to a uniform 200-foot width, removing accumulated sediment from the inundation area, installing embankment stability buttresses, and extending the primary spillway to accommodate the new buttresses. This alternative is the locally preferred alternative and the PR&G-recommended alternative. Project features were designed to utilize existing features as much as possible to reduce new disturbance and associated construction costs. The Preferred Alternative would meet current NRCS and California Division of Safety of Dams (DSOD) regulations and engineering standards associated with a high-hazard dam and ensure the useful life of the structure for at least 50 years. The storage capacity would be increased to route up to the 100-year 24-hour storm event without use of the auxiliary spillway. This alternative meets the purpose and needs of the project by providing flood protection and sediment reduction downstream of Matanzas Creek Dam in the most efficient manner. The Matanzas Creek Dam currently provides flood protection for the City of Santa Rosa and adjacent unincorporated areas in Sonoma County, California. It also captures approximately 3.8 acre-feet of sediment annually. Based on recent assessments, the dam does not meet NRCS and DSOD standards. The need is to bring the dam into compliance with current standards while maintaining flood protection and sediment reduction services. The Final Plan-EA is available in hard copy form at the NRCS Petaluma Service Center, 5401 Old Redwood Highway, Suite 100, Petaluma, CA. 94954 and the Sonoma County Water Agency at 404 Aviation Blvd., Santa Rosa CA. 95403. Basic data developed during the environmental assessment are on file and may be reviewed through coordination with NRCS by calling (530) 792-5510 (Attn: Small Watershed Program Matanzas Creek Dam Rehabilitation) or emailing at commentsCAwatersheds@usda.gov. The 30-day public review period for the Finding of No Significant Impact begins October 15, 2026. To request further information regarding this notice, email commentsCAwatersheds@usda.gov or contact Richard Rivas, Natural Resources Conservation Service, 430 G Street #4146, Davis, CA 95616, by telephone and leave a voicemail at (530) 792-5510 (Attn: Small Watershed Program, Central Sonoma Watershed Matanzas Creek Dam Rehabilitation Project).

Santa Rosa

Sonoma

Northern California

Other (Dam for Flood Control)

Finding of No Significant Impact for rehabilitation of a Dam used for flood control

Agriculture and Forestry Resources, Air Quality, Biological Resources, Coastal Zone, Cultural Resources, Drainage/Absorption, Flood Plain/Flooding, Geology/Soils, Hazards & Hazardous Materials, Hydrology/Water Quality, Mineral Resources, Public Services, Tribal Cultural Resources, Utilities/Service Systems, Vegetation, Wetland/Riparian