Matanzas Creek Dam Rehabilitation

Project overview

The Sonoma County Water Agency (Sonoma Water), with the assistance of the United States Department of Agriculture Natural Resources Conservation Service (NRCS), is considering alterations to the Matanzas Creek Dam to bring the structure into compliance with current NRCS and Division of Safety of Dams (DSOD) design criteria. The proposed actions will improve dam safety and increase flood storage capacity for the City of Santa Rosa and surrounding communities. Matanzas Creek Dam was designed and constructed under the Central Sonoma Watershed Work Plan that was approved by Congress in 1958. Alterations funded by NRCS to bring the dam into compliance with current NRCS and DSOD criteria will require a supplement to the 1958 Plan. 

NRCS has prepared a supplemental watershed plan and environmental document (Plan-EA) to evaluate environmental impacts associated with rehabilitation of the Matanzas Creek Dam and comply with the National Environmental Policy Act (NEPA). Technical studies are complete, and the Plan-EA will be distributed for a 30-day comment period.

Project video

Project documents

The NRCS has completed this Draft Supplemental Plan-Environmental Assessment (EA) in accordance with the National Environmental Policy Act (NEPA) and NRCS guidelines and standards. Reviewers should provide their comments to NRCS during the allotted Draft Supplemental Plan-EA review period. Comments must be submitted by August 25, 2026, to become part of the Administrative Record.

The following documents open as PDFs and are available for public review. Physical copies of the Draft Plan-EA are available for review and inspection at Sonoma Water’s main office at 404 Aviation Blvd, Santa Rosa, CA 95403 and at the NRCS Petaluma Service Center at 5401 Old Redwood Highway North, Petaluma, CA 94954.

• Final Supplemental Watershed Plan No. 1 and Environmental Assessment (PDF)
• Draft Supplemental Watershed Plan No. 1 and Environmental Assessment (PDF)

Appendices

• Appendix A: Project Actions, Alternatives, Comments and Responses (PDF)
• Appendix B: Project Map (PDF)
• Appendix C: Supporting Maps (PDF)
• Appendix D: Investigation and Analysis Report (PDF) – Appendix D references the nine technical memorandums (below) and includes the following reports: D.2 PR&G Report, D.3 USFWS IPaC Species List, D.4 Summary of Environmental Conditions, D.5 Botanical Assessment, D.6 Aquatic Resources Delineation Report, D.7 Cultural Resource Survey Reports, D.8 Endangered Species Act Section 7 Consultation Documents, and D.9 Air Quality Analysis.
  • Technical memorandum 001: Existing Conditions Assessment (PDF)
  • Technical memorandum 002: Geologic Investigation Report (PDF)
  • Technical memorandum 003: Soil Mechanics Report (PDF)
  • Technical memorandum 004: Hazard Classification (PDF)
  • Technical memorandum 005: Existing Hydrology Analysis (PDF)
  • Technical memorandum 006: Spillway Stability and Integrity Analysis (PDF)
  • Technical memorandum 007: Alternative Development and Evaluation (PDF)
  • Technical memorandum 008: Frequency Flood Routing (PDF)
  • Technical memorandum 009: Economic Analysis (PDF)
• Appendix E: Other Supporting Information (PDF)

Supporting Documents

Final-Plan Environmental Assessment (EA)

• Notice of Availability
• Finding of No Significant Impact (FONSI)

Draft-Plan Environmental Assessment (EA)

• Matanzas Creek Dam fact sheet (PDF)
• Notice of Availability

Public Meeting

View the presentation

In-person public meeting

Date: Tuesday, August 11, 2026

Comment Period Closed

Comments were accepted during the public meeting and by email or mail from July 26 through August 25, 2026.