Matanzas Creek Dam rehabilitation: final plan out, federal review runs to November 14

By The Downspout · Posted Oct 15, 2026 · 3 min read

The U.S. Natural Resources Conservation Service has published the final plan to rehabilitate Matanzas Creek Dam above Santa Rosa, with a finding of no significant environmental impact. The documents are available for 30 days, October 15 to November 14, 2026, and no action to carry out the plan will be taken before then.

What happened

On October 15, 2026, the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) issued a Notice of Availability. It covers the Final Supplemental Watershed Plan and Environmental Assessment for the Matanzas Creek Dam Rehabilitation Project, plus a Finding of No Significant Impact. Sonoma Water is the project sponsor.

The dam protects the City of Santa Rosa and nearby unincorporated areas from flooding and traps about 3.8 acre-feet of sediment a year. The plan says it does not meet NRCS or California Division of Safety of Dams standards. The principal spillway can pass only the 25-year, 24-hour storm. The embankment is predicted to be vulnerable to liquefaction and seismic deformation in an earthquake.

NRCS selected the preferred alternative, called Alternative 3.

Raise the dam crest by 7.4 feet.
Raise the auxiliary spillway crest by 11.4 feet.
Grade the spillway to a uniform 200-foot width.
Remove built-up sediment from the area behind the dam, which adds about 50 years of sediment storage.
Add embankment stability buttresses and extend the primary spillway.

Flood storage would grow to 2,322 acre-feet. That is enough to pass the 100-year, 24-hour storm through the principal spillway without using the auxiliary spillway. About 48.2 acres would be disturbed.

The plan prices the project at $32,025,000 in 2022 dollars. Federal funds are $21,509,000 (67 percent). Other funds are $10,516,000 (33 percent).

What it means for you

The plan says the dam protects Santa Rosa and nearby unincorporated areas from flooding. It describes the work as bringing the dam up to current standards.

If you live near the dam, the plan says daytime construction noise would increase for up to 24 months. The nearest home is about 300 feet west of the dam. The plan also says more land behind the dam would need flood easements. The maximum flooded area would grow by 16.3 acres.

For the creek, the plan lists losses and fixes. About 0.2 acres of waters, about 0.62 acres of wetlands and 0.3 acres of streamside vegetation would be lost for good. That needs a Clean Water Act Section 404 permit from the U.S. Army Corps of Engineers. Disturbed areas would be reseeded. The California red-legged frog was not detected. The plan says the project “may affect but is not likely to adversely affect” it.

For how local agencies fit together, see who makes the water rules and local water agencies.

What happens next

The notice says no action to carry out the plan will be taken until 30 days after publication. The review window is October 15 to November 14, 2026.

The notice does not set a comment deadline or name a comment method for this final document. It lists these contacts for more information.

Email commentsCAwatersheds@usda.gov.
Call NRCS, Small Watershed Program, (530) 792-5510. Voicemail is accepted.
Mail: Richard Rivas, NRCS, 430 G Street, Davis, CA 95616.
Read hard copies at the NRCS Petaluma Service Center, 5401 Old Redwood Highway, Suite 100, Petaluma, or at Sonoma Water, 404 Aviation Blvd., Santa Rosa.

The finding says the work will go ahead only after final design and permitting, and depends on funding and permit approvals. The documents give no firm construction start date. The plan's funding schedule lists construction in 2026 and 2027. The documents do not say who pays the non-federal share.

Where this came from

The NRCS Notice of Availability and Finding of No Significant Impact, both dated October 15, 2026, and the Final Supplemental Watershed Plan No. 1 and Environmental Assessment, all posted on Sonoma Water's environmental documents page. The earlier draft was open for comment July 26 to August 25, 2026.

Sources

Checked 2026-10-15. Rules and programs change. If something here is out of date, the source above wins. Spot an error? See our corrections policy.