Decaying leaves in a drying streamflow could be the cause of the water in Nathanson Creek turning black over the weekend, according to experts with Sonoma Ecology Center.

Sonoma community members began noticing the unusual darkening of the water in the portion of Nathanson Creek that runs through the corner of Second Street East and East Napa Street on April 27, and began expressing concerns for fish and wildlife that reside in and around the creek. Sonoma Ecology Center staff were alerted and began investigating the situation.

On Facebook, a community member posted about black water, saying she didn’t smell anything, but her partner thought he got a whiff of wine. The post also stated the water running by the Larry Murphy France Street Bridge appeared to be normal.

Sonoma Ecology Center Executive Director Richard Dale said he evaluated the situation on April 28.

“What I saw [Monday] afternoon, I’d say was inconclusive,” Dale said.

The Sonoma Ecology Center manages the ecological health of Nathanson Creek, which is part of the Nathanson Creek Preserve, through contract with the City of Sonoma.

The executive director said he sent photos to the center’s Research Program Manager Steven Lee for input.

Lee said something could have been released in the water, but the discoloration was probably a result of a natural phenomenon.

“It’s likely just the normal phase that occurs as streamflow starts to decline and dry up, where the tannins leaching out of the fallen leaves start to concentrate in the diminishing flow, turning the water dark,” Lee said. “The organic matter breaking down causes a layer of hydrophobic oils to accumulate on the surface in pooled areas, leaving a sheen.”

After examining photos of the creek, Lee said he did not see any accumulated foreign material on the partially submerged vegetation, which would likely happen if something was dumped into the creek.

Sonoma Ecology Center staff are awaiting a report from the California Department of Fish and Wildlife.

“This can be a normal phenomenon for this time of year, so the report didn’t cause me to immediately jump out of my chair and run up there,” Lee said. “But I’m ready to be wrong if something proves nefarious.”

Sonoma Ecology Center Senior Project Manager Caitlin Cornwall said, “Our first hypothesis was that the water was dark because of tannins.”

The preserve is home to native plant species, which could be the reason for the creek’s discoloration.

“Tannins are chemicals that some native plants produce. Oak trees are rather famously rich in tannins, to the extent they are used in tanning leather,” Cornwall said.

The Sonoma Ecology Center hosts volunteer efforts to clean up Sonoma Valley waterways, for more information, visit sonomaecologycenter.org.

This is a developing story, check back for updates.

You can reach Staff Writer Emma Molloy at emma.molloy@sonomanews.com.