A slow-moving fire in the Santa Maria Riverbed burned approximately 21 acres on Thursday afternoon, but crews achieved 80% containment by 6 p.m.

At approximately 1:40 p.m., personnel from the Santa Maria Fire Department and Cal Fire responded to the incident north of Santa Maria and west of Highway 101.

Firefighters said the blaze was moving at a slow pace in grass and heavy brush in the middle of the dry riverbed.

No structures were threatened by the blaze, Cal Fire said.

The fire had the potential to grow to 10 acres, according to firefighters’ initial report.

By 2:15 p.m., incident commanders canceled some of the resources including a helicopter.

At approximately 3:45 p.m., crews had stopped the fire’s forward progress and reportedly revised size estimate.

The cause of the fire remained under investigation Thursday.