Several grass fires burning on an island in the Sacramento Delta are causing lots of black smoke and prompting health officials to issue a health advisory for communities in East Contra Costa.

The fire was first thought to be burning on Sherman Island, but officials say its location has since been isolated to another, undetermined, island. The fire created huge plumes of thick black smoke and also created ash fall out in the Pittsburg and Bay Point area.

Health officials are warning people with pre-existing respiratory problems should stay indoors to avoid the ash, although no formal "shelter-in-place" warning has been issued. The flames were visible from Antioch's Rivertown district, where a scattering of people watched black columns of smoke rise on the opposite side of the Delta.

The wind, blowing in Antioch's favor, pushed the smoke away from town. In the mid afternoon, smoke began to encroach upon eastern Walnut Creek. No structures are threatened and no injuries have been
reported. Officials say they will let the fire burn out in its own.