Sonoma County Water Agency crews are scheduled to breach the sandbar at the mouth of the Russian River on Thursday morning to guard against the risk of flooding low-lying areas in the Jenner area.

The estuary was closed off in mid-October, a frequent occurrence in fall and spring, when ocean swells shift large quantities of sand across the mouth, blocking the river outflow and permitting water to pond behind the dam, agency officials said.

"The water surface elevation is rising pretty slowly, but we anticipate it's going to hit a level where it's going to start flooding things, maybe over the weekend," said Pam Jeane, the agency's assistant general manager.

Spokeswoman Ann Dubay said wildlife observers would be on-hand to ensure the safety of harbor seals and other marine mammals commonly seen around the beach, where they haul out by the dozens.

Heavy equipment is used in the process.