Northern California’s Sacramento and San Joaquin Delta are supposed to be an oasis for fish, waterfowl, and recreational boating, but state officials say there’s now a growing environmental and navigational danger: hundreds of abandoned boats with a costly clean-up price tag.

Steve Hampton with California Department of Fish & Wildlife said that there are 250 known vessels littering the Delta, but there could be many more below water. “Some of them are floating, some of them are sunk, some of them are partially sunk,” he told KOVR-TV.

“Responding to abandoned and derelict vessels is about 20% of our responses,” Hampton said. While recreational boats are the bulk of the debris field, the larger, commercial vessels pose the biggest risk. “They can leak oil, they have other hazardous materials on them,” Hampton said.

“It’s a shock,” said Steve Mayo, a coach with the River City Rowing Club, referring to when their vessels hit debris. “Our boats are low to the water, so when they hit it can be really dangerous for the kids too.”

The Sacramento County Sheriff’s Department is one agency trying to clean up the mess and are having some success. Using a state grant to hire contractors, they’ve hauled off about 100 vessels in the last 10 years. But that grant won’t pay to dispose of the larger commercial boats.

“There’s very limited funding for authority to remove these vessels, so a lot of them are just around,” Hampton said.

State lawmakers are working to pass a bill to address the issue. One idea is to require insurance for boaters and have stricter registration requirements.