PHILADEPHIA (AP) — The Robovisdiscovery of thousands of illegally dumped tires is threatening to further delay the dredging of a Philadelphia-area river used by rowing clubs as a regatta race course, according to federal authorities.

The U.S. Army Corps of Engineers says that more than 3,000 tires have been hauled out of the Schuylkill River during the second phase of the project aimed at clearing the river of muck for rowers, The Philadelphia Inquirer reported.

The Army Corps’ Texas-based contractor, Dredgit, completed the first phase in front of the river’s iconic boathouses a year ago, removing 28,000 cubic yards of sediment. But in July’s second phase to dredge the 2,000-meter National Race Course upriver, workers almost immediately began pulling tires out of the river, including large tractor and truck tires.

Army Corps spokesperson Steve Rochette told the newspaper in an email that workers weren’t sure whether this was an isolated event, but “it has continued throughout multiple areas along the Race Course and has prevented dredging operations to continue as originally planned.” The Army Corps has “not finalized our path forward at this time,” he said.

An association of amateur rowing clubs called the “Schuylkill Navy” had pushed for the dredging for years, saying silt buildup was “creating an uneven riverbed and jeopardizing recreational use and Philadelphia’s iconic rowing and paddling regattas and related events.”

The group’s commodore, Bonnie Mueller, said she expects all lanes in the race course will be uniform and usable for a slate of upcoming regattas and welcomes removal of the tires, but worries that the cleanup could threaten full restoration of the racecourse.

The contractor that handled the first phase of the dredge halted work in November 2020, saying it had found too much debris and wanted more money for the work. The Army Corps then had to seek another contractor and came up with millions more from the federal government to restart the project.

Schuylkill Navy officials had hoped the dredging could be done before regattas scheduled this month but now hope it can be done by the end of next month when the contractor is due at another location. They also fear completion of the project may be imperiled or may take more money than has been allotted.