Removing a couple feet from a portion of the top of the dam dropped water levels in the sediment basin while leaving trapped sediments in place.

With roughly 4 1/2 miles of stream already remediated below the sediment basin, a washout would have been a major setback in the project.

"It would have been a mess," Chavez said.

Constructed in 2007, the sediment basin enabled reconstruction work to progress simultaneously above and below Durant Canyon, where water from German Gulch enters Silver Bow Creek and significantly increases flows, said Ken Brockman, an engineer with the U.S. Environmental Protection Agency.

The dam and the contaminants it traps will be removed in the coming years as part of the Silver Bow Creek cleanup, he said.

Breaching the dam is expected to help prevent a washout for the rest of this year's runoff, even if water flows haven't yet peaked.

"It's my gut feeling that we haven't seen the peak yet," Brockman said.

Tuesday's flows on Silver Bow Creek were at or near a 50-year event - a rare occurrence for the stream, said Chavez.

Peak flows on Silver Bow Creek in recent years have topped about 300 cfs.