POWAY, Calif. — The weeklong boil water advisory that impacted Poway residents last month was caused by a piece of rope, according to the city’s official incident report.

In the report, which city officials created for the State Water Resources Control Board, a rope somehow clogged a valve that was meant to separate a storm drain from the Poway reservoir (also known as a clearwell).

Days before the suspected contamination was reported, a storm dumped rain and snow on communities throughout San Diego County. “It is theorized that the storm drain temporarily backed-up due to a surge with storm water (1.95″ of rain was recorded at the WTP from 11/28-11-29) collected in the storm drain system, and caused a backflow over the weir and into the Clearwell,” the report said.

The valve in question typically remains closed to keep storm water out of the reservoir in the case of overflow. In this case, the city’s report says the rope created an obstruction that prevented the valve from closing all the way, allowing a path for storm water to leak into the reservoir.

City officials said it was unclear how the rope got stuck in the valve.

Poway residents first reported seeing discolored water coming out of faucets on Nov. 29. The reports led to Poway’s first citywide boil water advisory, which lasted until Dec. 6 as officials investigated the cause of the discoloration.

During the advisory, a state official told the San Diego Union-Tribune Poway would be cited for its use of an out-of-compliance water system.