POWAY — A piece of rope “inexplicably” became lodged in a valve separating a 10-million gallon reservoir from a storm drain in late November, causing a nearly week-long, costly boil-water advisory in Poway, a report prepared by the city for the state concludes.

Officials say it is unknown how the sizable piece of rope got stuck in the valve, which was supposed to remain closed but instead became stuck open, thereby allowing muddy water to enter the city’s reservoir — called a clearwell — near Lake Poway during a storm.