The Boulder Reservoir's swim beach reopened at 2 p.m. today following a two-day closure enacted when water quality testing revealed elevated levels of E. coli bacteria in the water, according to city officials.

The closure began at 5 p.m. Wednesday when staff members at the reservoir, 5565 N. 51st St., performed state-mandated testing of the water, according Boulder Parks and Recreation spokeswoman Jennifer Bray.

According to state guidelines, natural swimming areas must have their water tested for safety once every seven days and five times each month, at minimum, Bray said.

It is unclear what caused the E. coli spike, but the Colorado Department of Public Health and Environment on its website indicates bacteria can originate from runoff, malfunctioning septic systems or waste from other swimmers, pets or birds.

Last summer, the Boulder Reservoir swim beach was closed from July 12 to 17 for heightened bacteria levels attributed to animal waste that had washed into the water.

People with questions or concerns about bacteria levels in the reservoir are asked to contact the Boulder County Public Health at 303-441-1564.

The swim beach will be open from 10 a.m. to 6 p.m. Saturday and Sunday.

Contact Camera Staff Writer Joe Rubino at 303-473-1328 or rubinoj@dailycamera.com.