The swim beach at Boulder Reservoir will be closed until at least 10 a.m. Friday after routine testing revealed elevated levels of potentially harmful bacteria in the water, according to Boulder officials.

The closure was announced over the Boulder Parks and Recreation Department Twitter account just after 5 p.m. today.

According to Parks and Recreation spokeswoman Jennifer Bray, the elevated bacteria levels were detected during state-mandated water testing performed late this afternoon.

Bray said reservoir staff members must wait at least 24 hours to retest the waters at the swim beach, 5565 N. 51st St., and may only reopen it once bacteria levels have receded.

The swimming closure has forced the cancellation of Thursday's Boulder Stroke & Stride race event. Without Limits Productions announced the cancellation on its Twitter account and website this evening, adding that pre-registered racers have the option to move their registration to a future week of the race series. A makeup race has been scheduled for Aug. 22.

It is not clear what caused the heightened bacteria levels, Bray said, but it is important that people stay out of the water until it can be confirmed the bacteria levels pose no health risk.

"This happens periodically, and we just want all our swimmers to be safe and healthy out there," she said.

Heightened bacteria levels forced swim beach closures at Boulder Reservoir twice last summer; once from July 12 to 17 -- a closure attributed to animal waste washed into the reservoir from rain runoff -- and a second time on July 26 and 27.