SAN FRANCISCO (CBS/AP) — At least 30 fish were killed after a drinking water pipe burst and sent thousands of gallons of chlorinated water into a Peninsula creek.

The city’s Public Utilities Commission discovered the leak on Saturday near Crystal Springs Reservoir, part of a drinking water system that serves 2.5 million Bay Area residents.

READ MORE: 'Fire Hose' Of Subtropical Moisture Bearing Down On Northern California

Officials said they expect the fish death toll — which may have included threatened steelhead trout — to rise, possibly to hundreds.

READ MORE: Kid-Size Doses Of Pfizer’s COVID-19 Vaccine More Than 90% Effective

San Mateo Creek flows about five miles from the reservoir to San Francisco Bay.

Steven Ritchie, assistant general manager for water at the commission, said the cause of the break in the 1932-era pipe is still unknown.

MORE NEWS: Support Builds for KPIX Reporter Targeted in YouTuber's Racist Remarks

(Copyright 2013 by CBS San Francisco. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)