A major artery supplying water to Cambridge sprung a leak yesterday, leading to a miniature flood on the sidewalk at the corner of Memorial Drive and River Street.

Water from the pipe flowed into the street, leading the police to stop traffic in the area early yesterday morning. By mid-afternoon, three of four lanes were open. Salt and sand truck were standing by to keep the road from freezing over after nightfall.

The 48" pipe, which crosses the river over the River Street bridge, is maintained by the Massachusetts Water Resources Authority (MWRA). It has provided Cambridge with its water for the past two years as a water treatment plant is constructed in Alewife.

Jonathan L. Yeo, a spokesperson for the MWRA, said the likely cause of the leak was the malfunction of a seal at a joint in the pipe, although he said he would not know for sure until workers tore up the sidewalk to fix the leak. He said they were planning to conduct repairs last night.

In response to the leak, the MWRA began bringing water into Cambridge through another water main. In order to do so, MWRA workers had to reverse the flow of water in the pipe.

As a result, Yeo cautioned that residents might find that their water had a "slight discoloration."

He said the discoloration, caused by tiny flecks of iron that are stirred up when water changes directions in a pipe, was a not a health hazard.

However, "it's not aesthetically pleasing," he said. The MWRA made efforts to notify residents of the possibility of discoloration, he said.