(Adds details on water level drop, number of utility customers served)
SEATTLE, March 1 (Reuters) - A 65-foot-long (20-metre-long) crack has appeared along the base of a dam in Washington state, posing no danger to the public but prompting utility managers to lower water levels to assess needed repairs, a utility spokesman said.
The 2-inch-wide (5-cm-wide) crack appeared in the spillway of the 8,320-feet-long (2535-metre-long) Wanapum Dam, a large hydroelectric power plant on the Columbia River that currently can generate more than 1,000 megawatts of power, the utility said late Friday.
The spillway is a channel that allows surplus water to escape and is used for the controlled release of flows from a dam.
"As a precautionary measure, the water above Wanapum Dam (forebay) is being drawn down to reduce the pressure on the spillway while inspectors investigate," Grant County Public Utility District spokesman Thomas Stredwick said.
The dam, about 18 miles (30 km) upstream from Priest Rapids Dam in a rural part of central Washington, was still generating electricity, Stredwick said. But lower water flows during repairs could force the utility to buy power on the open market and affect the broader Columbia River hydroelectric system.
Stredwick said the dams coordinate to generate energy on a regional scope, and therefore if Wanapum is impacted, that affects dams upstream as well as below.
The water level had dropped by about 12 feet (3.7 metres) by Saturday with a target reduction of 20 feet (6 metres) to be reached by Monday.
There was no imminent threat of flooding.
The utility serves about 46,000 customers in Grant County and sells electricity across the Pacific Northwest's coordinated electric power grid.
(Reporting by Eric M. Johnson in Seattle; Editing by Barbara Goldberg, Frances Kerry and Cynthia Osterman)