With the western United States now in its second year of a widespread drought, the extraordinarily low water levels at some of the critical reservoirs are adding a layer of uncertainty to electric reliability this summer.

The power generated from the West’s fleet of hydroelectric generating facilities is on pace to be much lower than normal during what could be a record hot summer. Some generators could shut down if water levels at reservoirs fall under certain thresholds needed for safe operation.

Lake Mead, the reservoir formed by the Hoover Dam in Nevada, has already fallen to the lowest level in its 85-year history, curbing the amount of power being produced to serve customers in Arizona, Nevada and Mexico, according to the Bureau of Reclamation Lower Colorado...