1990 could wind up as the third-worst water year for Idaho Power Co.

The utility said in a newsletter the drought will boost its cost of generating electricity in a system heavily dependent on hydroelectric facilities.Because of a long hot, dry spell, Idaho Power said it expects its annual electric generation mix for 1990 to be 53 percent hydro and 47 percent coal-fired. In a normal water year, Idaho Power usually generates two-thirds at hydro plants, which gives the utility has one of the lowest electrical rates in the nation.

The flow into Brownlee Dam, the storage reservoir for the Hells Canyon system, is estimated to average 12,069 cubic feet per second this year.

The lowest flow since the three-dam complex was completed 22 years ago was in 1988, 10,227 cubic feet per second. The second-worst flow on record was 1977, 10,653 cfs.