Valley County is a rural county located in west central Idaho in the United States. Established in 1917, it was named after the Long Valley of the North Fork of the Payette River, which extends over 30 miles from Payette Lake at McCall south to Cascade to Round Valley. The valley was formerly a summer pasture for livestock from the Boise Valley. Since the completion of the Cascade Dam in 1948, much of the northern valley has been covered by the Cascade Reservoir. The population was 7,651 at the 2000 census; it was estimated at 9,862 in 2010. The county seat is Cascade, and the largest city is McCall.