About Ada County
Boise, Idaho's capitol and largest city lies at the heart of Ada County. The foothills of the Boise Mountains begin at the north edge of Boise and cover the boundary along the northeast part of the county. Southward from Boise the lowlands of the Snake River Plain extend for many miles to the edge of the Snake River gorge, a portion of which forms the county's southwest boundary. Lucky Peak Lake, a reservoir on the Boise River, lies in the hills east of Boise. After passing the Luck Peak Dam, the Boise River flows northwest along the base of the mountains, through Boise, and and then westward out of the county. Some distance to the northwest it flows into the Snake River.
The heavily populated area of the county lies at the base of the mountains, and close to the Boise River. The southern part of the county is rural. The Birds of Prey National Wildlife Area covers areas close the the Snake River.
With a population of 300,904, as of the year 2000, Ada County is Idaho's most populous county. It covers 1,060 square miles. Interstate 84, which is also U.S. Highway 30, traverses the county along the south edge of Boise. U.S. Highway 20 and 26, a combined route, pass through Boise from the southeast towards the west. Idaho Highway 55 passes through the northwest corner of the county, heading northward into the mountains, and westward towards Oregon. Idaho Highway 21 follows the Boise River into the mountains and continues northeast.
Ada County is named after Ada Riggs, the first pioneer child born in the area and the daughter of H.C. Riggs, a cofounder of Boise.