 It is named for Ada Riggs, the daughter of H.C. Riggs, a member of the legislature; he established the county and was a co-founder of Boise.Canyon County, which originally included Payette County and most of Gem County, was partitioned from western Ada County in 1891.According to the US Census Bureau, the county has a total area of , of which is land and (0.7%) is water.