Matanuska-Susitna Borough (often referred to as the Mat-Su Borough) is a borough located in the U.S. state of Alaska. The borough is part of the Anchorage Metropolitan Statistical Area, along with the city and borough of Anchorage on its south. The Mat-Su Borough is so designated because it contains the entire Matanuska and Susitna Rivers. These rivers empty into Cook Inlet which is the southern border of the Mat-Su Borough. This area is one of the few agricultural areas of Alaska.