Just to be an a**hole, because it is so much fun, but the largest island in the Lower 48 is the Olympic Peninsula. When they dug Black Lake Ditch from the lake to Percival Creek they essentially made the OP and the Kitsap Peninsulas an island as they are surrounded by water. Black Lake to Black River to Chehalis River to the Pacific to the Straits, down the Sound to Capitol Lake, up Percival Creek to Black Lake Ditch to Black lake and point of beginning.