The lands south of the City of Vancouver, including the cities of Richmond and Delta, sit on the flat flood plain. The islands of the delta include Iona Island, Sea Island, Lulu Island, Annacis Island, and a number of smaller islands. While the vast majority of the river's drainage basin lies within British Columbia, a small portion in the drainage basin lies across the international border in Washington in the United States, namely the upper reaches of the tributary Chilliwack and Sumas rivers.