In southwest Alaska, the waters of the Yukon and Kuskokwim rivers flow through a vast "treeless plain," or tundra that forms the heart of the Yukon Delta National Wildlife Refuge. Almost 70% of this 19 million acre refuge is below 100 feet in elevation, and consists of a broad, flat delta stitched through with rivers and streams and dotted with countless lakes, sloughs and ponds. Bordering this expanse of tundra and wetlands are forest and shrub habitat and uplands sporting mountains more than 4000 feet high. The refuge also includes two large islands - Nelson and Nunivak.