St. Croix Ash Swamp features a range of forest types from mesic uplands adjacent to the St. Croix River through extensive low swamp to droughty uplands on the sandy plain above the river valley. The hardwood swamp is composed of basswood, black ash, American elm, yellow birch, white oak, and red maple with scattered white cedar, balsam fir, and white pine. On the forest floor are small pockets of standing water between the mossy hummocks. The flat sandy uplands are wooded with young oaks. The river valley contains very steep sides that rise nearly 100 feet above the swamp. Small spring fed streams and seepages have eroded small pockets and tributary valleys providing diverse microhabitats. The primary soils are Omega sand, Cathro muck, Rifle mucky peat, and Emmett loamy sand. Common nesting birds include winter wren, scarlet tanager, veery, ovenbird, red eyed vireo, and cerulean warbler, the latter here at its northern most range limit. St. Croix Ash Swamp is owned by the DNR and National Park Service. It was designated a State Natural Area in 1979 and later expanded to include the St. Croix National Scenic Riverway.