Floodplains are primarily located along the shores of the Potomac and Shenandoah Rivers, sometimes reaching inland along stream banks that drain into these rivers. Many forests on the lower floodplain are dominated by silver maple ( Acer saccharinum ), sycamore ( Platanus occidentalis ), and green ash ( Fraxinus pennsylvanica ). Upper floodplains are commonly covered with tulip poplar ( Liriodendron tulipifera ), sugar maple ( Acer saccharum ), hackberry ( Celtis occidentalis ), and white ash ( Fraxinus americana ). During a recent vegetation inventory, Shumard oak ( Quercus shumardii ) was recorded for the first time in the park along the banks of the Potomac. This tree is one of the largest oaks, preferring moist, well-drained, bottomland soils on stream and river banks.
Other herbaceous vegetation found along the water's edge include mistflower eupatorium ( Eupatorium coelestinum ), water-willow ( Justicia americana ), and halberd-leaved rose mallow ( Hibiscus militaris ). When walking higher on the floodplain, pawpaw ( Asimina triloba ), spicebush ( Lindera benzoin ), American bladdernut ( Staphylea trifolia ), and wingstem ( Verbsina alternifolia ) are some of the species likely to be encountered.