Something that is riparian is something that is next to, or directly associated with a river or other watercourse. In ecology or horticulture, a plant or ecosystem is 'riparian' if it grows in the thin corridor of moistness along a watercourse. Riparian habitats may not stand out in moist areas, but in the desert they are seen as thin sinuous strings of green winding across alluvial fans and through the bottoms of valleys. Riparian habitats in the western US are often associated with trees like Cottonwood, Birch, Willow, Alder, and sometimes oak, as well as grasses, sedges, and reeds. Many animals depend heavily on these areas since not only do they offer water but dense vegetation as well. Riparian habitats are one of the most endangered types around because they are threatened by development, siltation, diversion, dams, and other modifications of hydrology. Obviously, once the river is removed, the riparian ecosystem dies.