Water, or the wetland's hydrology, is why the wetland exists. But it also is very elusive. It also varies between years. So scientists look for signs that water was there at some recent time and for extended periods of time. Indicators of hydrology include leaves that have turned black from being in the water for a long time, or silt marks on tree trunks.
Vegetation is a more dependable and useful indicator that a wetland is present.
Typical obligates include cattails, pond lilies, and skunk cabbage. Common facultative species include red maple and green ash. Finally, other types of plants, known as upland species, cannot grow and survive in situations where their roots are wet for long periods of time in the growing season. Examples are black locust, black oak, and multiflora rose. Soils are the other commonly used indicator that wetlands may be present.