Find any of these three species and you've also found water, either at or just below the surface. This is a result of cottonwoods' inclination to drop their fluffy, floating seeds just in time for spring runoff. The seeds travel downstream, lodge on exposed gravel as the river recedes and begin to grow. In return for the free ride, cottonwoods stabilize river banks. Aspen grows in pockets where groundwater accumulates rather than near sources of surface water.
Willows are among the most abundant small trees in Greater Yellowstone.
At higher elevations, they take the place of cottonwoods, stabilizing riverbanks and lakes and providing cover for small animals. Willow, one of the moose's favorite food sources, is particularly common on the marshy, flat valley floors that glaciers tend to leave behind. Also included are tiny ponds such as Kelly Hot Springs and the lakelike waters of the Snake River Oxbow in Grand Teton National Park. Then there are the natural lakes that have been enlarged by dams to store more water, including Henry's, Jackson and Hebgen, as well as man-made reservoirs such as Palisades and Island Park. The fish-bearing waters of Greater Yellowstone make up one of the world's most productive fisheries.