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and supply food for insects, which are food for fish. When they die, the trees fall into the stream, creating "organic dams"-and the pools and rimes fish and insects rely on. Tree roots also help hold the streambanks in times of high water. As the river drops in elevation, the vegetation that grows along its banks changes. From about 7,000 to 10,000 feet, willow wetlands are an important riparian wetland type. Ileavers live and feed in many stands of willows and alders along the river, building dams and creating ponds. In many meas they have great- ly expanded the size of a wetland by spreading water out across the land. scape, keepi ng t he soil saturated and low in oxygen, so that wetland plants can thrive. During the dry season, part of the water stored in the wetland seeps back ioto the river, maintaining base river levels even in late summer and fall. These wetlands also provide food and cover for a variety of animals, from trout, moose, elk and otter to water- fowl, warblers and snipe. At about 8,500 feet, narrowleaf cottonwood forests begin to appear- often with an understory of red-osier dogwood. At about 6,500 feet, the Fremont cottonwood takes over as the dominant tree along the river. At this elevation, stream power rules the Yampa Valley. Over decades and cen- turies, the river "dances" back and forth across its floodplain, eroding banks, cutting new channels, dropping sediment to fonn new land and leaving behind oxbows-bends in the river cut off from the main channel as the Yampa seeks the easiest course down- hill. This dynamic is critical to the life cycle of three endangered fish found in the Yampa: the Colorado squawfish, the razorback sucker and the hump- back chub. Cottonwoods also depend on this cycle of flooding and erosion in order to reproduce. To germinate, cot- tonwood seeds must make contact with bare, wet soil produced by spring flood- ing. Once the new plants sprout,they grow very fast. They can send roots :l feet into the earth during their first (.- '? :t- '7' ~' r 71w blue ,If.msh('(/k, Ollt' of the species of birds f!lot depend 01/ Ihe YllIIl/Hl \; njHlriall area.\'. PhOfO by Wcw/r SllI/lIil ilI/d Holi H/dmki season, and they can be established trees 10 feet tall in a few years. Cottonwoods can survive the periods of/ow water that are also typical of low- land riparian areas if their roots reach underground water supplies. Then the next time a flood comes along, the force of the water rearranges enough of the floodplain that there arc bare spots for new cot- tonwoods to become established.