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... Native plants are adapted to deal with extreme variations in flow and soil saturation, conditions that do not occur in the dynamic fashion that characterize unregulated hydro graphs in the Colorado River system. That is, in the predam environment, the riparian zone was large and only periodically or seasonally flooded. Hence, the natural plant succession that followed scouring flood events has , . been curtailed or lost along regulated streams as reflected in the narrow, undisturbed riparian corridor along the wetted perimeter of the river and its backwaters (Gregory et al. 1991). Maintenance of cottonwood (Populus deltoides; P. fremontil) gallery forests, that once characterized the floodplains of the pristine Upper Colorado River Basin, were dependent upon seasonal flooding and drying in the riparian zone. Seeds produced by cottonwoods in the spring were deposited with debris on the floodplain surfaces as flows declined after the spring spate. Gradually drying soils of fine riverine alluvium provided ideal substratum and water supply for germination and growth of seedlings. As a result of this unique coupling of the tree's life cycle with the annual hydrograph, trees of even age can be used to date the extent of past high flow events. Moreover, cottonwood leaves dropped in the fall and blown into the river provide an important allochthonous source of nutrients for riverine food webs. Only remnant forests remain today along the rivers of the Upper Colorado River Basin, owing to regulation of flow which limits distribution of seeds and conditions required for gennination. Agricultural activities such as grazing and tillage, and floodplain revetments also prevent establishment of cottonwood seedlings. Replacement of riparian forests of naturally reproducing cottonwoods and associated native plants by nonnative plants in a narrow fringe along the river corridor is a classic symptom of the severing of dynamic spatial and temporal connections between the river channel and its floodplain (Stanford and Ward 1986a, 1992a, 1993). Two questions require resolution with regard to riparian ecology and imposition of re- regulated flows in the Upper Colorado River Basin. First, how much flooding and what frequency of flooding does the riparian zone require in order to maintain native riparia? Fisher et al. (1983) showed that the Yampa corridor remains largely unchanged, although salt cedar has invaded throughout the lower half of the river. The 1983-84 high floods allowed cottonwoods to reseed 33