Cottonwoods (Populus sp.) and willows (Salix sp.) are generally limited to riparian landscapes in semi-arid regions of western North America. Water availability is a major determining factor for the establishment, growth and survival of these plants. Willows generally occur closer to the stream and at lower elevations than cottonwoods, suggesting reduced drought tolerance and increased flood tolerance. In the present thesis project, three related studies were conducted to investigate this hypothesis. Firstly, tolerable rates of water table decline and the impacts of the corresponding drought stress were investigated by growing cottonwoods and willows under water table decline rates from 0 to 12 cm/d. Willow saplings responded similarly to cottonwood saplings, but willow seedlings were more vulnerable than cottonwood seedlings to rapid rates of water table decline. In the second study, will saplings tolerated elevated water tables of 0 to 7.5 cm below substrate surface and the resulting flood stress for 152 days slightly better than cottonwood saplings. Finally, mature cottonwoods along Willow Creek, Alberta experienced water table decline from 1996 to 1998 due to water pumping in a nearby gravel pit; the water table recovered in 1999. The cottownwoods displayed physiological changes indicating drought stress in 1998 and recovered following restoration of the water table. This confirmed the cottonwoods' reliance on the water table as their primary moisture source. These studies indicate that the spatial separation of willows and cottonwoods may be particularly related to reduced drought tolerance of willows and these display only slightly increased flood tolerance of willos and these display only slightly increased flood tolerances. The studies confirm that both willows and cottonwoods are physiologically dependent on a sufficient riparian water table. / 85 leaves : ill. ; 28 cm.