Effects of Regulated Water Flows on Regen- eration of Fremont Cottonwood

The reduction in extent of riparian forests in the southwestern United States has been a topic of recent concern. The effect of dams on downstream river flow and the consequent modification of the riparian habitat was studied along the lower Salt River in central Arizona. Dams were found to change the magnitude of river flows and change the seasonal timing of flows in such a way that the habitat appeared less adapted for regeneration ofPopulusfremontii. Modification of river flow patterns, therefore, appears likely to have been a significant factor causing change in vegetation along the Salt River. Riparian forests in the southwestern United States have been reduced in extent in recent history (Johnson and Jones 1977). These habitats provide food and cover for many wildlife species and shade and forage for livestock because of the relatively wet conditions in an otherwise arid to semiarid environment. This study investigated the effects of releasing water from dams on the establishment of riparian vegetation. The hypothesis examined was that dams modify local conditions to the extent that some plant species find the riparian zone less favorable or even unfavorable as habitat. Of particular interest was the effect of dams on patterns of seasonal flooding and how this influences seedling establishment. Seasonal flooding is an important factor in Fremont cottonwood (Populus fremontii) regeneration because of the tree’s short seed viability.’ Research on the effects of dams on riverbed morphology also has been conducted (Priest and Shindala 1969a, 1969b, and 1969~; Black 1973; Blench 1972); however, few studies related changes in riverbed morphology to subsequent vegetative changes (Warren and Turner 1975, Turner and Karpiscak 1980).