The Emergence of Reservoir Deltas in the Regulated Missouri River Opportunities for Cottonwood F
Most of the world’s large river flows are regulated by dams, altering the processes that sustain riparian ecosystem biodiversity and function. Numerous studies have documented declines in riparian vegetation extent and diversity along regulated rivers. Many reservoirs in large river systems, now decades old, are beginning to show their ages by the appearance of expanding delta formations at river/reservoir and tributary/reservoir junctions. These deltas are novel habitats that were not present in the former river system, and are governed by both river flow and sediment regimes and managed reservoir level fluctuations. Although largely unstudied, available evidence suggests that these delta habitats may support native riparian vegetation that is in decline elsewhere along regulated rivers. The delta formed at the confluence of the White River and Fort Randall Reservoir on the Missouri River in South Dakota represents a novel habitat where riparian forest has expanded during the post-dam era; however, expansion may be curtailed at times by high stages of Fort Randall Reservoir that cause forest mortality. Time-series analysis of riverine cross-sections indicated that there has been a trend of channel and floodplain aggradation within the post-dam delta, facilitating expansion of delta surfaces into and above the reservoir pool. Likewise, GIS analysis of historic aerial photography showed that forest area on the delta increased by about 50 percent during the post-dam era. Field inventories determined that a heterogeneous mixture of riparian forest exists within the White River delta, and that these forests are similar in structure and composition to those along natural river reaches. Current research will improve understanding of: (1) the contribution of novel deltas to biodiversity along regulated rivers and (2) how reservoir management and restoration efforts could improve rates of native riparian vegetation expansion and survival in novel delta habitats along regulated rivers.