Vegetation dynamics on deltas developing at tributary-reservoir confluences on the Missouri River in South Dakota

Background/Question/Methods

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 where tributaries enter reservoirs. 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, field reconnaissance indicates that these novel habitats can support riparian vegetation similar to that of the former river and floodplain. The White River delta, formed at the confluence of the White River and Fort Randall Reservoir on the Missouri River in South Dakota, is an example of a novel riparian habitat. A time-series (1948-2012) of aerial photographs was classified using GIS to quantify changes in the age-area distribution of riparian woodland following closure of Fort Randall Dam in 1956. Digital elevation models and stream gage data also were used to investigate the time and space dynamics of the delta.

Results/Conclusions

The areal extent of riparian woodland within the White River delta has consistently increased during the postdam time period, with the exception of the interval spanning 1991 to 1998. Woodland area was 886 hectares in 1948, 8 years prior to complete closure of Fort Randall Dam. By 1983, woodland area had grown to 1,082 hectares, and continued to expand to 1,493 hectares by 2012. The rate of woodland expansion appears to have been driven by reservoir levels, sediment discharge from the White River, rising groundwater, and decreased woodland clearing since the late 1990s. Although woodland area increased between most photo intervals, there was turnover of some woodland patches, especially those at the lowest elevations vulnerable to prolonged reservoir inundation. Consequently, turnover of woodland in the lower delta has maintained a younger age component in the delta woodland. Future field studies will determine the structure and composition of riparian woodland within in the White River delta, as well as how novel habitats may help compensate for riparian habitat declines along regulated rivers. Further study will also reveal how reservoir pools may be managed to favor the establishment and growth of delta riparian habitats over the long-term.