Riparian vegetation diversity along regulated rivers: contribution of novel and relict habitats

Abstract

1. The creation and maintenance of spatial and temporal heterogeneity by rivers flowing through floodplain landscapes has been disrupted worldwide by dams and water diversions. Large reservoirs (novel ecosystems) now separate and isolate remnant floodplains (relict ecosystems). From above, these appear as a string of beads, with beads of different sizes and string connections of varying lengths. 2. Numerous studies have documented or forecast sharp declines in riparian biodiversity in relict ecosystems downstream from dams. Concurrently, novel ecosystems containing species and communities of the former predam ecosystems have arisen along all regulated rivers. These result from the creation of new environments caused by upper reservoir sedimentation, tributary sedimentation and the formation of reservoir shorelines. 3. The contribution of novel habitats to the overall biodiversity of regulated rivers has been poorly studied. Novel ecosystems may become relatively more important in supporting riverine biodiversity if relict ecosystems are not restored to predam levels. The Missouri River of the north‐central U.S.A. is used to illustrate existing conditions on a large, regulated river system with a mixture of relict and novel ecosystems.

Dynamics of Plains Cottonwood (Populus deltoides) Forests and Historical Landscape Change along Unchannelized Segments of the Missouri River, USA

“…Rising groundwater and surface water levels linked to bed aggradation (USACE 2004) have converted cottonwood forest, cropland, and other land cover in the adjacent floodplain to emergent marsh, wet shrubland, and wet grassland; forced the relocation of the town of Niobrara, Nebraska in 1977 (Schneiders 1999); and currently threaten an existing state highway and other infrastructure (Coker and others 2009). Similar processes are occurring on other river-reservoir transitional areas (Johnson 2002), including between the Bad River confluence and Lake Sharpe near Pierre, SD (segment 6), between Bismarck, ND and the upper end of Lake Oahe (segment 4), and between the Yellowstone River confluence and the upper end of Lake Sakakawea near Williston, ND (segment 2) (USACE 1990(USACE , 1999(USACE , 2004NRC 2002).…”

“…Below Gavins Point Dam (segment 10), annual peak flows have been strongly reduced, seasonal patterns of high and low flows have been shifted, and flows in the historically lowflow seasons of fall and winter have increased, with similar changes noted on other segments (Galat and Lipkin 2000). Flows have been much less altered on segment 0, the most upstream segment in our study area, and considered a ''lightly-regulated'' remnant floodplain reach by Johnson (2002). This segment occurs downstream of smaller, mostly run-of-the-river dams on the mainstem Missouri (Canyon Ferry Dam) and on the Marias River (Tiber Dam), a major tributary.…”

“…Deltas have formed at the river-reservoir interface of the mainstem reservoirs, as sediments carried by the Missouri or tributaries are deposited in the slackwaters of the upper end of the reservoir and the adjacent river-reservoir transitional reach (Johnson 2002;Galat and others 2005;Coker and others 2009). This process is particularly notable downstream of where the Niobrara River, historically one of the largest providers of sediment to the upper Missouri, enters the Missouri just upstream of Lewis and Clark Lake (segments 8 and 9) (see Elliott and Jacobson 2006).…”

“…As noted by Johnson (2002), reservoir deltas are novel habitats resulting from the spatial patterning of tributaries and reservoir infrastructure. While these areas are dynamic and provide wetland and shallow water habitat, initial assessments suggest that they are low in plant diversity and are too wet to support significant areas of successful, longterm cottonwood establishment.…”

“…During multi-year periods of drought, delta areas and the shallow, upstream portions and margins of reservoirs that become exposed from declining reservoir levels (e.g., up to 70 km on upper end of Lake Oahe) may become suitable for recruitment of cottonwood and other woody riparian species. These areas are lost, however, during subsequent wetter periods when reservoirs can be filled to their full operating capacity (Johnson 2002). …”