Abstract
The majority of the world's floodplains are dammed. Although some implications of dams for riverine ecology and for river channel morphology are well understood, there is less research on the impacts of dams on floodplain geomorphology. We review studies from dammed and undammed rivers and include influences on vertical and lateral accretion, meander migration and cutoff formation, avulsion, and interactions with floodplain vegetation. The results are synthesized into a conceptual model of the effects of dams on the major geomorphic influences on floodplain development. This model is used to assess the likely consequences of eight dam and flow regulation scenarios for floodplain geomorphology. Sediment starvation downstream of dams has perhaps the greatest potential to impact on floodplain development. Such effects will persist further downstream where tributary sediment inputs are relatively low and there is minimal buffering by alluvial sediment stores. We can identify several ways in which floodplains might potentially be affected by dams, with varying degrees of confidence, including a distinction between passive impacts (floodplain disconnection) and active impacts (changes in geomorphological processes and functioning). These active processes are likely to have more serious implications for floodplain function and emphasize both the need for future research and the need for an “environmental sediment regime” to operate alongside environmental flows.
Highlights
The riverine landscape is generally defined as a longitudinally continuous corridor consisting of those parts of the landscape directly affected by the river
Geomorphic processes affecting the formation of floodplains have been well studied and indicate that floodplains are formed by a combination of relatively frequent within-channel or bankfull flows driving lateral channel migration and overbank flow processes driving vertical accretion and channel switching (e.g., [6,7,8,9])
This review has attempted to address this knowledge gap by drawing together literature from both dammed and undammed rivers to assess the potential for dams to impact on the geomorphology of floodplains
Summary
The riverine landscape is generally defined as a longitudinally continuous corridor consisting of those parts of the landscape directly affected by the river (i.e., channel, riparian zones, and floodplains). In the United States, dams have reduced peak discharges by an average of 67% [16], and 55% of large rivers in the United States have had a greater than 25% reduction in the mean annual flood [17] The importance of these events for ecological functioning of floodplains is well recognized (e.g., [18]). It is acknowledged that this does not encompass either the full range of floodplain types, or the full range of floodplain types which have been affected by dams This is justified in that meandering rivers form by far the greater part of the rivers across the globe [30], and as discussed below, these floodplain types are highly likely to be impacted by dams and flow regulation. Using this approach allows this paper to examine an interrelated set of floodplain processes
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