Downstream effects of dams on channel geometry and bottomland vegetation: Regional patterns in the great plains

Abstract

Abstract:The response of rivers and riparian forests to upstream dams shows a regional pattern related to physiographic and climatic factors that influence channel geometry. We carried out a spatial analysis of the response of channel geometry to 35 dams in the Great Plains and Central Lowlands, USA. The principal response of a braided channel to an upstream dam is channel-narrowing, and the principal response of a meandering channel is a reduction in channel migration rate. Prior to water management, braided channels were most common in the southwestern Plains where sand is abundant, whereas meandering channels were most common in the northern and eastern Plains. The dominant response to upstream dams has been channelnarrowing in the southwestern Plains (e.g., six of nine cases in the High Plains) and reduction in migration rate in the north and east (e.g., all of twelve cases in the Missouri Plateau and Western Lake Regions). Channel-narrowing is associated with a burst of establishment of native and exotic woody riparian pioneer species on the former channel bed. In contrast, reduction in channel migration rate is associated with a decrease in reproduction of woody riparian pioneers. Thus, riparian pioneer forests along large rivers in the southwestern Plains have temporarily increased following dam construction while such forests in the north and east have decreased. These patterns explain apparent contradictions in conclusions of studies that ft-mused on single rivers or small regions and provide a framework for predicting effects of dams on large rivers in the Great Plains and elsewhere. These conclusions are valid only for large rivers. A spatial analysis of channel width along 286 streams ranging in mean annual discharge from 0.004 to 1370 cubic meters per second did not produce the same clear regional pattern, in part because the channel geometries of small and large streams are affected differently by a sandy watershed.

Riverbank changes along the Mekong River: Remote sensing detection in the Vientiane–Nong Khai area

“…The decrease in suspended sediment concentration in the lower Mekong River (Lu and Siew, 2006;Kummu and Varis, 2007) may increase erosional power or transport capacity (Brandt, 2000), and hence would eventually change the river's morphology. As sediment load has decreased, the river in the lower basin may increase its transport capacity (Friedman et al, 1998), and cause riverbank erosion. However, the decrease of suspended sediment rates in Nong Khai is not statistically significant (Kummu and Varis, 2007).…”

Multiple states in river and lake ecosystems

“…River regulation has facilitated the invasion of saltcedar in several ways. Upstream dams often cause a narrowing of braided stream channels that is associated with increased recruitment of both native and exotic species on the former riverbed (Friedman et al 1998). …”

The relation between geomorphological features and species richness in the low flow channel of the Warche, downstream from the Bütgenbach dam (Ardennes, Belgium)

“…Sometimes certain terrestrial species may adapt to these conditions of excessive hydromorphy (Riis et al, 2001;Bernez et al, 2004). However, when the hydrological impacts of dams result in a significant decrease in flood and low water discharges, the low flow channel may be invaded by non-aquatic vegetation, reducing its width (Williams and Wolman, 1984;Chang and Crowley, 1997;Friedman et al, 1998). On the other hand, when www.elsevier.com/locate/aquabot Aquatic Botany 85 (2006) [112][113][114][115][116][117][118][119][120] the discharge increases downstream from the dam, channel widening generally results (Church, 1995), which causes riparian vegetation to recede (Dominick and O'Neill, 1998).…”