Downstream effects of dams on channel geometry and bottomland vegetation: Regional patterns in the Great Plains

The response of rivers and riparian forests to upstream dams shows a region al pattern related to physiographic and climatic factors that influence cha nnel 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 pri ncipal response of a braided channel to an upstream dam is channel-narrowin g, and the principal response of a meandering channel is a reduction in cha nnel migration rate. Prior to water management, braided channels were most common in the southwestern Plains where sand is abundant, whereas meanderin g channels were most common in the northern and eastern Plains. The dominan t response to upstream dams has been channel-narrowing in the southwestern Plains (e.g., sis of nine cases in the High Plains) and reduction in migrat ion race in the north and east (e.g, all of twelve cases in the Missouri Pl ateau and Western Lake Regions). Channel-narrowing is associated with a bur st 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. Thu s, riparian pioneer forests along large rivers in the southwestern Plains h ave temporarily increased following dam construction while such forests in the north and east have decreased. These patterns explain apparent contradi ctions in conclusions of studies that focused on single rivers or small reg ions anti provide a framework for predicting effects of dams on large river s in the Gnat Plains and elsewhere. These conclusions are valid only for la rge rivers. A spatial analysis of channel width along 286 streams ranging i n 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 geomet ries of small and large streams are affected differently by a sandy watersh ed.