Adjustment of Riparian Vegetation to River Regulation in the Great Plains, USA

The Missouri River and the Platte River provide opposite examples of the way riparian vegetation responds and adjusts to regulation by dams and diversions.Populus-Salix woodland has expanded rapidly into Platte River channels, while it has failed to regenerate in gaps between reservoirs along the upper Missouri River. This divergent response is the result of different geomorphologies and water-use patterns. The Platte River is a braided-type stream with a significant portion of its flow diverted for cropland irrigation. The Missouri is a meandering-type stream with low irrigation usage. I developed a graphical model that characterizes the different ways that riparian vegetation has adjusted to regulation. The model identifies two time periods: pre-regulation and post-regulation adjustment, with the latter divided into phase 1 and phase 2 subperiods. In the pre-regulation period, woodland composition shifts according to weather extremes and climate change. During phase 1, braided rivers adjust by channel-narrowing and expansion of pioneer woodland (Populus-Salix), while meandering rivers cease meandering. During phase 2, after major geomorphic adjustments are complete, both types of rivers, show sharp declines in pioneer woodland Replacement communities in the new equilibrium (post-adjustment period) will be dominated by later successional woodland or grassland species. Geomorphic factors of importance to vegetation establishment adjust relatively quickly (decades), but the subsequent adjustment of vegetation through succession is relatively slow (century or more)