Adjustment of riparian vegetation to river regulation in the great plains,USA

The Missouri River and the Platte River provide opposite examples of the wa y riparian vegetation responds and adjusts to regulation by dams and divers ions. Populus-Salix woodland has expanded rapidly into Platte River channel s, while it has failed to regenerate in gaps between reservoirs along the u pper Missouri River. This divergent response is the result of different geo morphologies and water-use patterns. The Platte River is a braided-type str eam with a significant portion of its flow diverted for cropland irrigation . The Missouri is a meandering-type stream with low irrigation usage. I dev eloped a graphical model that characterizes the different ways that riparia n vegetation has adjusted to regulation. The model identifies two time peri ods: pre-regulation and post-regulation adjustment, with the latter divided into phase 1 and phase 2 subperiods. in the pre-regulation period, woodlan d composition shifts according to weather extremes and climate change. Duri ng phase 1, braided rivers adjust by channel-narrowing and expansion of pio neer woodland (Populus-Salix), while meandering rivers cease meandering. Du ring phase 2, after major geomorphic adjustments are complete, both types o f rivers show sharp declines in pioneer woodland. Replacement communities i n the new equilibrium (post-adjustment period) will be dominated by later s uccessional woodland or grassland species. Geomorphic factors of importance to vegetation establishment adjust relatively quickly (decades), but the s ubsequent adjustment of vegetation through succession is relatively slow (c entury or more).