Effects of Moderate and Extreme Flow Regulation on Populus Growth along the Green and Yampa Rivers,Colorado and Utah

River regulation induces immediate and chronic changes to floodplain ecosystems. We analysed both short‐term and prolonged effects of river regulation on the growth patterns of the keystone riparian tree species Fremont cottonwood (Populus deltoides ssp. wislizenii) at three upper Colorado River Basin rivers having different magnitudes of flow regulation. We compared cottonwood basal area increment on (i) the regulated Upper Green River below Flaming Gorge Dam; (ii) the adjacent free‐flowing Yampa River; and (iii) the partially regulated Lower Green River below their confluence. Our goal was to identify the hydrologic and climatic variables most influential to tree growth under different flow regimes. A dendrochronological analysis of 182 trees revealed a long‐term (37 years) trend of declining growth during the post‐dam period on the Upper Green, but trees on the partially regulated Lower Green maintained growth rates similar to those on the reference Yampa River. Mean annual, mean growing season, and peak annual discharges were the multicollinear flow variables most correlated to growth during both pre‐dam and post‐dam periods at all sites. Annual precipitation was also highly correlated with tree growth, but precipitation occurring during the growing season was poorly correlated with tree growth, even under full river regulation conditions. This indicates that cottonwoods rely primarily on groundwater recharged by river flows. Our results illustrate the complex and prolonged effects of flow regulation on floodplain forests, and suggest that flow regulation designed to simulate specific aspects of flow regimes, particularly peak flows, may promote the persistence of these ecosystems. Copyright © 2016 John Wiley & Sons, Ltd.     

Floodplain forest dynamics: Half‐century floods enable pulses of geomorphic disturbance and cottonwood colonization along a prairie river

In dry ecoregions, trees are restricted to river valley floodplains where river water supplements the limited local precipitation. Around the Northern Hemisphere, cottonwoods, riparian poplars, are often predominant trees in floodplain forests and these ecological specialists require floods that create and saturate sand and gravel bars, enabling seedling recruitment. By pairing the interpretation of aerial photographs at approximately decade intervals with dendrochronology, we explored the coordination between river floods, geomorphic disturbance and colonization of plains cottonwoods (Populus deltoides) over eight meanders along the Red Deer River in the semi‐arid prairie of western Canada. This river has a relatively natural flow regime and minimal human alteration through the World Heritage Site of Dinosaur Provincial Park. We found that the 50‐year flood of 1954 increased channel migration and produced extensive accretion with downstream expansion of meander lobes and some channel infilling, which was followed by prolific cottonwood colonization. Those processes accompanied the major flood, while bank erosion and cottonwood losses were more gradual and continuous over the past half‐century. Results indicated even greater floodplain and woodland development after an earlier 100‐year flood in 1915. Each flood produced an arcuate band of mature cottonwoods and there were five to seven progressively older woodland bands across the floodplain, with each cottonwood age grouping increasing by about a half‐century. The 700 m wide floodplain was progressively reworked by the river through pulses of channel movement and floodplain and woodland development over approximately 250 years and correspondingly, the oldest cottonwoods were about 250 years old.     

Riparian vegetation and channel change in response to river regulation: a comparative study of regulated and unregulated streams in the Green River Basin,USA

The effects of river damming on geomorphic processes and riparian vegetation were evaluated through field studies along the regulated Green River and the free‐flowing Yampa River in northwestern Colorado, USA. GIS analysis of historical photographs, hydrologic and sediment records, and measurement of channel planform indicate that fluvial processes and riparian vegetation of the two meandering stream reaches examined were similar prior to regulation which began in 1962. Riparian plant species composition and canopy coverage were measured during 1994 in 36, 0.01 ha plots along each the Green River in Browns Park and the Yampa River in Deerlodge Park. Detrended correspondence analysis (DCA) of the vegetation data indicates distinctive vegetation differences between Browns Park and Deerlodge Park. Canonical correspondence analysis (CCA) indicates that plant community composition is controlled largely by fluvial processes at Deerlodge Park, but that soil chemical rather than flow related factors play a more important role in structuring plant communities in Browns Park. Vegetation patterns reflect a dichotomy in moisture conditions across the floodplain on the Green River in Browns Park: marshes with anaerobic soils supporting wetland species (Salix exigua, Eleocharis palustris, Schoenoplectus pungens, and Juncus nodosus) and terraces having xeric soil conditions and supporting communities dominated by desert species (Seriphidium tridentatum, Sarcobatus vermiculatus, and Sporobolus airoides). In contrast, vegetation along the Yampa River is characterized by a continuum of species distributed along a gradual environmental gradient from the active channel (ruderal species such as Xanthium struminarium and early successional species such as S. exigua, Populus deltoides subsp. wislizenii, and Tamarix ramossissima) to high floodplain surfaces characterized by Populus forests and meadow communities. GIS analyses indicate that the channel form at Browns Park has undergone a complex series of morphologic changes since regulation began, while the channel at Deerlodge Park has remained in a state of relative quasi‐equilibrium with discharge and sediment regimes. The Green River has undergone three stages of channel change which have involved the transformation of the historically deep, meandering Green River to a shallow, braided channel over the 37 years since construction of Flaming Gorge Dam. The probable long‐term effects of channel and hydrologic changes at Browns Park include the eventual replacement of Populus‐dominated riparian forest by drought tolerant desert shrublands, and the enlargement of in‐channel fluvial marshes. Copyright © 2000 John Wiley & Sons, Ltd.     

黄河下游造床流量的变化及其对河槽的影响

本文通过计算黄河下游的第一造床流量和第二造床流量,分析了黄河下游河槽的萎缩过程。第一造床流量与年水沙量的关系分析表明,黄河下游的深槽平浅滩流量700～1000m3/s是相对稳定的,恒定流的平衡输沙能力约为18～21kg/m3。通过小浪底水库拦粗排细年平均拦沙约3亿t,并在汛期强化调水调沙运用,基本可以控制黄河下游河槽的淤积萎缩。第二造床流量和平滩流量的关系分析表明黄河下游恢复和维持平滩流量约4000m3/s的中水河槽是比较合理的。根据小浪底水库调水调沙运用状况,提出了恢复和维持黄河下游稳定中水河槽的措施,并探讨了强化小浪底水库调水调沙运用的方案。