Novel plant communities limit the effects of a managed flood to restore riparian forests along a large regulated river

Dam releases used to create downstream flows that mimic historic floods in timing, peak magnitude and recession rate are touted as key tools for restoring riparian vegetation on large regulated rivers. We analysed a flood on the 5th‐order Green River below Flaming Gorge Dam, Colorado, in a broad alluvial valley where Fremont cottonwood riparian forests have senesced and little recruitment has occurred since dam completion in 1962. The stable post dam flow regime triggered the development of novel riparian communities with dense herbaceous plant cover. We monitored cottonwood recruitment on landforms inundated by a managed flood equal in magnitude and timing to the average pre‐dam flood. To understand the potential for using managed floods as a riparian restoration tool, we implemented a controlled and replicated experiment to test the effects of artificially modified ground layer vegetation on cottonwood seedling establishment. Treatments to remove herbaceous vegetation and create bare ground included herbicide application (H), ploughing (P), and herbicide plus ploughing (H + P). Treatment improved seedling establishment. Initial seedling densities on treated areas were as much as 1200% higher than on neighbouring control (C) areas, but varied over three orders of magnitude among the five locations where manipulations were replicated. Only two replicates showed the expected seedling density rank of (H + P) > P > H> C. Few seedlings established in control plots and none survived 1 year. Seedling density was strongly affected by seed rain density. Herbivory affected growth and survivorship of recruits, and few survived nine growing seasons. Our results suggest that the novel plant communities are ecologically and geomorphically resistant to change. Managed flooding alone, using flows equal to the pre‐dam mean annual peak flood, is an ineffective riparian restoration tool where such ecosystem states are present and floods cannot create new habitat for seedling establishment. This problem significantly limits long‐term river and riparian management options. Copyright © 2010 John Wiley & Sons, Ltd.     

Large wood debris recruitment on differing riparian landforms along a Gulf Coastal Plain (USA) stream: a comparison of large floods and average flows

In southeastern Coastal Plain streams, wood debris can be very abundant and is recruited from extensive forested floodplains. Despite importance of wood debris, there have been few opportunities to examine recruitment and redistribution of wood in an undisturbed setting, particularly in the southeastern Coastal Plain. Following extensive flooding in 1994, measurements of individual downed trees (species, dbh, orientation, distance from base‐flow channel and condition) were made across replicated riparian landforms in a Gulf Coastal Plain 5th‐order stream. Annually, the fate of these trees was determined and newly recruited trees were noted. More than 300 downed trees have been recorded. Recruitment varied across landforms with more constrained reaches having greatest mortality. Total tree mortality varied substantially across years. Generally, tree recruitment was greatest in years with substantial floods (1994 and 1998). For each riparian landform type, tree mortality was correlated with the maximum daily flow during the period preceding annual debris surveys. This relationship was particularly strong for sand ridges (r² = 0.942) and low terraces (r² = 0.915), but was significant for floodplains (r² = 0.413). Greatest rates of debris recruitment per maximum daily flow were observed for sand ridges followed by low terraces. Flood characteristics also influenced debris recruitment. The 1994 flood was caused by a tropical storm and resulted in a rapid rise in streamflow. Much of the debris recruited during this flood was from toppled trees and was oriented parallel to the stream channel. In contrast, the 1998 flood was preceded by a wetter than average winter with more gradually rising flows and there was no relationship between riparian landform and debris characteristics. These results indicate that wood recruitment dynamics in Coastal Plain streams are complex. Wood recruitment rates are controlled by cyclical variations in climate interacting with riparian geomorphology. Infrequent high flows appear critical in the maintenance of the instream debris pool. Copyright © 2007 John Wiley & Sons, Ltd.     

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.     

Large dam removal and early spontaneous riparian vegetation recruitment on alluvium in a former reservoir: Lessons learned from the pre‐removal phase of the Sélune River project (France)

The restoration of ecological continuity along the Sélune River (Normandy, France) involves the removal of two tall hydroelectric dams (36 m removed in 2019 and 16 m in 2021), a project without precedent in Europe. During the pre‐removal phase (2014–2018), we performed scientific monitoring of the vegetation that was colonizing alluvium in the former dam reservoir (length: 19 km; surface area: 151 ha). Our study aimed to analyse if spontaneous vegetation could ecologically restore the riparian zone and help maintain fine sediment after dam removal. We used colonization indicators related to vegetation structure, taxonomic richness and diversity, and composition. These indicators were calculated at two spatial scales (local, at a single site, and broad, along the reservoir). The aim was to (a) characterize the spontaneously established species pool; (b) analyse longitudinal patterns in vegetation colonization; and (c) assess temporal changes in the species community. Our results show that diverse plant communities have developed. Slight differences in longitudinal and lateral patterns existed; they were linked with habitat heterogeneity and the reservoir's slow pace of draining. We observed fast spontaneous terrestrialization, which has resulted in cover stabilization, decreased diversity, and the development of herbaceous riverbank communities, with very few invasive species. This finding suggests stabilization potential is high and passive ecological restoration could occur, at least locally. Further analyses focusing on functional traits could help inform future management decisions regarding revegetation on reservoir alluvium.     

Hydrosedimentary classification of natural and engineered backwaters of a large river,the lower Rhône: possible applications for the maintenance of high fish biodiversity

At the end of the last century, the shorelines of the Lower River Rhône were embanked and submersible dikes were built in the main channel, delimiting dike fields. We compared the morphological, hydrological and sedimentary variables of nine dike fields, four sites in dead arms and one in the main channel, in order to describe the fluviosedimentary functioning and to produce a classification of these different natural and engineered backwaters. Within these ‘artificial floodplains’, the different types of backwaters and their long-term changes bear a strong similarity to those observed within a natural floodplain. The long-term frequency of submersion by the water of the main river channel is the first factor that controls the erosion and sedimentation within the different backwaters. Three types of site can be distinguished: the lotic sites are the main channel and one dike field, five dike fields comprise the semi-lotic sites and the cluster lentic sites include the three remaining dike fields and the four dead arms sites. These three groups also differ in their riparian habitats and in their juvenile fish assemblages. As juvenile fish were most abundant in the semi-lotic dike fields, the use of such structures can be envisaged in order to restore high fish diversity in rivers where the former floodplain has disappeared or is no longer accessible to fish because of human modifications. © 1997 John Wiley & Sons, Ltd.