Living landscapes: Muddy and vegetated floodplain effects on fluvial pattern in an incised river.
ABSTRACT: Cohesive floodplain sediment and vegetation are both thought to cause meandering river patterns. Our aims are to compare the isolated and combined effects of mud and vegetation on river planform and morphodynamics in the setting of intermediate-sized valley rivers. We use a numerical model for century-scale simulation of flow, sediment transport and morphology coupled with riparian vegetation settlement, growth and mortality as functions of species traits on which flow resistance depends. Mud fluxes were predicted by excess shear stress relations in combination with the active layer formulation. We found that valley-flooding water levels increase with vegetation density, causing a higher braiding intensity rather than meandering tendency. The shear stress during floods carves channels through the muddy floodplain surface. Higher mud concentration, on the other hand, increases floodplain aggradation, reduces the overbank flow frequency and ultimately causes formation of a single-thread channel. Vegetation causes mud to deposit closer to the river channel as a levee, showing that mud sedimentation and vegetation settling mutually enhance floodplain formation. However, mud and vegetation counteract in two ways. First, vegetation enhances floodplain accretion, which ultimately increases plant desiccation for high mud concentrations. Second, vegetation increases the tendency of periodic chute cutoffs in valleys. The chute cutoffs locally reset the landscape and create new windows of opportunity for the vegetation. Surprisingly, in systems with a high mud concentration this causes hysteretic loops of vegetation cover and delayed mud deposition. Ramifications for the interpretation of Palaeozoic fluvial facies are that even rootless vegetation, capturing cohesive mud closer to the river channel to form thicker floodplain on the point bar, can enhance the tendency to meander and, under high mud supply, form stable channels. However, meandering is more unlikely in narrower valley rivers with higher vegetation density.
Project description:Meandering rivers are common on Earth and other planetary surfaces, yet the conditions necessary to maintain meandering channels are unclear. As a consequence, self-maintaining meandering channels with cutoffs have not been reproduced in the laboratory. Such experimental channels are needed to explore mechanisms controlling migration rate, sinuosity, floodplain formation, and planform morphodynamics and to test theories for wavelength and bend propagation. Here we report an experiment in which meandering with near-constant width was maintained during repeated cutoff and regeneration of meander bends. We found that elevated bank strength (provided by alfalfa sprouts) relative to the cohesionless bed material and the blocking of troughs (chutes) in the lee of point bars via suspended sediment deposition were the necessary ingredients to successful meandering. Varying flood discharge was not necessary. Scaling analysis shows that the experimental meander migration was fast compared to most natural channels. This high migration rate caused nearly all of the bedload sediment to exchange laterally, such that bar growth was primarily dependent on bank sediment supplied from upstream lateral migration. The high migration rate may have contributed to the relatively low sinuosity of 1.19, and this suggests that to obtain much higher sinuosity experiments at this scale may have to be conducted for several years. Although patience is required to evolve them, these experimental channels offer the opportunity to explore several fundamental issues about river morphodynamics. Our results also suggest that sand supply may be an essential control in restoring self-maintaining, actively shifting gravel-bedded meanders.
Project description:The crayfish fauna of West Virginia consists of 23 species and several undescribed taxa. Most survey efforts documenting this fauna have been conducted in lotic waterways throughout the Appalachian plateau, Allegheny Mountains, and Ridge and Valley physiographic provinces. Bottomland forests, swamps, and marshes associated with large river floodplain such as the Ohio River floodplain historically have been under-surveyed in the state. These habitats harbor the richest primary burrowing crayfish fauna in West Virginia, and are worthy of survey efforts. In an effort to fill this void, the crayfish fauna of West Virginia's Ohio River floodplain was surveyed from 2004 through 2009. From this survey, nine species from four genera were documented inhabiting the floodplain. Zoogeography, biology, and conservation status is provided for all nine crayfishes. The dominant genus along the floodplain is Cambarus, which includes Cambarus (Cambarus) carinirostris, Cambarus (Cambarus) bartonii cavatus, Cambarus (Procambarus) robustus and Cambarus (Tubericambarus) thomai. Cambarus (Tubericambarus) thomai is the most prevalent burrowing species occurring along the floodplain. The genus Orconectes consists of two native species, Orconectes (Cambarus) obscurus and Orconectes (Cambarus) sanbornii; and two invasive taxa, Orconectes (Gremicambarus) virilis and Orconectes (Procambarus) rusticus. Orconectes (Cambarus) obscurus has experienced a range extension to the south and occupies streams formerly occupied by Orconectes (Cambarus) sanbornii. Both invasive taxa were allied with anthropogenic habitats and disturbance gradients. The genera Fallicambarus and Procambarus are represented by a single species. Both Fallicambarus (Cambarus) fodiens and Procambarus (Orconectes) acutus are limited to the historic preglacial Marietta River Valley.
Project description:One of the most dramatic events in river environments is the natural diversion, or avulsion, of a channel across its floodplain. Though rarely witnessed, avulsions can cause massive floods, and over geologic time they create most of the fluvial stratigraphic record. Avulsions exhibit behavior ranging from reoccupying abandoned channels to constructing new channels and splay complexes. To quantify avulsion behavior, or style, we measure avulsion-related floodplain disturbance in modern environments. We show that for 63 avulsions from Andean, Himalayan, and New Guinean basins, avulsion style correlates with channel morphology and changes systematically downstream. Avulsions in braided rivers reoccupy abandoned channels, whereas avulsions in meandering rivers often produce flooding and sediment deposition during channel construction. These downstream changes in avulsion style can explain the abrupt transition from channel-dominated to floodplain-dominated facies commonly observed in foreland basin stratigraphy. These dynamics also explain why some avulsions are more hazardous than others.
Project description:High-elevation mountain streams are commonly viewed as erosive environments, but they can retain sediment along river corridors for thousands of years. In 2013, an extreme flood evacuated floodplain sediment in the Colorado Front Range, USA. We use fifty-two 14C ages collected along four streams prior to the flood to estimate mean residence time of floodplain sediment. Here we show that mountain streams above the elevation of the Pleistocene terminal moraine retain floodplain sediment for longer durations than those at lower elevation, but that wildfires may decrease floodplain sediment residence time at high elevations. Comparison of field sites and differencing of pre- and post-flood lidar show that valley confinement is a significant predictor of residence time, sediment flux, and floodplains disturbed by the 2013 flood. Elevational trends in floodplain disturbance regime also reflect differences in forest type, precipitation pattern, and wildfire regime, which are expected to shift under a changing climate.
Project description:Background:Conventional river engineering operations have a substantial influence on the fluvial ecosystem. Regulation and channelization generally reduce the physical heterogeneity of river beds and banks and the heterogeneity of habitats. They determine the character, diversity and species richness of plant communities. The effect of river regulation on vegetation has been repeatedly investigated, but few studies have been conducted within reaches of previously regulated rivers. The aim of this work is to expand and current knowledge about the impact of dredging on the vegetation of a regulated section of a lowland river. Materials & Methods:The study included pre-dredging (1 year before) and post-dredging surveys (results 1 and 2 years after dredging). The vegetation was analysed in terms of species composition, origin of species, life forms, distribution of Grime's life strategies, and selected ecological factors. The Shannon-Wiener biodiversity index (H) and evenness were also analysed in each year of the study. The impact of dredging on the vascular flora was assessed by 'before-after-control-impact' (BACI) analysis. Results:The number of species and biodiversity as measured by the Shannon-Wiener index (H) increased in the analysed section of the river valley. However, enrichment of the flora was observed only on the floodplain, on the surface of the deposited dredging material, while the number of species in the river channel decreased, as dredging of the river bed and levelling of the banks had markedly reduced habitat diversity. Although species richness in the second year after the dredging approached the values recorded before the intervention, the absence of particularly species or phytocenoses associated with shallow river banks and sandbars was still observed. The change in habitat conditions and the destruction of the vegetation cover during the dredging enabled penetration by numerous previously unrecorded alien species of plants and apophytes. There was a perceptible increase in the role of therophytes in the flora. It is worth noting that the number of alien species and therophytes declined significantly in the second year after the dredging. Analysis of the proportions of species representing various life strategies showed that previously unrecorded species with the type R (ruderal) life strategy had appeared, representing by pioneer species occurring in frequently disturbed habitats. There was also a marked increase in the share of species representing the mixed C-R (competitive-ruderal) strategy, occurring in habitats with low levels of stress, whose competitive abilities are limited by repeated disturbances. By the second year after the dredging, however, these changes were largely no longer observed. Conclusions:Through appropriate maintenance of the regulated river, it can be rapidly recolonized by vegetation after the procedure, but it may lead to the loss of some species and phytocoenoses.
Project description:When inundated by floodwaters, river floodplains provide critical habitat for many species of fish and wildlife, but many river valleys have been extensively leveed and floodplain wetlands drained for flood control and agriculture. In the Central Valley of California, USA, where less than 5% of floodplain wetland habitats remain, a critical conservation question is how can farmland occupying the historical floodplains be better managed to improve benefits for native fish and wildlife. In this study fields on the Sacramento River floodplain were intentionally flooded after the autumn rice harvest to determine if they could provide shallow-water rearing habitat for Sacramento River fall-run Chinook salmon (Oncorhynchus tshawytscha). Approximately 10,000 juvenile fish (ca. 48 mm, 1.1 g) were reared on two hectares for six weeks (Feb-March) between the fall harvest and spring planting. A subsample of the fish were uniquely tagged to allow tracking of individual growth rates (average 0.76 mm/day) which were among the highest recorded in fresh water in California. Zooplankton sampled from the water column of the fields were compared to fish stomach contents. The primary prey was zooplankton in the order Cladocera, commonly called water fleas. The compatibility, on the same farm fields, of summer crop production and native fish habitat during winter demonstrates that land management combining agriculture with conservation ecology may benefit recovery of native fish species, such as endangered Chinook salmon.
Project description:Information concerning the dynamics of river meandering is embedded in their planforms. Here, we focus on how bend skewing varies with increasing sinuosity, and how flow direction is embedded in bend skewing. It has often been thought that upstream-skewed bends are dominant within a sufficiently long reach. These bends may allow a reasonable inference as to the direction of flow. Here we consider this issue using 20 reaches of freely meandering alluvial rivers that are in remote locations, generally far from human influence. We find that low-amplitude bends tend to be downstream-, rather than upstream-skewed. Bends with sinuosity greater than 2.6, however, are predominantly upstream-skewed. Of particular interest are the neck cutoffs, all chosen to be relatively recent according to their position related to the main channel: 84% of these are upstream-skewed. Neck cutoffs, which have likely evolved directly from bends of the highest sinuosity, represent the planform feature most likely to have flow direction embedded in them. The field data suggest that meander bends without external forcing such as engineering works tend to evolve from downstream-skewed low-sinuosity bends to upstream-skewed high-sinuosity bends before cutoff. This process can be reproduced, to some extent, using models coupling sedimentary dynamics with flow dynamics.
Project description:Tidal channel networks mediate the exchange of water, nutrients and sediment between an estuary and marshes. Biology feeds back into channel morphodynamics through the influence of vegetation on both flow and the cohesive strength of channel banks. Determining how vegetation affects channel networks is essential in understanding the biological functioning of intertidal ecosystems and their ecosystem services. However, the processes that control the formation of an efficient tidal channel network remain unclear. Here we compare the channel networks of vegetated salt marshes in Massachusetts and the Venice Lagoon to unvegetated systems in the arid environments of the Gulf of California and Yemen. We find that the unvegetated systems are dissected by less efficient channel networks than the vegetated salt marshes. These differences in network geometry reflect differences in the branching and meandering of the channels in the network, characteristics that are related to the density of vegetation on the marsh.
Project description:The majority of the world's floodplains are dammed. Although some implications of dams for riverine ecology and for river channel morphology are well understood, there is less research on the impacts of dams on floodplain geomorphology. We review studies from dammed and undammed rivers and include influences on vertical and lateral accretion, meander migration and cutoff formation, avulsion, and interactions with floodplain vegetation. The results are synthesized into a conceptual model of the effects of dams on the major geomorphic influences on floodplain development. This model is used to assess the likely consequences of eight dam and flow regulation scenarios for floodplain geomorphology. Sediment starvation downstream of dams has perhaps the greatest potential to impact on floodplain development. Such effects will persist further downstream where tributary sediment inputs are relatively low and there is minimal buffering by alluvial sediment stores. We can identify several ways in which floodplains might potentially be affected by dams, with varying degrees of confidence, including a distinction between passive impacts (floodplain disconnection) and active impacts (changes in geomorphological processes and functioning). These active processes are likely to have more serious implications for floodplain function and emphasize both the need for future research and the need for an "environmental sediment regime" to operate alongside environmental flows.