The evolution of meandering river floodplains is predominantly controlled by the interplay between overbank sedimentation and channel migration. The resulting spatial heterogeneity in floodplain deposits leads to variability in bank erodibility, which in turn influences channel migration and planform development. Despite the potential significance of these feedbacks, few studies have quantified their impact upon channel evolution and floodplain construction in dynamic settings (e.g., locations characterized by rapid channel migration and high rates of overbank sedimentation). This study employs a combination of field observations, GIS analysis of satellite imagery and numerical modelling to investigate these issues along a 375 km reach of the Rio Beni in the Bolivian Amazon. Results demonstrate that the occurrence of clay-rich floodplain deposits promotes a significant reduction in channel migration rates and distinctive styles of channel evolution, including channel straightening and immobilisation of bend apices leading to channel narrowing. Clay bodies act as stable locations limiting the propagation of planform disturbances in both upstream and downstream directions, and operate as 'hinge' points, around which the channel migrates. Spatial variations in the erodibility of clay-rich floodplain material also promote large-scale (10-50 km) differences in channel sinuosity and migration, although these variables are also likely to be influenced by channel gradient and tectonic effects that are difficult to quantify. Numerical model results suggest that spatial heterogeneity in bank erodibility, driven by variable bank composition, may force a substantial (c. 30%) reduction in average channel sinuosity, compared to situations in which bank strength is spatially homogeneous.This article is protected by copyright. All rights reserved. KeywordsMeander migration; floodplain heterogeneity; bank erosion; numerical model; planform evolution This article is protected by copyright. All rights reserved.

Soils deliver crucial ecosystem services, such as climate regulation through carbon (C) storage and food security, both of which are threatened by climate and land use change. While soils are important stores of terrestrial C, anthropogenic impact on the lateral fluxes of C from land to water remains poorly quantified and not well represented in Earth system models. In this study, we tested a novel framework for tracing and quantifying lateral C fluxes from the terrestrial to the aquatic environment at a catchment scale. The combined use of conservative plant-derived geochemical biomarkers n-alkanes and bulk stable δC and δN isotopes of soils and sediments allowed us to distinguish between particulate organic C sources from different land uses (i.e. arable and temporary grassland vs. permanent grassland vs. riparian woodland vs. river bed sediments) (p<0.001), showing an enhanced ability to distinguish between land use sources as compared to using just n-alkanes alone. The terrestrial-aquatic proxy (TAR) ratio derived from n-alkane signatures indicated an increased input of terrestrial-derived organic matter (OM) to lake sediments over the past 60years, with an increasing contribution of woody vegetation shown by the C/C ratio. This may be related to agricultural intensification, leading to enhanced soil erosion, but also an increase in riparian woodland that may disconnect OM inputs from arable land uses in the upper parts of the study catchment. Spatial variability of geochemical proxies showed a close coupling between OM provenance and riparian land use, supporting the new conceptualization of river corridors (active river channel and riparian zone) as critical zones linking the terrestrial and aquatic C fluxes. Further testing of this novel tracing technique shows promise in terms of quantification of lateral C fluxes as well as targeting of effective land management measures to reduce soil erosion and promote OM conservation in river catchments.

SUMMARY1. Substratum stability and shear stress exerted by flowing water can have a strong influence on the structure of benthic communities. Bed stability can be characterised in a variety of ways, e.g. flow competence, threshold of particle entrainment, measures of erosion and deposition, particle transport distance, abrasion and bedload transport rate. This paper reviews methods for the quantification of bed stability and shear stress in streams and rivers that are relevant for the examination of the relationships between stream biota and bed stability. 2. The most suitable method for a research project depends mainly on the objectives. The targeted group of biota, spatial and temporal scale of investigation, as well as hydraulic conditions and substratum characteristics at the study site(s) determine the choice of a technique for the assessment of bed stability. 3. Indirect measurement of shear stress can be more accurate than calculations based on the DuBoys equation. However, the latter is preferred for reach-wide applications within the limits imposed by hydraulic conditions. The entrainment of the substratum is most effectively assessed using a combination of shear stress and competence equations, but the latter require careful parameterisation. At the patch-scale, direct measurement of entrainment force is a valid alternative. 4. Morphometric budgeting is the most comprehensive and least invasive technique for the assessment of rates of erosion and deposition. The transport of substratum particles is efficiently monitored with in situ marked or active tracer particles which allow for rapid and non-invasive identification and high recovery rate. As the assessment of bedload transport rate by formulae can be inaccurate, direct measurement is preferred. However, bedload traps interfere with the substratum and continuity of measurement with samplers is limited. Thus developments in the sector of acoustic and piezoelectric devices offer a potential alternative. 5. The abrasive forces by suspended sediments on stream biota are effectively evaluated with artificial blocks that are fixed on the stream bed. Descriptive surveys that assess bed stability offer an alternative to direct measurement and calculations. They are straightforward and non-invasive but can be observer-biased. If single methods do not provide useful links with biological data this may be improved by the application of a multivariate approach. 6. Many of the methods assessed have not yet been applied in research on benthic communities, but these hydraulic and geomorphologic techniques offer considerable potential for the assessment of bed stability in stream ecology.

Digital elevation models (DEMs) of river channels, built by interpolation between samples of topographic survey points, are widely used to represent surfaces and to derive land‐surface parameters. Differencing between successive DEMs permits quantification of change, which in gravel‐bed rivers is used to construct a morphological budget of lower bound estimates of sediment flux and bed‐stability surrogate. Choice of DEM interpolation method strongly influences DEM quality and realistic representation of channel forms. When comparing morphological budgets between multiple contrasting reaches, e.g. for rapid ecological appraisal, an effective and consistent means of DEM construction is required to avoid digitally generated inconsistencies. An appropriate interpolation method should be suitable for accurate representation of channels contrasting in substrate and hydraulic conditions, surveys of varying data density and distribution, and avoidance of site‐specific parameterization. This paper investigates representation of channel form using a series of DEMs generated within Surfer® by triangulation with linear interpolation, natural neighbours, point kriging, universal kriging, multiquadratic radial basis function, modified Shepard's method and inverse distance to a power on the example of four reaches of mountain streams in New Zealand. These reaches represent a diversity of channel forms, substrate and hydraulic properties. DEMs from triangulation with linear interpolation revealed consistently the best results and this method is recommended for geomorphological and ecological studies of multiple reaches. The main advantage over point kriging and radial basis function is better representation of channel margins and bedforms without introduction of breaklines, while it outperforms natural neighbours in honouring measured points. Copyright © 2010 John Wiley & Sons, Ltd.

Research at York St John (RaY) is an institutional repository. It supports the principles of open access by making the research outputs of the University available in digital form.