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Meanders are oscillatory systems characterized with multiple spatial frequencies in their planforms. Although the concept of dominant meander wavelength (DMW) is central to morphodynamic research on river meandering, shorter and longer spatial scales of oscillations are also relevant. Using Continuous Wavelet Transform (CWT), we present an objective quantitative characterization of the relevant wavelengths of meandering rivers. Analysis of synthetic meanders generated through Kinoshita curves and real meandering rivers show that DMW can be better detected by applying CWT to the spatial series of the direction-angle, instead of curvature, because the latter shifts the meander oscillation energy toward shorter harmonics. This is related to the typical observed high ratio of meander arclength to channel width, as emerging from field observations and from morphodynamic modeling of meanders, implying the dimensionless wavenumber of meander oscillations to be an O 10 − 1 number. The capability of CWT of capturing local as well as global information in a spatial series is exploited by analyzing two CWT-based indicators, which express the relevance of other wavelengths with respect to the local DMW within the same reach ( M λ ) and the variability of the local DMW with respect to the global DMW ( S λ ). Thresholding the M λ indicator shows that the global DMW corresponds to local DMW for only the most regular meanders. At longer scales, the S λ indicator also reveals the existence of spatial modulations of the direction-angle oscillations in real meandering rivers, which have been previously detected only in synthetic planforms generated by morphodynamic models. This correspondence between observations and modeling indicates the potential of CWT analysis of meanders to provide further insight into the connections between the form and processes of channel meandering, as it suggests that spatial modulations might be inherent in meander-planform dynamics and related to the tendency of meanders to evolve in wavegroups, a potential cause of the so called ‘cutoff avalanches’ observed in natural and modeled meandering streams.

Meandering channels is a vast research field, spanning a broad variety of time and space scales, environmental domains, and conceptual and methodological approaches. This paper serves as an introduction to this special issue of Geomorphology “Meandering Channels”, which addresses the need for sustained scientific dialogue on the dynamics of meandering channels. In an effort to place this issue in the broad context of this rapidly changing and advancing research field, we begin by discussing the motivation behind this issue. Then, we continue by summarizing the main novel research contributions of each paper. Finally, we conclude by proposing five major research directions that directly develop from the ensemble of the scientific contributions to this special issue. These research directions emphasize the critical importance of the coupling of near-bank geomorphic and flow processes; the characterization of co-evolution of meandering rivers and their floodplains; the need to improve linkages between meandering rivers research and river management and restoration; the potential of expanding laboratory-based research; and the integration of holistic and reductionist approaches.

The extent and pattern of width variations along a meandering channel and its association with variation in bed topography and development of mid-channel bars have been examined through field survey evidence for a reach of the River Bollin in NW England. The wet width has been quantified along the reach by applying a hydraulic model to surveyed cross sections under a range of discharges between low flow and defined bankfull conditions. This approach allows an objective, modelling-based method to compute channel width. The high spatial resolution of the topographical survey allows capture of significant variations in the cross-sectional morphology at the meander wavelength scale. Results disclose some features of longitudinal and stage-dependent width variability in meanders. Width variation is shown to be highly correlated with curvature: at bankfull conditions width peaks in bend apex sections exceed those at inflection sections and can be up to twice greater. The width-curvature behavior is correlated with the pattern of bed and banks morphology, which is different in bend apices and in meander inflections. The survey shows that the bedform morphology can be characterized by a mid-channel bar pattern that is initiated at the inflection section and that the bedform dynamics can be closely associated with channel width variations.