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Meanders, or frequent sinuous reversals of channel curvature, are a fundamental characteristic of river planform. Although the cause of meandering has attracted attention since the beginning of civilization, as yet there is no completely satisfactory explanation of this phenomenon. The self-similarity of meander geometry over a wide range of scales and environmental conditions suggests that meandering reflects the influence of some general control (Davy and Davies, 1979). Various arguments have been advanced to explain why rivers meander, including dissipation of excess energy (Jefferson, 1902;Inglis, 1947), minimization of energy expenditure (Yang, 1971;Chang, 1988), and minimization of the variance in bed shear stress and boundary friction (Langbein and Leopold, 1966). These theories provide predictions that conform well with observed forms, but they are primarily teleological in nature and thus difficult to evaluate scientifically; minimization of energy or shear stress could just as easily be the result of meandering as it could the cause (Richards, 1982: 28). More important, these arguments do not describe how meanders develop.Over the past 25 years, considerable effort has been devoted to identifying the physical processes that initiate the development of meanders in straight channels. This research has focused on inter-relationships among fluid dynamics, sediment transport, bed morphology, and bank erosion; the goal is to develop a hydrodynamic explanation of meandering. Although much of this work has occurred outside geography, it has relevance to physical geographers, particularly fluvial geomorphologists, for at least two reasons. First, it relates directly to geomorphic theory concerning the basic nature of meandering rivers and their sedimentary deposits. Secondly, geomorphologists are in a position to contribute substantively to theory development because many postulates of hydrodynamic theories have not been rigorously tested in the field. The purposes of this paper are: 1) to review current hydrodynamic concepts related to meander initiation; and 2) to provide direction for future geomorphologic investigations so that they may contribute directly to theory enhancement. The approach taken here is to summarize the salient ideas embodied in various theories. For detailed reviews of mathematical formulations the reader is referred to Diplas et al. (1988) and Seminara and Tubino (1989). , at WEST VIRGINA UNIV on June 20, 2015 ppg.sagepub.com Downloaded from 128 I Flow oscillation and meander initiationThe development of a meandering stream from an initially straight channel requires retreat of the banks at regularly-spaced intervals along alternate sides of the channel. Although bank erosion is a necessary condition for meander initiation (Friedkin, 1945), it is clearly the effect of some original cause, not the cause itself (Ackers and Charlton, 1970). Because bank erosion results primarily from removal of sediment at the base of the bank by hydraulic action (Thorne and Tovey, 1981), meandering is ofte...

This study investigates the fluvial dynamics of straight natural stream channels. In particular, this experimental field study quantitatively assesses a physically based non-linear mathematical theory of alternate bar formation under unsteady natural flow conditions within a straight alluvial stream. The study site is an artificially straightened section of the Embarras River located approximately 16 km south of Champaign, Illinois. Data were collected on channel form, gradient, alternate bar dimensions, bed sediment size and flow stage over a 2 year study period.Both linear and non-linear steady flow hydrodynamic theories suggest that alternate bars are critical to the process of meander development. But these theories do not predict bar development for unsteady flow conditions, which typically occur in natural alluvial channels. Tubino (1991) suggests that bar evolution for a flood hydrograph can be divided into three parts: (1) a period of limited bar growth during the rising stage of the flood; (2) a stage of modest bar decay near the peak of the flood; and (3) a stage of non-linear bar growth during the prolonged falling stage of the flood.Bars developed during the falling limb of a hydrograph, and exhibited sequential development rather than the uniform growth along the reach predicted by Tubino's model. As flow stage decreased, short, low, fine-grained bars were superimposed on long, high and coarser-grained bars that developed under preceding high flow stages. These results suggest that the process of bar formation in artificially straightened natural streams with heterogeneous bed material may occur under different flow conditions and in a different manner than predicted by theoretical models. Further work should focus on attempting to isolate the physical mechanisms responsible for alternate bar formation in straight natural streams with heterogeneous bed material and flashy hydrologic flow regimes.

BackgroundRecent studies have noted myriad qualitative and quantitative inconsistencies between the medieval Black Death (and subsequent “plagues”) and modern empirical Y. pestis plague data, most of which is derived from the Indian and Chinese plague outbreaks of A.D. 1900±15 years. Previous works have noted apparent differences in seasonal mortality peaks during Black Death outbreaks versus peaks of bubonic and pneumonic plagues attributed to Y. pestis infection, but have not provided spatiotemporal statistical support. Our objective here was to validate individual observations of this seasonal discrepancy in peak mortality between historical epidemics and modern empirical data.Methodology/Principal FindingsWe compiled and aggregated multiple daily, weekly and monthly datasets of both Y. pestis plague epidemics and suspected Black Death epidemics to compare seasonal differences in mortality peaks at a monthly resolution. Statistical and time series analyses of the epidemic data indicate that a seasonal inversion in peak mortality does exist between known Y. pestis plague and suspected Black Death epidemics. We provide possible explanations for this seasonal inversion.Conclusions/SignificanceThese results add further evidence of inconsistency between historical plagues, including the Black Death, and our current understanding of Y. pestis-variant disease. We expect that the line of inquiry into the disputed cause of the greatest recorded epidemic will continue to intensify. Given the rapid pace of environmental change in the modern world, it is crucial that we understand past lethal outbreaks as fully as possible in order to prepare for future deadly pandemics.

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