Bed‐material size and the riffle‐pool sequence

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Abstract
Relationships between the bed‐material size distribution and riffle‐pool geometry of a small coarse‐bedload upland stream channel are described. Downstream sequences of bed‐material size are highly variable, although series analysis reveals some persistence in the data over short distances. Cross‐correlation of bed‐material series with bed‐elevation series indicates riffles tend to coincide with coarser bed material sequences.Comparison of the bed‐material size distributions of individual riffles, pools and bars is, however, more revealing. Mean particle size was found to be the most useful criterion for distinguishing between bed forms, and in these terms bars are the most distinctive features. Moreover, riffles and pools can be viewed as simply downstream oscillations in elevation of a single coarser‐grained sedimentary unit. Coarse sediment supply from erosion scars created where the channel meets confining bluffs in valley‐fill deposits does mean that the average particle size of adjacent and downstream bed forms is increased. However, conclusions remained unchanged after reanalysis following the identification and subtraction of immobile particle diameters.Plan geometry and coarse sediment supply in the upland environment are also shown to influence bed‐material size distributions. Tightly curved pools tend to have the finest pool sediments but also the least difference between bar and pool sediments. Size of material in pools is also negatively related to an index of cross‐section asymmetry.