Dimensions of fluvial-tidal meanders: Are they disproportionally large?

Many of the world's major river systems seemingly have one or a few disproportionally large meanders, with tight bends, in the fluvial-tidal transition (e.g., the Thames in the UK, and the Salmon River in Canada).However, quantitative studies on meanders have so far primarily focused on rivers without tidal influence or on small tidal meanders without river inflow, providing relations between channel geometry and meander characteristics (length, amplitude, and sinuosity).Physics-based predictions of meander size and shape for the fluvialtidal transition zone remain untested for a lack of data.Therefore, it remains unclear whether the dimensions of meanders in the fluvial-tidal transition zone are indeed disproportionally large, and whether meander characteristics can be used as an indicator for tidal influence.Here, data from 823 meanders in 68 fluvial-tidal transition zones worldwide are presented that reveal broad-brush relations between channel geometry and meander dimensions.Our results show that fluvial-tidal meanders indeed become larger in the seaward direction, but the dimensions are proportional to local channel width, as in rivers.Sinuosity maxima are an exception, rather than the rule, in the fluvial-tidal transition zone.Surprisingly, the width of the upstream river correlates with estuarine channel width and tidal meander size even though river discharge constitutes only a fraction of the tidal prism.The new scaling relations can be used to constrain dimensions of rivers and estuaries and their meanders.