Abstract

A gravel-bed river often changes abruptly into a sand-bed river. The causes of such so-called gravel–sand transitions have been elucidated in recent years, but detailed observations of gravel–sand transitions in large rivers are scarce. The present study fills this knowledge gap by investigating the sedimentary characteristics of the gravel–sand transition in the river Rhine. Data on bed composition and flow characteristics are used, in combination with a novel method for the evaluation of the bed structure. The analysis shows that, in contrast to common belief, the bed structure in the gravel–sand transition zone does not change from fully clast supported to fully matrix supported. Instead, there is a strong cross-stream variation in bed structure, especially upstream of the transition zone. The analysis also shows that the downstream decrease in gravel mobility in the gravel–sand transition zone, which is usually attributed to the change in bed structure and the so-called patchiness effect, is at least partly caused by the larger-scale bend sorting process, which concentrates gravel in outer bends where it is only infrequently mobile. Dune sorting, which has been suggested to have an effect on the gravel mobility similar to that of bend sorting, was found to be relatively unimportant in the Rhine, partly because it is strongly counteracted by armoring processes that affect an uncommonly thick sediment layer. The gravel–sand transition in the Rhine, with its length of 50 km, is much less abrupt than the gravel–sand transitions of smaller rivers reported on in the literature. Although the dependence of the transition length on river size intuitively is correct, the exact causes remain unclear.