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Abstract. Thegrain-size distribution of ancient alluvial systems is commonly determinedfrom surface samples of vertically exposed sections of gravel deposits. Thismethod relies on the hypothesis that the grain-size distribution obtainedfrom a vertical cross section is equivalent to that of the riverbed. Such anhypothesis implies first that the sediments are uniform in size in the riverbed, and second that the sampling method implemented on a vertical sectionleads to a grain-size distribution equivalent to the bulk one. Here, wereport a field test of this hypothesis on granulometric samples collected inan active, gravel-bedded, braided stream: the Urumqi River in China. Wecompare data from volumetric samples of a trench excavated in an activethread and from surface counts performed on the trench vertical faces. Basedon this data set, we show that the grain-size distributions obtained from allthe samples are similar and that the deposit is uniform at the scale of theriver active layer, a layer extending from the surface to a depth ofapproximately 10 times the size of the largest clasts. As a consequence, thegrid-by-number method implemented vertically leads to a grain-sizedistribution equivalent to the one obtained by a bulk volumetric sampling.This study thus brings support to the hypothesis that vertical surface countsprovide an accurate characterization of the grain-size distribution ofpaleo-braided rivers.

earth surface dynamics

Uniform grain-size distribution in the active layer of a shallow, gravel-bedded, braided river (the Urumqi River, China) and implications for paleo-hydrology