Extended Data Fig. 5: Schematic diagram of laboratory flume.
![Extended Data Fig. 5](https://cdn.statically.io/img/media.springernature.com/full/springer-static/esm/art%3A10.1038%2Fs41586-022-05564-6/MediaObjects/41586_2022_5564_Fig8_ESM.jpg)
Measurements of bed and water surface slope were made in the middle 2.5 m of the flume, where there were no visible entry or exit effects on grain motion. The flume is inclined 3°, but the sediment bed can develop a slope that is either steeper or less steep than the flume.