Conventional explanations for bimodal grain-size distributions with a gap somewhere in the 1-10-mm range, as found in some river deposits, have commonly invoked processes which occur within the channel itself, such as abrasion or selective transport. Field data from two lithologically distinct watersheds suggest that the size distribution of the input material may be largely responsible for the size distribution of the channel deposits. Data from a small, second-order basin of sandstone indicate that the gap is present in the most headward channel deposits as well as in material at the base of the hillslopes along the channel. It is not present in samples taken at the ridge crests which mark the boundary of the watershed. Data from a slightly larger second-order basin of basalt exhibited no gap in the stream deposits.

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