Drainage reversals, an end-member case of drainage reorganization, often occur toward cliffs. Reversals are commonly identified by the presence of barbed tributaries, with a junction angle >90°, that preserve the antecedent drainage geometry. The processes that form reversed drainages are largely unknown. Particularly, barbed tributaries cannot form through a spatially uniform migration of the cliff and drainage divide, which would be expected to erase the antecedent drainage pattern, and tectonic tilting toward the cliff that could reverse the flow direction is inconsistent with geodynamic models of large-scale escarpment, where many reversals are documented. Here, we propose a new mechanism for drainage reversal, where the slope imbalance across a cliff, together with the high erodibility of sediments that fill cliff-truncated valleys, result in faster divide migration along valleys compared to interfluves. We demonstrate this mechanism along channels that drain toward the escarpment of the Arava Valley in Israel. Reversal is established by observations of barbed tributaries and opposite-grading terraces. We show that drainage reversal occurs when erodible valley fill exists, and that the reversal extent correlates with the thickness of this fill, in agreement with the predictions of the proposed mechanism. This new reversal mechanism demonstrates that valley fill could play an acute role in fluvial reorganization processes, and that reversals could occur independently of tectonic tilting.

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