RAPID RIVER REORGANIZATION BY EROSION THROUGH LAYERED STRATIGRAPHY

Interaction between the crust and atmosphere forms a diversity of drainage basin shapes. These shapes form from drainage divide migration, which is driven by horizontal components of erosion that are difficult to measure. We show that exhuming stratigraphy with variable resistance to erosion can lead to horizontal erosion rates that exceed vertical erosion rates by 2-3 orders of magnitude, leading to 50-100 km-long river reversals and substantial drainage network reorganization. Between the Rhine and Danube rivers of southwestern Germany, vertical rock uplift and catchment averaged erosion rates were quantified using uplifted paleoshorelines and in situ produced ¹⁰Be concentrations in river sand, whereas horizontal divide migration rates were quantified using the position and age of fluvial deposits that record divide positions prior to river reversal. We find divide migration rates of 2.2-33.6 km/My greatly exceed regional rock uplift and catchment averaged erosion rates (0.02-0.1 km/My) and require enhanced bedrock erodibility to reconcile. Fast divide migration rates are localized in valley bottoms of former rivers (wind gaps) where underlying weak bedrock units are exposed. Evaporite and clay layers are particularly weak, possibly obscuring detection of these erosion hotspots with catchment-averaged sampling strategies that use in situ produced ¹⁰Be in quartz sand. Rapid divide migration along in-valley wind gaps creates barbed tributaries that form acute junction angles with mainstem rivers, which have reversed flow directions. These watershed structures and associated divide migration rates indicate that divide migration follows preferential pathways along bedrock weaknesses, and reveal cases where drainage network structure reflects bedrock strength properties instead of regional tectonic or hydrologic drivers.