Attempts to understand the causes of gully erosion have been hampered by a poor understanding of quantitative changes to the force of flows and the resistance to scour. We used flume experiments on an unincised valley floor to determine flow resistance and the critical shear stress for scour under natural and degraded vegetation covers. Applying the results to sites of gully formation in southeastern Australia demonstrates the crucial role that reduced vegetation cover plays in increasing the susceptibility of valleys to channel incision. Widespread and rapid gully formation in the 19th century required degradation of valley-floor vegetation and was not solely the result of land use or climatically induced increases in discharge.

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