Alluvial fan evolution and morphology are often considered to respond primarily to external forcing (e.g., tectonics, climate, and base-level change). Here we present a numerical model of alluvial fan evolution that shows that dramatic and persistent fan entrenchment may occur in the absence of such forcing. This process is driven by positive autogenic feedbacks between flow width, sediment transport, and rate of fan aggradation. Entrenchment is initiated where sediment accommodation space limits continued fan growth. Our results highlight a need to rethink both the representation of fluvial width adjustment in landscape evolution models and the established framework for the interpretation of fluvial landforms as archives of environmental change.