The Lava Creek B ash bed, erupted from the Yellowstone caldera ca. 0.64 Ma, provides a datum for measuring long-term fluvial incision west of the Mississippi River. The ash is widely preserved due to its substantial volume, broad initial dispersal, and the aggrading environment into which the ash fell. Drainages incised soon after Lava Creek B deposition, isolating the ash from fluvial erosion and preserving it in fill terraces. Calculated rates of incision since ca. 0.60 Ma range from ≤2 to ∼30 cm k.y.−1. Rates are high in most areas near the Rocky Mountains and downstream along rivers draining mountainous terrain, and are lowest east of the High Plains and along the Snake River. Incision rates along many rivers decrease downstream. Rates of downcutting increased in the late Pleistocene along several major rivers, indicating that climate change altered sediment budgets. Regional and temporal data suggest that fluvial incision records increased middle and late Pleistocene runoff from the southern Rocky Mountains, rather than epeirogenic uplift, but regional rock uplift cannot be excluded as a significant factor.