Abstract

Cosmogenic-burial and U-series dating, identification of fluvial terraces and lacustrine deposits, and river profile reconstructions show that capture of the Gunnison River by the Colorado River and abandonment of Unaweep Canyon (Colorado, USA) occurred between 1.4 and 0.8 Ma. This event led to a rapid pulse of incision unlike any documented in the Rocky Mountains. Following abandonment of Unaweep Canyon by the ancestral Gunnison River, a wave of incision propagated upvalley rapidly through Mancos Shale at rates of ∼90–440 km/m.y. The Gunnison River removed 400–500 km3 of erodible Mancos Shale and incised as deep as 360 m in 0.17–0.76 m.y. (incision rates of ∼470–2250 m/m.y.). Prior to canyon abandonment, long-term (ca. 11–1 Ma) Gunnison River incision averaged ∼100 m/m.y.

The wave of incision also caused the subsequent capture of the Bostwick–Shinn Park River by the ancestral Uncompahgre River ca. 0.87–0.64 Ma, at a location ∼70 km upvalley from Unaweep Canyon. This event led to similarly rapid (to ∼500 m/m.y.) but localized river incision. As regional river incision progressed, the juxtaposition of resistant Precambrian bedrock and erodible Mancos Shale within watersheds favored the development of significant relief between adjacent stream segments, which led to stream piracy. The response of rivers to the abandonment of Unaweep Canyon illustrates how the mode and tempo of long-term fluvial incision are punctuated by short-term geomorphic events such as stream piracy. These short-term events can trigger significant landscape changes, but the effects are more localized relative to regional climatically or tectonically driven events.

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