Cosmogenic 26Al and 10Be in alluvial gravel on a strath 150 m above the San Juan River, Utah, reveal a depositional age of

\(1.36\ ^{{+}0.20}\ _{{-}0.15}\)
Ma. This gravel is correlative with a series of terraces that grade to Glen Canyon on the Colorado River, indicating a similar age for incision there. The calculated incision rate, 110 ± 14 m/m.y., is somewhat slower than that of the Colorado River in the eastern Grand Canyon and suggests active steepening of the Colorado River. The cosmogenic nuclides also indicate rapid erosion in the sediment source area and are consistent with alluviation due to enhanced Pleistocene erosion in the San Juan Mountains.

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