Alluvial history of the Porcupine River, Alaska: Role of glacial-lake overflow from northwest Canada

Abstract

The stratigraphy and morphology of alluvial terraces in the lower Porcupine Valley permit the definition of twelve river stages, each marked by distinctive surface characteristics, sediment composition, and regional gradient. Terraces that exhibit characteristics suggestive of extremely high discharge, such as coarse, bouldery, braided gravel surfaces and intense scouring, formed-at times when the Porcupine River at the Ramparts acted as an overflow outlet for glacial lakes in northern Yukon Territory which had been impounded by the Lauren-tide ice sheet. Terraces capped by sediment suggestive of relatively low discharge meandering streams, and which were strongly affected by Coleen River drainage, probably formed when glacial-lake overflow did not occur. Ten radiocarbon dates on alluvial sediments from the lower Porcupine River range from greater than 35,000 to 2,350 ± 55 yr B.P. When combined with geomorphologic interpretations on terraces in Alaska and with radiocarbon dates from northern Yukon Territory, these dates suggest repeated glaciolacustrine innundations of the Old Crow and Bluefish Basins during Wisconsinan time.