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Abstract

The Scott and Yana outwash fans on the northeastern Gulf of Alaska exhibit a succession of facies from glacier terminus to tidewater that are each characterized by differences of gradient, clast size, bar morphology, and sedimentary structures. The upper fan has steep gradients (as much as 17.6 m/km), large maximum clast size (>10 cm), and longitudinal bars. The midfan has gentler gradients (2 to 6 m/km), clast size ranging from less than 10 cm to sand, and predominantly longitudinal bars. The lower fan, a sand area, has gradients less than 2m/km, longitudinal and linguoid bars in braided reaches, and point and lateral bars in meandering reaches.

The longitudinal bars of the upper fan consist mainly of well imbricated, poorly sorted gravel that have clast long-axes oriented transverse to the flow direction. Bars are often covered with transverse ribs, here interpreted as an upper flow regime bedform, and perhaps as relict antidunes. The midfan area is characterized by a decrease in gravel, with a corresponding increase in sand. Sand is deposited as flat upper regime beds interbedded with gravel and as lower regime megaripples in channels forming trough cross-beds. The longitudinal bars of the lower fan show planar cross-beds formed by migration of the bar slipface topped by flat beds on the bar-surface and low-angle ripple-drift cross-lamination. Linguoid bars exhibit large-scale planar to tangential cross-beds topped by ripple-drift cross-lamination. Point and lateral bars are characterized by large-scale planar to trough cross-beds caused by migration of the bar surface and bar slipface. Overbank deposits of silty sand ripple-drift cross-lamination and draped lamination increase in importance downfan.

Stream regimen is governed by early summer floodng. Measurements taken during a rising stage and a declining stage indicate gravel movement in channels on the upper fan (bars were mostly emergent), but little gravel movement on the midfan. Megaripple migration in midfan channels and linguoid-bar migration on the lower fan continue at low flow stages.

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