Active stream systems on the glacial outwash plain of southeastern Alaska are building individual fans that exhibit a systematic variation in gradient, morphology, and suites of sedimentary structures from glacier terminus to ocean. The upper fan is characterized by a single, incised stream channel. The central fan, delta-shaped in plan view and occupied by braided streams, is the locus of most active deposition. It is subdivided into a gravel-depositional (upper) area and a sand-depositional (lower) area. A marsh or swamp area, with both braided and meandering streams, may be present at the fan terminus.

Bar morphology changes downstream from sheet bars to longitudinal bars to a complex of longitudinal and linguoid bars. Side and point bars are found in meandering streams. Mega-ripples are common in channels on the lower fan and the sand-depositional central fan, but are rare elsewhere.

A downstream succession of sedimentary structures is (1) well-imbricated, poorly sorted, coarse gravel

End_Page 604------------------------------

with crude bedding, (2) interbedded fine gravel and plane-bedded sand and some large-scale planar crossbeds, and (3) a repeated sequence of large-scale festoon crossbeds overlain by sandy ripple-drift capped by a silt drape. Although absent on the upper-central fan, overbank deposition is present as sandy ripple drift on the lower-central fan, and is important on the lower fan as silty ripple drift, undulatory bedding, and silty-clay laminations.

The outwash plain is in a positive tectonic area (except possibly the Copper River delta) and may be viewed as a regressive series of coalescing fan-deltas.

End_of_Article - Last_Page 605------------