Abstract

Three anastomosed river systems are described. Each reach consists of an interconnected network of low-slope, narrow and deep, straight to sinuous, stable channels that transport coarse sand and gravel. Channels are separated by levees and wetlands composed of silt/mud and vegetation. Gravel-bed braided channels occur upstream from each anastomosed system, joined by a transitional reach comprising stable, elongate, silt islands within braided channels. The three anastomosed reaches have formed upstream from elevating base levels caused by deposition of alluvial fans across trunk valleys. Rapid aggradation of floodplain alluvium is confirmed by buried volcanic ash layers. Channel migration is inhibited by root-stabilized banks which, combined with rapid vertical aggradation, results in production of stringer-like, coarse-grained channel deposits surrounded by overbank fines in stratigraphic cross sections. Although it is unlikely that such small base-level controls (alluvial fans) could produce extensive anastomosed deposits, other mechanisms such as glacial moraines, isostatic rebound, or marine transgressions could provide plausible controls for yielding important contributions to the stratigraphic record.

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