Braided streams dominate environments characterized by high sediment load and flashy discharge. Although coarse-grained braided alluvium is most abundant today in association with semiarid to arid alluvial fans, several authors have speculated about the effects that a lack of terrestrial vegetation may have had on sedimentation prior to the late Paleozoic. It has been suggested that the increased flashiness of discharge and sediment yield associated with the lack of vegetation probably biased the pre-Carboniferous record toward braided alluvial deposits formed in humid areas.

A model for alluvial-fan sedimentation in a humid environment is based on the investigation of six fans formed in response to flooding associated with hurricane passage along the Sierra de

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Omoa of northwest Honduras, Central America. This model and the similarities between the facies herein described and braided alluvial deposits formed in other humid settings (proglacial) suggest that the deposits of ancient humid alluvial fans may be readily distinguished from those of arid fans.

Humid fans differ from arid fans with respect to slope, gravel roundness, downfan changes in roundness, the patterns of imbrication and long axis orientation, and the abundance of debris flow deposits. Humid fans display a gentle, smoothly sloping concave-upward longitudinal profile, whereas arid fans are steeper and typically consist of segmented straight sections, producing a profile which is concave upward overall. Deposits consist largely of subangular to subrounded gravels and there is typically little change in gravel roundness downfan. Unlike arid fans, angular clasts are rare in humid fans. Imbrication and long-axis orientation transverse to flow are each well developed and, although each may be present on arid fans, their development in a humid setting is more striking. The p incipal difference, however, is the complete lack, among proximal sediments, of evidence for debris-flow deposition.

Proximal-fan deposits of humid fans are very poorly sorted, clast supported, and have a matrix of granular sand. Deposits generally have a crude horizontal stratification. Distally, there are transitions from clast-supported fine gravel, through sand matrix-supported gravel, to granular sands. In distal-fan areas, horizontal laminations are the dominant sedimentary structure, although high- and low-angle planar cross-stratification and trough cross-bedding may also be present.

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