Abstract

Witnessing a two-peak flash flood in west-central Arizona enabled us to make the following observations: (1) Sheet flow and associated streamlet flow on pediment surfaces began shortly after a period of heavy rain. The flood crest in the wash lagged several tens of minutes behind the initiation of streamlet flow. (2) The flash flood was preceded by an accumulation of standing water in the deepest and most active part of the channel. Pools of standing water gradually merged to form the first flood, which then receded. The second arrived suddenly in the form of a foamy tongue. (3) The rise time of the flood was measured in seconds, and the fall time was measured in seconds in the channel slip-off-slope but in minutes for the channel bottom. (4) Magnetite-ilmenite black sand was deposited at the top of the alluvial unit left by the flood over substantial parts of the channel, chiefly in those areas that emerged most rapidly. The same layer was covered by as much as 2 cm of quartz-feldspar sand in the lower, more active, and slowly emergent parts of the channel. We suggest that this distribution of black sand resulted from the extremely rapid fall of flood waters, a characteristic of flash floods. Consequently, heavy-mineral layers preserved near the top of alluvial units is one criterion suggesting that such deposits were laid down in an aggrading, probably arid environment subject to flash flooding.

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