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<br />of mass and lichen are a function of climate (and microclimate) and <br />geochemical weathering of rocks; hence, detailed studies are needed to <br />determine reliable dates since a flood. The use for this <br />investigation is to provide a relative time frame of flooding between <br />the different streams in the study area. As a general rule moss can <br />take 10 or more years to reestablish following a flood; lichen can <br />take 10 to 20 years to recolonize and then hundreds of years far the <br />colonies to merge together and completely caver a surface. A near <br />completely covered lichen surface is indicative of a very old surface <br />(and und~sturbed by flooding). <br />Depos~tional features, which occur where streamflow velocity can <br />not transport the sediment in motion, include large boulder bars in <br />the channel and alluvial fans at tributary junctions, at rapid channel <br />expansions, and at decreases in channel slope. Along many streams <br />terraces on the channel margins are formed during the flood. Flood <br />boulders can easily be distinquished from boulders that have rolled <br />off valley walls as they are more rounded because of their movement <br />and abrasion by flawing water and sediment. Generally the mare <br />frequent the floods (and particle movement) the more rounded the <br />particle. Magnitude of floods can be determined based an the size of <br />boulders moved. In these basins there are boulders of essentially all <br />sizes available far transport (boulders as large as houses occur in <br />these basins). The larger a boulder that was transported by a flood, <br />the larger the flood velocity and depth. Interpretation of these <br />sediment deposits can provide both the number and the magnitude of <br />past floods. <br />It has been shawn (Jarrett, 1987) that debris flaws generally <br />occur above 7,500 feet in Colorado because of steep slopes and that <br />there has been a problem misidentifying debris flaws as water floods. <br />Because debris-flow deposits were identified in one basin <br />investigated, a brief overview is provided. Debris flaws are defined <br />as mass-movement intermediate between landsliding and water flooding <br />that contain very large quantities of sediment. Debris flows are a <br />heterogeneous mixture of sediment sizes and water, because the <br />processes differ, mitigation of their hazards differ. Debris-flow <br />deposits can be differentiated from water flood deposits. Debris-flow <br />deposits have characteristic levees on the sides of the channels. <br />These depositsj are poorly sorted and the largest boulders are on the <br />top surface, and there is no evidence of "flooding" downstream from a <br />characteristic terminal lobe marking the end of the debris flaw. <br />Water-flood depos~ts are sorted by grain size, cross-bedded, <br />stratified, and with gradational boundaries; sediments often are <br />deposited as bars or alluvial fans. The largest boulders are <br />concentrated near the bottom center of the deposits (where velocity <br />and depth are largest). <br />When gravel bars are deposited, vegetation regenerates on top of <br />the bars (or on other flood deposits). Recent flood deposits support <br />little vegetation; older deposits have increasingly more vegetation. <br />Trees growing on the surface can be cored, annual tree rings counted, <br />and approximate date of the flood determined. (A tree corer was not <br />available far this survey, but the diameter and types of these trees <br /> <br />3. <br />