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<br />2964 <br /> <br />JARRETT AND TOMLINSON: REGIONAL INTERDISCIPLINARY PALEOFLOOD METHOD <br /> <br />. , <br />in steep mountain basins [Boner and Stemtitz, 1967; Snipes et' <br />af., 1974; Schwarz el af., 1975; McCain el al., 1979; Cosla, 1983; <br />Jarrell and Cosla, 1986; Jarrett, 1987, 199Ob, 1991; Grimm el al., <br />1995; Waylhomas and Jarrett, 1994] is unequivocal (e.g., Figures <br />3-5). Paleoflood evidence is relatively easy to recognize and <br />long lasting because of the quantity, morphology, and structure <br />and size of sediments deposited by floods. Lack cf movement <br />of all sizes of clasts on a streambed with an unlimited size of <br />clasts is a direct measure of flow competence [Costa, 1983; <br />Komar, 1987; Bull, 1990; Wilcock, 1992] and an indirect mea- <br />sure of flood discharge. In many channels in Colorado, there is <br />an "unlimited" size of clasts present in channels because of <br />past glacial outwash or rockfall that are available for transport. <br />Point, longitudinal, and transverse bars are built up of layers of <br />silt, sand, gravel, cobble, and boulders from bed load to about <br />the height of maximum flood water [Jarrett el af., 1996J. For a <br />given reach of channel the relation of maximum clast size in a <br />flood deposit to the available clast size in the upstream channel <br />was noted. Clasts show little reworking on the stream bed <br />(random pattern due to winnowing of finer ma1:erials) when <br />floods are small; however, large floods tend to produce well- <br />developed, fluvial-depositional features. In adjition, when <br />flows exceed the main channel by even small dep:hs (<0.5 m), <br />coarser-grained bed material is transported onto the floodplain <br />and deposited [Jarrett el ai., 1996J such as shown in Figure 3; <br />thus lack of coarse material on floodplain surfaces indicated <br />minimal inundation. <br />There are three main sources of uncertainty in paleoflood <br />reconstructions [Jarrett and Maide, 1987]: selection of flow- <br />resistance coefficients, channel changes, and representative- <br />ness of PSIs of flood height Selecting flow-resistance coeffi- <br />cients, whether Manning's n value as in this s':udy or other <br />resistance forms (e.g.. Darcy's or Chezy's), can be problematic <br />for flood studies, particularly for paleoflood e~ timates when <br />riparian vegetation may have been different and unquantifi- <br />able, Jarrett [1986] and TrieSIe alld Jarrett [198~'] indicate for <br />most higher-gradient streams and many lower~gradient rivers, <br />flow is about critical or slightly subcritical even in channels <br />having substantial channel and floodplain vegetation prior to a <br />flood, Large floods tend to remoVe most vegetation prior to the <br />peak stage [Matthai, 1969; McCain el ai., 1979; Phillips and <br />Ingersoll, 1998], and because flow is nearly critical, some of the <br />uncertainty in estimating n values due to unquantifiable past <br />vegetation is removed. <br />To minimize effects due to channel changes, the second <br />sourCe of uncertainty, cross sections were loca:ed in bedrock <br />reaches or in relatively stable alluvial reaches, !'>Jthough most <br />of the Yampa River is alluvial, there are good constraints on <br />relative stability or slight rates of incision during the Holocene <br />[Madole, 1991a]. For alluvial tributary streams that may have <br />undergone cyclical aggradation and degradation or if these <br />cycles were difficult to ascertain, then very restrictive (conser- <br />vative) ages were assigned to the paleoflood estimate, usually <br />less than 100 years. <br />The third source of uncertainty in paleoflood reconstruc- <br />tions is maximum flood height inferred from PSIs. In most <br />paleoflood studies, PSIs have been assumed to be slightly lower <br />than maximum flood height [Kochel and Baker, 1982; NRC, <br />1988; Baker et ai., 1988], New research on the ,levation of the <br />top of flood-deposited sediment (new PSIs) and high-water <br />marks (HWMs) of recent flooding in 90 streams primarily in <br />the western United States that has been dore [JalTett et ai., <br />1996J challenges this assumption. HWMs are the evidence of <br /> <br />the highest stage reached by a flood and primarily consist of <br />fine woody debris, leaves, grass, needles, other floatable mate- <br />rials and mud lines. These streams have drainage areas that <br />range from about 0.004 lan2 to more than 6000 lan'. Peak <br />discharge ranged from about 0.03 to 2500 m3 s", and the <br />majority .were iarger than 100-year floods. Stream gradient at <br />these sites ranges from about 0.0007 to 0.35 m m ". Tbe size of <br />flood~deposited sediments ranged from silt to boulders more <br />than 4 m in diameter; only those deposits considered to have <br />long-term preservation potential were documented. Analysis <br />of the differences in PSIs and HWMs indicates that the eleva- <br />tions of tbe top of flood-deposited sediments (PSIs) generally <br />are within ::!:O.2 m of flood HWM elevations, Therefore use of <br />the top of flood-deposited sediments as PSIs for streams in this <br />study provides a reliable estimate of the maximum paleoflood <br />depth that is used to reconstruct tbe discharge of paleofloods. <br />Good HWMs for the 1995 near-record peak flows during <br />fieldwork were established, which were not that much smaller <br />than maximum paleofloods. Paleoflood techniques were used <br />to estimate peak discharge for the 1995 flood where stream- <br />flow-gaging station estimates were available but without prior <br />discharge knowledge, Although this comparative approach <br />does not consider all the uncertainties, the comparison does <br />provide an estimate of the uncertainty of the critical depth and <br />slope-conveyance methods for estimating peak discharge in the <br />study. <br /> <br />4.2. Paleoflood Chronology <br /> <br />In this study, a variety of relative-dating (RD) techniques <br />were used to estimate the approximate length of time (age) <br />corresponding to the highest flood deposits emplaced during <br />the Holocene for subsequent use in flood-frequency analysis, <br />RD techniques based on landform modification, rock- <br />weathering features, and soil development have long been used <br />to differentiate and map Quaternary deposits in the western <br />United States with emphasis on dating glacial deposits [Birke- <br />iand el ai., 1979; Colman and Pierce, 1983; Birkeland, 1990]. <br />Rock-weathering, soil, and geomorphic parameters all change <br />with time [Coiman and Pierce, 1983; Birkeland el ai., 1979J. <br />Although mucb dating has involved fluvial deposits, ages pri- <br />marily are determined with absolute-dating methods such as <br />14C, thermoluminesce, and dendrochronology [Kochel and <br />Baker, 1982; Hupp, 1988; Coiman and Pierce, 1991J. Some <br />investigators have used a variety of RD methods, which utilize <br />the collective strengths of each method [Burke and Birkeland, <br />1979; Harden, 1986, 1990; Waythomas and Jarrett, 1994; Mills <br />and Allison, 1995J. The chances of arriving at a valid approxi- <br />mate age is greatly enhanced if a variety of RD methods are <br />measured [Birkeland et ai., 1979]. ' <br />RD methods appiied to surficial deposits are based on post- <br />depositional modifications that vary with age [Birkeland el aI., <br />1979J. Field evidence of age is usually derived from soil prop- <br />erties, rock-weathering characteristics, changes in landform <br />morphology, and lichenometry. Their usefulness for character- <br />izing surficial deposits is related to the degree to which they <br />can be quantified and to their rate of change [Birkeland et ai., <br />1979]. Episodic flooding produces deposits of different ages <br />that can be separated by long periods; thus the deposits have <br />distinctive properties that are amenable to measurement by <br />RD methods. It is assumed that when flood~deposited sedi. <br />ments undergo transport and reworking during high~energy <br />flood transport, the weathering "clock" essentially is reset to <br />zero [Waythomas and Jarrett, 1994). Certainly, a limitation is <br /> <br /> <br />." <br /> <br />l <br /> <br />, <br />~ <br />~. <br />. <br /> <br />I <br />, <br /> <br />;; <br />'. <br />, <br /> <br />l <br />:1 <br />i. <br />i, <br />t <br />~ <br />, <br />~ <br />" <br />