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<br />296-2 <br /> <br />JARREIT AND TOMLINSON: REGIONAL INTERDISCIPLINARY PALEOFLOOD METHOD <br /> <br />-'. <br /> <br />," <br />" <br />~J <br /> <br /> <br />~ <br />j <br /> <br />Figure 2. Diagrammatic section across a stream channel showing a hypothetical maximum paleoflood and <br />various flood features preserved as paieostage indicators [JalTelt, 1991]. <br /> <br /> <br />risan and Reid, 1967; Hupp, 1988; Gattesfeld, 1996]. A lack of <br />scarring is an indicator that flooding has not occurred since <br />establishment of trees on the floodplain. In semiarid streams, <br />woody flood debris typically lasts 60 years or longer before <br />completely decaying (D. Levish, Bureau of Reclamation, per- <br />sonal communication, 1998). <br />Paleoflood discharge was determined from estimates of <br />flood widtb and depth corresponding to the elevation of the <br />top of flood-deposited sediments (or PSis) and channel siope <br />obtained during on-site visits to streams. Hydraulic calcula- <br />tions are similar to indirect discharge estimates using high- <br />water marks following floods, except PSI usually are older and <br />may bavc greater uncertainty. Flood depth was estimated by <br />using tbe PSis in the channel or on tbc floodplain above tbe <br /> <br />channel-bed elevation. Using the estimated flood depth and <br />channel geometry. the width and cross-sectional area below the <br />PSI elevation was determined. Because most streams in the <br />study area are higher gradient (>0.002 m m-'), paleoflood <br />discharge was estimated using tbe criticai-depth method <br />[Barnes and Davidian, 1978], which has been suggested for use <br />because flow in these streams usually is very ne8r critical or <br />slightly subcritica!, particuiarly for large floods [Jarrett, 1984, <br />1986; Triesee and Jarrett, 1987J. The slope-conveyance method <br />[Barnes and Davidian, 1978] was used to estimate paleoflood <br />discharge in the few lower-gradient rivers (mostly mainstream <br />Yampa River downstream from Craig). Flow.resistance coef- <br />ficients for lower-gradient rivers were estimated from analysis <br />of data for Colorado rivers (JalTett, 1984, 1985]. Although flow- <br /> <br />, <br />,~, <br />t <br /> <br />, <br /> <br />Figure 3. Upstream view of flood-bar deposit for Leftband Creek near Boulder, Colorado, with 1995 flood <br />waters about 20 em helow the peak stage. Peak discharge was 34 m' S-I, and the eievation of the top of flood <br />bar (and a separate slack-water deposit of sand in far right center) equaled maximum flood elevation. <br /> <br />1 <br />.. <br />.. <br />1 <br />~ <br />