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<br />draft 3120/98'
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<br />Large differences in maximum paleoflood and PMF values and the large cost of retrofitting possible
<br />spillway deficiencies for thousands of dams in the United States demonstrate the importance of
<br />additional hydrometeorologic and paleoflood research and using risk-based assessments for dam
<br />safety. Most state dam-safety agencies in the Rocky Mountain region have ongoing
<br />hydrometeorologic and paleoflood studies to better estimate extreme rainfall and flooding for dam
<br />safety. The Bureau of Reclamation recently began using a risk-based assessment for some of their
<br />dams, which incorporates paleoflood investigations to provide estimates of the magnitude and
<br />frequency of extreme floods, to assist with dam safety decision making (Bureau of Reclamation,
<br />written commun., 1997).
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<br />Floods leave distinctive fluvial-sediment deposits (Kochel and Baker, 1982; Costa, 1987; Knox, 1988;
<br />Jarrett, 1991; Ely et al., 1993; O'Connor et al., 1994), botanic evidence (Hupp, 1988), erosional
<br />features on channel margins (Baker, 1987; Jarrett and Malde, 1987; O'Connor, 1993), and landforms
<br />(Costa, 1978; Jarrett, 1991; Hupp, 1988; Levish et al., 1994) in streams and on floodplains. These
<br />features, sometimes termed paleostage indicators (PSIs), can be used to infer the maximum stage of
<br />past floods (fig. I). In paleoflood studies, the most commonly used PSIs are slack-water deposits of
<br />silt and sand (SWDs) (Kochel and Baker, 1982, 1988; Baker, 1987; Webb et al., 1988; Enzel et al.,
<br />1993; Ely et al., 1993; O'Connor et al., 1994) and bouldery flood bars (FBs) (Baker, 1984; Jarrett
<br />and Malde, 1987; Jarrett, 1990, in review; Waythomas and Jarrett, 1994; WOhl, 1995) and erosional
<br />features (Costa, 1978; Jarrett and Costa, 1988; Levish et al., 1994). PSIs of flood-deposited sediments
<br />composed of sand. gravel, and boulders can be preserved for many thonsands of years (Kochel and
<br />Baker, 1982; Jarrett and Malde, 1987; Knox, 1988; Ely et al., 1993; O'Connor et al., 1994;
<br />Waythomas and Jarrett. 1994; Springer and Kite, 1997). Sites where flood deposits commonly are
<br />preserved include: (I) locations of rapid energy dissipation, where sediments would be deposited,
<br />such as tributary junctions, areas of decreased channel gradient, abrupt channel expansions, or areas
<br />of increased flow depth; (2) locations along the sides of valleys in wide, expanding areas where fine-
<br />grained sediments or slack-water deposits would likely be deposited; (3) backwater areas upstream
<br />from channel-width constrictions; and (4) locations downstream from glacial moraines across valley
<br />floors where large floods would likely deposit sediments eroded from the moraines. Optimal
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