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<br />Data Sources
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<br />1987; Xu and Ye, 1987).
<br />Paleoflood hydrology primarily is concerned with determining the
<br />magnitude and frequency of individual paleofloods (Baker, 1987; Baker et aI., 1988;
<br />Costa, 1987; Gregory, 1983; Hupp, 1988; Jarrett, 1991; Kochel and Baker, 1982;
<br />Stedinger and Baker, 1987). Although most paleoflood studies involve prehistoric
<br />floods, the methodology is applicable to historic or modem floods (Baker, 1987).
<br />Two approaches are in current use. The geomorphic approach is based on the sizes
<br />of flood transported boulders (Costa, 1983; Gregory, 1983; Williams, 1984;
<br />Stedinger and Baker, 1987). The hydraulic approach, which is more commonly used
<br />today, is based on paleostage indicators that provide indirect evidence of the
<br />maximum stages in a flood (Baker, 1987; Hupp, 1987; Jarrett and Malde, 1987).
<br />There are many. kinds of paleostage indicators, including evidence of
<br />vegetation damage, accumulations of woody debris, and sedimentologic evidence.
<br />The latter includes erosional and depositional flood features along the margins of
<br />flow in a channel (Figure 2.1). Slack-water deposits of sand-sized particles (Figure
<br />2.2) and bouldel)' flood bar deposits commonly are used as paleostage indicators.
<br />The strategy of a paleoflood investigation is to visit the places where evidence of out-
<br />of-bank flooding is most likely to be presetved. The types of sites where flood
<br />deposits commonly are found include: (1) locations of rapid energy dissipation where
<br />flood transported sediments would be deposited, such as tributary junctions, reaches
<br />of decreased channel gradient, abrupt channel expansions, or reaches of increased
<br />flow depth; (2) locations along the sides of valleys in wide, expanding reaches where
<br />fme-grained sediments or slack-water deposits would likely be deposited; (3) ponded
<br />areas upstream from channel contractions; and (4) locations downstream from
<br />moraines across valley floors where large floods would likely deposit sediments
<br />eroded from the moraines. Lack of evidence of extraordinary floods may be as
<br />important as tangible onsite evidence of flooding (Jarrett and Costa, 1988; Levish et
<br />aI., 1994). Knowledge of the nonoccurrence of floods for long periods oftime has
<br />great potential value in improving flood frequency estimates (Stedinger and Cohn,
<br />1986). The actual value depends on the correctness of the assumed probability
<br />distribution and of the assumption that flood flows are independent and identically
<br />distributed. Paleoflood evidence is generally relatively easy to recognize and long
<br />lasting (e.g., Figure 2.3). because of the quantity, morphology, structure, and size
<br />distribution of sediments deposited by floods:
<br />Once paleostages have been estimated, a hydraulic analysis must be
<br />conducted to estimate the corresponding discharges. The step backwater method
<br />(Chow, 1959) is a commonly used and reliable method for discharge estimation in
<br />which a one-dimensional gradually-varied flow analysis is used to calculate water-
<br />surface elevations as a function of discharge. For a given site, the discharge that
<br />produces the obsetved paleostage elevations is selected as the peak discharge. The
<br />analysis readily allows for evaluation of critical assumptions, such as choice of
<br />roughness coefficients, and for estimation of uncertainties. For complex channel
<br />reaches, two-dimensional hydraulic models are coming into use (Stockstill and
<br />Berger, 1994; Miller, 1994).
<br />A third step in the analysis of a paleoflood is dating of the event A
<br />commonly used and relatively accurate dating technique is radiocarbon dating
<br />(Baker, 1987; Kochel and Baker, 1982, 1988), by which absolute ages are
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