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<br />=------~ <br /> <br />Table 7. Re~iona! regression equations for streamflow flood frequency used for <br />ungaged tributaries of the Colorado River in Grand Canyon, Arizona. <br /> <br />Flood frequency region' Recurrence Inte,vel (yr.) Flood-frequency 'eletlon <br /> <br />Thomas and others (1997) <br /> <br />2 <br />5 <br />]0 <br /> <br />2 <br />5 <br />10 <br /> <br />2 <br />5 <br />10 <br /> <br />Roeske (1978) <br /> <br />2 <br />5 <br />10 <br /> <br />2 <br />5 <br />10 <br /> <br />8 <br /> <br />10 <br /> <br />II <br /> <br />4 <br /> <br />Q= 598 Ao.so1 (E,II.000)'1.02 <br />Q = 2,620 AO.449 (E,/I ,000)'1.28 <br />Q = 5.310 AO.425 (E ,II ,000)' 1.40 <br /> <br />Q= 12 Ao.s8 <br />Q = 85 Ao.59 <br />Q = 200 Ao.62 <br /> <br />Q = 26 AO.62 <br />Q= 130 Ao.s6 <br />Q= 0.IOAo.52 E/'o <br /> <br />Q = 19 Ao.660 <br />Q = 66.3 AO.600 <br />Q = 127 AO.566 <br /> <br />Q = 1.35 AO.491 (E}II.000)2.25 <br />Q = 0.319 AO.446 (E"I,OOO)J.60 <br />Q = 0.143 AO.423 (E,II.000)4.31 <br /> <br />Q = peak discharge (f't3/s); A = drainage area (mi2); E, = mean basin elevation (ft); E2 = mean annual <br />evaporation (in.). <br />. For Thomas and others (1997), eastern Grand Canyon is mostly Region 8. western Grand Canyon is mostly <br />Region 10, and the central pan of Grand Canyon is mostly Region II. For Roesk.e (1918), drainage areas east <br />of the Colorado River and north of the Linle Colorado River lie in Region 4, while the remainder of Grand <br />Canyon falls within Region I. <br /> <br />regional-regression equations calculated by Roeske <br />(1978) for the state of Arizona (table 7). Values for <br />the variables listed in table 7 were determined <br />digitally using I :250,000 digital elevation models <br />and maps of free water-surface evaporation by <br />Farnsworth and others (1982). Multiple zones of <br />evaporation within any given tributary were <br />weighted by area and averaged to defme one <br />evaporation value for the tributary. <br />We assumed an expected value for the number <br />of floods to occur in a decade. This expected value <br />calls for five 2-yr floods, two 5-yr floods, and one <br />10-yr flood to deliver most of the sediment to the <br />Colorado River. Regional flood-frequency relations <br />do not produce annual floods, so we ba ve no means <br />of determining the effect of neglecting the smallest <br />events and we chose not to include the influence of <br />long r;currence-interval floods in the analysis. <br /> <br />Flood volumes and sediment-rating curves <br /> <br />Hydrographs for floods on Bright Angel Creek <br />are the only available data concerning the form of <br />streamtlow floods in Grand Canyon. We assumed <br />that the form of these hydro graphs could be <br /> <br />transferred to all ungaged tributaries if a method <br />could be developed to relate flow volume to peak <br />discharge. We digitized hydrographs from 22 <br />summer and 20 winter floods recorded in Bright <br />Angel Creek between 1924 and 1973. We <br />attempted to generalize these hydrographs and <br />apply them to all 768 tributaries of the Colorado <br />River using the regional flood-frequency relations. <br />Our attempt to scale these hydrographs according to <br />peak discharge and duration of the flood failed <br />because of the high degree of variability in the <br />shapes of the hydro graphs. Therefore, we used the <br />hydro graphs in combination with sediment-rating <br />curves to calculate a total sediment delivery per <br />event, as described in the next section. <br /> <br />Two sets of sediment-rating curves were <br />available to apply to the hydrographs from Bright <br />Angel Creek. Streamflow and sediment data were <br />collected for Bright Angel Creek during the period <br />from 1991 through 1993 (Rote and others, 1997). <br />One hundred sediment samples were used to <br />develop a rating curve, which had a ma;umum <br />discharge of 11.75 m3/s (table 9). However, the <br />rating curve for Bright Angel Creek estimates <br />sediment loads several orders of magnitude smaller <br /> <br />16 Sediment Delivery by Ungeged Trlbuterle. of the Colorado Rive, In Grend Cenyon <br />