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, FEB-25-97 TUE 11:49 BRR BLD 53 DFC ~I),lARI\ETr plFllilNO sr..TIA1.OCCURRENCi orPl.OOD6 Ie 1976 flood. This ,ABLE I , 976 flood was the eI - I' , r '. 1 -~ d flu',' .. J 'al +- t C: ',11 Upper evation IIDll 0 I'Il1llill -p......u.. ........ 1ft ...e ael re.....a rODl south Plalle River baslll cletcmlined by paleoflood 10,000 years ago !ill\'alisatioDS I Investiaations in 1- dicate that the Big ISU'ClIm I 'ark (drainage area I , J. 2) has not had a 'Cache I. Poucln RiV\."I" 2070 I t&e &reater than 8S ' ,i1St 80~Oto ! 0.000" ! Buckhorn Creek tent With estimates ~ 1987; larrettand BiaThomPlOlIlUver 2300 igations were con- inages of the South ,'othills and moun. ~ criteria were used Ice. The spatial dis- ~ depositional fea-' the boundary be- and the mountain lod deposits are not evation (Table I). hese streams (Fol. : indirect-discharae I fcological Survey, II, Colorado) are investigated, a very 15ition in the mor- , ,f flood deposits oc- ~mit of flooding in . 1ges from 2040 to ,:flects local topo. ,f1uences, These ; titional supportin have not' occurred ,Platte River basin , I . r basin and Tucker l highest elevations ontain geomorphie I le largest floods in 'rrence intervals of u, 1987). Storms lbly represent the ElcvaliOll DaleS of cIocumenled (m) f100cSs 2040 51. V rain Creele 2200 Boulder Creek 2160 Coal Creele 2160 Tucker GUlch 2300 Clear Creek 2130 Bear Creek 2130 Turkey Creek 2200 Deer Creek 2200 Soulb Platte River 2070 1844,1864,1876.1904, 1921,1923,1930, t938, ml,1976 1923,1938,1948,19~I, 1976 1864,1894,1906,1919, 1921,1923,1938,1941, 1942, 194~, 1949, 19~I, 1976,1980 1864,1876,1894,1919, 1921,1938,1941,1949, 19~I,19~7,1969 1864,1876,1894,1914, 1923, 1938, 1969 1969, 1973 1872, 1896, 1948, 196~ 1864,1867,1888,1896, 1933, 1941,196~,1983 1876,1894,1896,1933, 1938,1969,1973 1946,1969,1973 196~, 1969,1973 1864,1867,1876,1894, 1921, 1933,196~, 1969, 1973 largest and most infrequent that have occurred in the foothills of Colorado in historic times. Dejining the contributing drainage area of a !lood One important factor in flood hydrology is determinina the size of the drainaae area over which the rainfall occurred that produced the flood. Because most storms rarely co"er an en. tire drainage basin in the study area, even for the very rare 1948 Tucker Gulch and 1976 Bia Thompson River storms, use of paleoflood techniques helps delineate the approximate 1, FAX NO, 2365034 P,03 , 191 . , storm limits and the collt1butina drainago area. , As an example, little precl/litation data were available for the storm thaf C'~used flooding on Sweetwater Creek in 1976,111e drainage basin covers 272 km~ at an cle~al~n of 1900 m in the Colorado River basin nett DoUero, Colo- rado. The peak discharge for:liIis flood was 209 m3 S-I. The unit discharge, 'fmputed using a total drainage areaof272 kmr was 0.77 m3 S-I km-> and does not indica[l~ very larae flood. Rill erosion on exposed siO'p:(s was substantial but was limited to a few ~lS below 2300 m, Erosional and depositiona;t features (as previ- ously described) were apparent primarily in Lucky Gulch, a small tribJ,lt(ry to Sweetwater Creek, about 3.2 kIn upstrea.P from iu conflu- ence with the Colorado R,~hr. and in nine nearby small tributaries. Ol~er streams (par- ticularly Sweetwater C~~f upstream from Lucky Gulch) indicated mh~mal acomorphic and sedimentologic evidenc'.t of storm runofT, Relatively low dischar&es (fl~ws within the ac- tive channel) are required tp leave flood evi. dence of storm runoff. Conv,fntional hydraulic methods were used to coin~te approximate discharges in the streams In fhe vicinity orthe storm, The approximate bpt:l1dary of the 1976 si&I1ificant storm runofT:""~ delineated by mapping the boundary ~.t"een clearly de- fined flood features and thf absence of such features (Fig, 12). This delipeation indicated that the contributing drain:aSf area for the 1976 Sweetwater Creek flood. iII~s approximately 11.7 lan', This contributii1i:ilrainage area was used to compute a unit di~c}~ge or 18 ml S-I km-2, which indicates ~allthis flood is the larzest known unit dischl\~f in the Colorado River basin in Colorado.' ~ Additional documentatip.~s of the contrib- uting drainage area of flood, ~sini geomorphic and sedimentologic infonna~on and hydraulic estimates of associated di!ll:~arges are needed for the evaluation ofIarge slpnns. These types of paleohydrologic investip:fDs could be used to help define and enhanc'tlsohyetal patterns ',.. ,- I . I ~ . I .~ '. . ,