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III. BASIN CHARACTERISTICS AND HYDROLOGY i Pawn~~ Creek FigureS Is <lmapwhich Shows each watershed considered in this study. Pawnee Creek drains an area of approximately 703 square miles upstream from its confluenc~ wit~ the South Platte River about 5 miles southwest of Sterling. Although Pawne~ Creek is located several miles southwest from Sterling, U.S. Highway 6 and the Burlington/Union Pacific Railroad tracks are major obstructions which divert runoff from the Pa,mee flood plain into Sterling via the so-called "PawneeOverflow"nl:'arRiv l:'rside Ce~tary. According to a recent analysis madl:' by thl:' U.S. Army Corps of Enginl:'l:'rs, the 100.y~ar flood at th~ Pawne~ 6v~rflow is approximately 5,000 CUbic feet per second (U.S. Army, 1982). In a r~cently published flood plain information report, the U.S. Army (1978) estimated the 100- year flood to be 41,000 c.f.s. for Pawnee Creek at U.S. Highway 6. This compares favorablJ with 35,200 c.f.s. measured at the bridge on Colorado Highway 14 some 13 miles upstream and west of Sterling on June 15, 1965 (see Matthai. 1969). Finally, a review of the flOOd history indicated that the South Platte River is generally at flood stage during najor flood events on Sand Creek and Pawnee Creek. Accordingly design floods on Pawnel:' Creek and Sand Creek were assumed to be coincident with 10.y~ar flood flows in the River. Figure 7- Sand Creek flood walers at Pioneer Ditch and Highway 138, June 14, 1968 Photo courtesy of Slarlir>g Joumal-Advoca/e Sand Cree~ Watersheds That portion of the Sand Creek drainage lying north of Highway 14 and upstream from the Springdale and Sterling No.1 ditches was resolved into three subwatershedsasfollows: Subwatershed OrainaqeArea(mi.2) Sand Creek 5-1 '.2 ,.J These watersheds are shown in 29.58 1.13 3.80 0.70 Figure 8. 12 "