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. . . Description of Flooding Problems Major flood events have occurred on Pawnee Creek in May 1935, June 1965, and July 1997. Minor flooding occurred in 1967, 1990 and 1992. Flows in 1935 damaged the railroad between the City of Sterling and Town of Atwood. Several hundred yards of HWY 14 west of Sterling were destroyed, and approaches to the Sterling bridge north of Atwood, HWY 6, and the bridge and tracks of railroads at the Pawnee bridge were also damaged. Discharge estimates from the 1935 event are not available, but the Pawnee Overflow did operate sending flows into Sterling. Flooding occurred on June 14 and 15 of 1965 as a result of heavy rainfall on the Pawnee Creek watershed. Considerable damage occurred throughout the lower reaches of the watershed. South of Sterling, near Town of Atwood, flood water overtopped HWY 6 and the railroad tracks. Pawnee Creek flood waters also flowed in a northeasterly direction along HWY 6 and inundated portions of the western section of Sterling. A peak flow estimate of 35,200 cfs was indirectly measured at the Colorado Highway 14 bridge approximately 10 miles upstream from Town of Atwood. The estimated Pawnee Overflow was 5,000 cfs. During the week of July 28, 1997, heavy rains caused signifiCllDt flooding in several areas including the communities of Fort Collins, Weldona, Town of Atwood, and Sterling. Rainfall over the Pawnee Creek watershed on the evening of July 29th saturated agricultural/range lands lIDd caused major storm runoff in the tributaries and main channel flood waters in Pawnee Creek crested in Town of Atwood at approximately 3 PM on July 30. Flood waters from the Pawnee overflow first reached the south side of Sterling at about 4 PM, and then crested in the downtown area around midnight on July 30th. Point rainfall amounts of up to 15 inches in a 4 to 8 hour time period were reported in the upper watershed areas of Pawnee Creek. Estimated rainfall depths over the drainage basin are shown on Figure 3. These depths were based upon a rainfall bucket survey conducted by the CWCB, Corps of Engineers, Omaha District, and Logan County staff. The peak flow estimate on the mainstem above Colorado HWY 14 is significantly higher thllD the 1965 peak flow estimate. Based upon field surveyed high water marks, flow estimates were made using indirect flow measurement techniques and are presented on the following table. Table 4. Peak Discharge Estimates - 1997 Flood Pawnee Creek and Pawnee Overflow . .. .... . ...... Location < ... ... . ...... Estimated Peak Flow (cfs) Total Pawnee Overflow into Sterling just 3,800* northeast of the Riverside Cemetery Portion of Pawnee Overflow into Sterling 2,500 Between the Riverside Cemetery and the Railroad Embankment Pawnee Creek Mainstream u/s ofHWY 14 65,000 Pawnee Creek mainstem at CR. 25 50,000** Pawnee Creek mainstem at CR. 29 45,000** * Asswnes that the Sterling No.1 Ditch is full prior to Pawnee Overflow peak dischargc. ** 1bis value is less than the upstream value due to valley storage and flow attenuation. III-2