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The surveyed cross-section a1: each bridge was repea1;-ed up- stream and downstream far enough to allow for expans~on and contraction of flow under the bridge. Each repeated section was adjusted for slope based <In the average stream bed slope taken from the U.S.G.S. quadrangle maps. Normal depth was used for the st:arting water surface elevation. Channel "n" values ranged from 0.035 to 0.045 and overbank "n" values ranged from 0.040 to 0.050. 1. RATING CURVES The models were run for flows from 200 to 2,000 cfs for the B:wy 52 bridge and from 200 to 7,000 cfs for the Weld County road bridge. The HEC-2 model for the Hwy 52 bridge reach was considered valid up to 2,000 cfs where flmrs begin to exmled the low chord of the bridge and the channel banks. Not enough cross section information was available t:o rate the reach for flows in excess of 2,000 cfs. A dike exists along the right bank just upstream of the Weld County road bridge. The top elevation of the dike is believed to he slightly higher tha.n the low chord of the bridge. Flows greater 'than 5000 cfs would overtop the dike and flow east along the road for approximately 2,000 feet be- fore overtoppj.ng the road. Since flows in excess of 5,000 cfs overtop thEl roadway a considera.ble distance away from the channel, the HEC-2 model for the We,ld County road reach is considered very approximate for flows greater than 5,000 cfs. The 2,000 and 5,000 cfs discharges for the Hwy 52 and Weld County road bridges respectfully, correspond to 'the discharge that resul ted in backwater elevations between 'the low chord of the bridge and below the top of the road on the upstream side of the bridge. A rating curve was developed just down- stream of each bridge from these models, in order to provide a rated section at each flood warning gage (See Figures 2 and 3) . 2. TRAVEL TIMES Traveltime,s were calculate,d for a 2,000 and a 5,000 cfs discharge using velocities from the models and the stream lengt:h measured from topographic maps (See Table 1). The 5,000 cfs travel times are not shown in Table 1, as 1:his discharge!. was not: rout:ed upstream of the county road gage site. For flows greater than "!, 000 cfs, including 10, 50, 100, and 500 year :Elood events, a third HEC-2 model was devel- oped. Cross s,~ctions were taken from topographic maps from the Hwy 52 bridge to 12,000 feet upstream of the Burlington Northern Rail Road (BNRR) bridge. The downstream cross sec- tion, which is located 7000 feet upstream from the BNRR bridge, was taken from the HEC-2 decks provided by the CWCB. Once the cross sections were in HEC-2 format, intermediate sections were produced by interpolation using the Fortran program SINTE:~P. EXE. Travel time!; were calculated by the HEC-2 program using reach lengths and average velocities (See Table 1). 30