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STUDY AREA DESCRIPTIO~ Draina~e Basin Characteristics The Big Thompson River flows casterly from the high mo~ntains of the Colorado Front Range in Rocky MountBin National Park to join the Sou th Platte River southwest of Greeley, Colorado. Basin elevations vary over 14,000 feet, mean sea level (m.s.l.) at the source to about 4,700 feet, m.s.l. at the mouth. Eight miles west of Loveland the river leaves the mountain canyons, passes through a short reach of foothills and enters the high plains area. Major tributaries below the canyon mouth are Buckhorn Creek which drains from the north and enters just west of Loveland, and the Little Thompson River Which joins the parent stream from the right bank near its mouth. The total drainago area above the mouth is 830 square miles. The reduced drainage area upstream from Loveland is 515 square miles as shown in Plate 1. Study Reach Description The portion of the Big ThQmpson River sOOied in this report extends from the eastern edge of the foothills to a point 8 miles do~~stream as shown On Plate 1. The Colorado & Southern Railway crOsses the flood plain south of Lovela",-d near tt". "d~!,oi,,~ of th.. st"rly rPR~_h. Tho> flood p1a:!.n is fair1yflatand"veragesl/2roileiriWidth. Tllechannelismoderate1y sinuous, the 8-mile length of river within the study reach covers a valley distance of only 6 mllo~. Channel dimensions vary appreciably from 4 to 10 feet in depth and 50 to 200 feet in ..idth. The streambed is enIVelly and has ~n average slope of 19 feet per mile through the Atudy r~ach. The banks of tho stream are vegetated with weeds, willows, snd an occasional clUlllpoftrees. The flood pl~in is used predominantly for agricu1tur" and gravel mininG. !lulldings aSliIociacudw1th the abQv"" use>l occur at intervals nlon 1\ the flood plain. A moderate d"!,r,,e "f urbanhat:l<!n occurll withi", th", l-~il~ strip of the flood plain whic~ is bjs~cred by U. s. Highway 287. 7 RE:\OURCE: CON\UlT~NT\ INC Ihe urbanization cOrlsisu of the fairgrounds, comrr.ercial facilities and a few residential acreages. Ten road bridges, one railroad bridge and two foot bridges span the Big Thompson River within the study reach. Railroad Avenue and the Colo- rado & Southern Railway each have an additional bridge which spans an overflo>lchannellocatedl,500feetaouthofthemainriverchanne1. All 11 the decks- of the bridges in the study reach are above the 100-year flood elevation vith the exception of the St. Louis Avenue and Railroad Avenue bridge. Portions of the approach r<!adways are overtopped at all bridges. I I Elevated rondways that cross the flood plain impede flood flows and raise the water surface to a degree dependent on the height of the roadway and the size of the waterway opening at the bridge. Likewise irrigation diversion strUctures may impede flood flo>ls. The diversion dans or weirs impede channel flows while the embankments adjacent to the diversion ditches impede overbank flows. The effects of the roadway and irrigation diversion obstructions ere shown in Table 5--Flood Frequency Elevation and Discharge Data, and on the Profile Plates J, 4, and 5 at the end of this report. Approximate elevations of the bridge deck and top of "ater- 1/ vay- are also shown on the profiles. There are no structures along the Bi~ Thompson River or its tribu- taries which provide a major reduction in flood flows. Numcrous existing levees along the river are able to contain frequent floods (I.e., 10-to 25-year floods), but provide little protection agatnst thp 100-ypsr floorl. 1/ - See Glosl\sry of TetJlls , 1 I, I' , , , " RE:\OURCE: CON\UlT~NT\ INC