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SU:e.MARYOffLOOOSITUATlON . The city of Pueblo, Pueblo County, Colorado, is lo- cated at the confluence of the Arkansas River and fountain Cre9k (~ee Plate 1). Among the flcod prcne dr..inages that contribute to tho Pueblo flood problem, the St. Ch~rles River is a right- benk tributary th~t enters the Arkansas River ~bout 8 miles east of Pueblo. This report covar~ ~ portion of the St. Ch~rle~ River flood pl~in that is within the planning jurisdiction of the Pueblo Regional Planning Com~ission. The ~tudy ~rea extends from Mile 0.0 to Mile 2?6 es sho~~ on Plate 3. Pueblo's principel reside~ti~l d~velopment~ are on high ground edjoining the various weterweys thet intersect the city, but soma residentiel and com~erciel davelopment axists on flood plain lands which are immedietely edjacent to flood prone channels. These la~ds, includi~g e large segment of th~ down- town commercial and industriftl arees. have been co~siste~tly in- und~t~d. by mejor floods of the p~st. As development of the flood pl~i~. co~tinuos and flow obstructio~s multiply, ever lerger ~reaS can become vulner"bh to the pote~tially greet floods of the fu- tu..._ . ThO St. Cherlcc River flood pleins specific~lly stud. ied i~ this report have rern~inod relatively undovclopod end ro- mote from Pueblo's urba~ growth until recent years. However, a new devolopme~t trend now appears to be 9~inin9 momentum ~~d more rapid urba~ization is oxpected 1~ the future. Only fragmentary streamflow records ore av~ilabln for the ~tudy roech. A stream gaging station WaS maintained near the . mouth of the St. C~arleB River duri~g the years 1921 throug~ 1925. Con~i~uous records of slreamflows, from 1942 t~roug~ 1955, were obtained at a 9~ging etetio~ located at the 2?t~ Le~e bridge. Historical documents a~d newsp~per files have been ~carc~ed for information concorning post fioods. T~e local flood . 3 situation on t~e St. C~arle6 River ha~ been developed from t~e6e investigations and studies of po~sibl" future floods. The follow- ing p~r~graphs su~arize significa~t findings which are discussed in more detail in succeeding sections of the report. . . . HISTORICAL FLOODS in the general~Pueblo region, particularly those occurri~g on t~e Ark~n"as River and Fou~tain Creek, have bee~ re_ ported with re~so~able eccuracy sinca the mid-1800's. Fragmentary records of flooding effects on t~e St. Charles River bagin with the major flood of June 4, 1921, however, a~d av~ilable newspaper aC- counte provide only abbreviated descripticns of later floods as they occurred in 1948, 1950, and 1955. Reliable information of other damaging floods has been lost in the obecurity of time or i6 generelly inco~clusive for study purposes. . . . INTER~EOIATE RECIONAL FLOODS are floods that have a~ average fre- quency of occurrence i~ the order of o~ce in 100 years. They are determined from a~a"yses of p~st floods that have occurred on the St. Charles River and ot~et streams in the general area. . - . STANDARD PROJECT FLOODS are floods whic~ would be exceoded only on rare occasions a~d are reasonably characteristic of the geograph- ical area in w~ich t~e drainago basin is located. In meny inata~ces, they mfty define the upper limit of flooding against whic~ proteo_ tion may be sought. The 5ta~dard Project Flood would average 3.8, 5.6, and 5.2 faet higher than the Intermediate Regional flood i~ . , the upper, middle, ~nd lower portions, respectively, of the tot~l st. Charles River study reach. . . . FLOOD OAMAGES resulting from eilher the Intermediate Regional flood or the Standerd Project flood would be more cxte~sive tha~ from past floods because of increased development in lhe flood plain and t~e consequent wider extent, greater depths, and higher velocities ,