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. agri~ultural purposes. Most of the new construction--primarily residantial__i~ located in the vicinity of ford Lana, U.S. Hign- ~ay 50, Or 27th Lare. This development trend no~ appears to be gai~ing momentum, but agrIculture and ~tock-gra~i~g Betlvitle, remain dominant. . Trensportetion facilities in the flood plain include portions of U.S. Highway 50, 85-87 (Interstate 25), various couo_ ty roads, and rei 1 lines of the Denver and Rio Granda Western Railroad Company (O&RGW) and the Colorado and Southern Railway Company (C&5) Five vehicular bridges and ona railroad bridge span the river channel and ~rOaS the flood plain. The railroad bridg~ i8 used jointly by the twe r~ilroad companies mentioned. TABLE: 1 DRAINACE AREAS IN WAT(RS~ED or ST. CHARLES RIVER Mile Above Drainage Loclltion ~ Area ". miles " river mouth 0.00 '" Below Creenhorn River cOrlfluerlc" 18.45 m Above Creerlhorn River co"fluence 18.80 189 Below Stirlki"9Hrroyo confluence 27.07 in . Bridoe,; across theStrearn Three vehicular bridges cross the St. Charles River at points within the lower 10 miles of the 27.6 mile study reach. The other two vehicular bridges and single railrOlld bridge are located within the upper 10 mile segment. Photographs of these bridges appear irl figures 1 through 3. Desoriptive struotural informatiorl and pertinent elevotiOrls are listed in Table~ 2 arld3. , " During the Intermediate Regional flOOd and Standard Projeot flood, the U.S. Kigh~~y 50 bridge would not be overtopped, although floodflows would top the high~~y as it crosees the flood plain. Tho South Road and Interstate 25 bridges would be Over_ topped by oOe or both flood events, while the 27th Laoe bridge would be overtopped by the Standard Project flood only. In Con_ trast, the tops of rails on the railroad bridge would be 22.2 feet and 42 feet above the respective flood ~re5ts. [~oess1ve head 10ss8s would cause increased depths of flow immediately upstream, from e~~h of the bridges. Depths of flow at bridges would be effected by the de_ gree of etruotural obstruction and clogging of waterway openings that occurs during floods. for etudy purposes herein, it has been assumed that limited clogging can be e~pected end ell bridge struc- tures would etand intact. Significant changes in this premise, imposed by difforing conditions of a future flood, could alter the estimated flood crests and related flood limite shown. Dbstructior,stoFloodflow Thseffe<:t ofobstroctiO'1S dCJS to bridges "ndtheir approach fille has been described in previous paragraphs. New development and futCJre land-filling in or nsar ths rivor Channel oould obstruct floodflowe ~nd c~use increasod depths. These con- ditions or greater hazard Oan also occur where trees and under_ brush are allowed to grow without reetraint within the flowage arca. F1oodRecords Stream stege and discharge records which derive from stream gagin9 stations on the study reach are fragmentary or other_ wise inadequate for accurate determinations ae to the characteris- tics of past floods. These records include limited flow dat~ ob- teined from 3 gaging station nmintained '1e~r lhB mouth of the 13