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b~sed on Channel ~nd flood plain conditions ~s represented by survey dat~ g~thered In ""ori I 1975 ~nd ~s suop lemented by later field investigations. est~bllshed by determining the water surface elevation from the profIle or reference tables and then locating that elevation by survey on the flood plain. The profiles on plates 9 through 23 show the water sur- face elevations and w"ter depth of the I r)-year, 50~year, IOO-year, and 500~year floods rel~tive to the streambed. In this particular case and throughout this study, tho streambed is actually the ch~n- nellowflowwatersurfaceelevatlonsincethesurveY5WereCOn- ducted by aerial photogrammetrlc method~. The water surface ele- vations of the !OO~year and 500~year floods were used to determine flood limits on t"e cross sections and topographic ,,"'ps. The results are Shown on the Flooded Areas, plates 3 through 8, An index to+he Flooded Areas plates Ison plate 2. Representative cross soctlons are Illustrated on plates 24~28, The cross sections show ground elevations across the valley on both sldos of the channel and the depths of overbank flOOding. Reference points, coinciding with the locations of the cross sections and pertinent planImetric features are shown on the Flooded Areas and profile forcorrellltion beheen the drawings. Tables 7llnd 8 provIde dat.!! at reference point locations, including distance along the main channel centerline, elevations of the streambed, and elevations of the 10-year, 50-year, IOO~ye~r, ~nd 500-year floods. A reference line is shown on the Flooded Are~s for both the Btg Thompson River and for the Little Thompson River. This line !s used for reference only and does not necessarily coincide with the existing stream ch~nnel even though it may be referred to as such in this report. The flood elevatIons shown on the prof tie ~nd In t~bles 7 and 8 apply laterally from the channel over most of the flood platn width. Rbad crosslngs and other topographic features can alter the lateral flood elevatIons. Depending on whether fhese te~tures divert or block flows, the flood elevatIons at the edge of the flood plaIn may be higher or lower th~n at the ch1\nnel . Flood plain widths vary from one area to another through~ out the study reach. The 100-year flood plaIn width of the Big Th~~~on Rlvor verle~ fro~ e ~Inl~~~ neer 80D feet to ~ ~exi~u~ The Flooded Areas show the area that would be flooded by the IOO-yearand500-yearfloods. The I I oed limits were located at each cross section and the Intervening flood outlines were drawn based upon de+alled toPographic mapping, engIneering judgment, and tleldobservatTons. It is, "-<>wever,posslbletha+ rI'Ore or less floodln') should be shown on the Ftood9d Areas. For ~ specl f Ic situation, wllere more delall..d accuracy 01 flooded arealsrequired,thefl<.>od limltscanbernoreaccurately near 4,000 feet with an avera'l9 of 1,200 feet. Likewise, the 100- year flood plain width of the Little Thompson River varies from a minimum near 100 feet to a ma~;mum near 900 feet with an average ot500feet.lngeneral,thefIQOdplainsofboththeBi'lThomtlson River end the Little Thompson River are of relatively uniform width. The m~jor portion of the IOO-year flood plain of both the 8ig Thompson River and the Little Thompson River in th<lstudy reaCh is inagricult'Jrnl uSe. Few buildings of any fyae exist on thi 5 flood plain. Tho~e that do exi st are primarily agrlcui tu;a Ily oriented. 25 26