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Peak flow~ this derived for the 100- and SOa-year flooda in the study area follow: I average about 10 feet per second in the main channel of Vsllecico Creek and sbout 3 feec per ~econd in overbank aress. Velocity of flow during a SOO-year flood ~uld ~ alightly higher than during a loo-year flood. Peak Flow ,,. Stream and Location lOa-year Flood SOO-year Flood ~acer flowing At 10 feet per aecond will cauae aevere eroaion of streAmways, deatroy stteam crossings, and tranaport large boulders. Streambanka and the fill around bridge abut~enta ~y be eroded, and large amounts of gravel, sand, snd slIt may be tranaported by water flowing at a rate of S-7 feet per second. Water flowing at about 2 feet per second or leas will depo~it sand, silt, and other debris. Vallecito Creek: at the "near Rayfield" gage 8,300 21,500 below Pdl Creek 9,200 23,800 local fl~ below BerriCreek 750 1,900 Gr1nles Creek near Vs1lecito Guard 1,100 2,900 Station Large objects of floating debris attain dangerously destructive force when transported by floodflows as swift a~ those characteriatic of Vallecito Creek. Swiftly flowi~ floodwater ~y cut new stre~ channels and endanger structures not normally subject to flood hazards. Plates 1-3 show the areas that would be inundated by the 100- and 500-year floods. As shown on these plataB, esaentially the entire study area is subject to flooding or sheet flaw. gheet flow b broad, over- land floodi~ generally less than 2 feet deep on the average and charac- terized by unpredictable flow paths. Water surface elevations of flooding in theae areas aTe easentially independe.nt of thoae along the streamways and are affected principally by obatructiona and local topography in the area flooded. Water surface elevation of the IOO-year flood on Grimes Creek sre indicated in whole-foot incrementa by flood elevation reference linea. They are drawn at right anglea to the direction of floodflow and are used in determining the elevation flooding is upected to reach. Water surface elevationaof the lOO-and SOO-yellr neods on Vallecito Creek are ahown on Plates 4-8. Depth of flood flow in the channel and adjacent overbank areaS can be dete~ned froQ these illustrations. The stream gradient of Vallecito Creek averages about 50 feet per 1II11e in tile lower section of th= study erea andsbout 100 feet PU !Illle 1n the upper section. During a tOO-year flood, velocity of flow would s 6 e~o ""'_900