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<br />. <br /> <br />16 <br /> <br />" <br /> <br />indicate general areas that may have flooded in the past, <br /> <br />The second source of uncertainty is introduced by land use <br /> <br />changes within the basin. Ditches and canals may tap off sufficient <br /> <br />water from streams to decrease flooding. Flood control measures may <br /> <br />eliminate flooding. Roads may behave as dams and cause water to <br /> <br />back up and flood areas never previously !looded. Roads 'heve, on <br /> <br />occasion, behaved as chennels and directed water into areas far <br /> <br />rer:::!.oved r-ro::.l flo.c'ding strea:::s. Canals carrying flood T~later cay fail <br /> <br />at any point along their length, flooding adjacent areas, ,Reservoirs <br /> <br />have failed under the stress of storms and'flooded areas never pre- <br /> <br />viously flooded, <br /> <br />Where air photo evide'nce was distinct ,or where eye witness <br /> <br />accounts were available, the boundaries are accurate in that they <br /> <br />indicate areas that have flooded in the recent past, Comparison of <br /> <br />air photo interpretation, Corps of Engineers Flood Plain Information, <br /> <br />(1971) mapping, and the areas mapped on the basis of surficial <br /> <br />deposits supports the assumption discussed above, (pages lZ-16). In <br /> <br />both cases the boundaries are mapped as solid lines, Hhere soil <br /> <br />and topographic maps were used to confirm vague atr p~oto evidence, <br /> <br />the map boundaries are least certain and are dashed. <br /> <br />In any case, future land use changes introduce uncertainties <br /> <br />into this statement that areas of past flooding are areas that' can <br /> <br />be expected to flood in the future. It is possible tha~ the effect <br /> <br />of modifications ~ay be to decrease or eliminate flooding, It is <br /> <br />more likely that changes in land use will increase flooding, The <br /> <br />geologic and historic evidence may not reflect these changes, Their <br />