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Floods on Mainstems Basin shape has a considerable affect on the magnitude of Fan-shaped basins, with well-developed drainage rapidly drain runoff. Long and narrow basins with few tributary streams yield flood peaks than basins of comparible size with different shapes. floods. excess smaller Many major rivE!rs in Iowa have fan-shaped basins along the upstream reaches and long, narrow basin shapes along the downstream reaches. The narrow basins along the downstream reaches markedly attenuate the magnitude of the flood peaks. A typical example of this is the Cedar River basin. Along the upstream reaches of the Cedar River, the Shell Rock River, Yest Fork Shell Rock RivE!r, and Beaver Creek all join the Cedar River upstream from Cedar Falls within a distance of about 7 river miles. About 11 river miles farther downstream, Blackhawk Creek enters the Cedar River at Yaterloo. The combined drainage area of these tributary streams is about 3,400 square miles. The total drainage area of the Cedar River increases from 1,66l square miles at the gaging station in Janesville (site 101). to 5,146 square miles at the gaging station in Yaterloo (site 116), about a 2l0-percent increase in drainage area in 19 river miles. Downstream from Waterloo, the basin is narrower with few tributary streams. The drainage area at the gaging station in Cedar Rapids (site 122) is 6.510 square miles, an increase i.n drainage area of about 26 percent in 182 river miles. The extent of flood attenuation between the gaging stations in Waterloo (site 116) and Cedar Rapids (site 122) can be assessed by comparing the magnitude of the 100-year flood at each station (table 1). The discharge of a lOO-year flood at Waterloo (44 years of record) is 97,100 cubic feet per second and that at Cedar Rapids (82 years of record) is 83,000 cubic feet per second, a 14 percent flood attenuation. Readers interested in comparing the effects of flood attenuation for a single flood may consult a companion report (Lara and Eash, 1987) where basic data collected in Iowa are tabulated. Data on the mag;nitude and frequency of floods for major streams in Iowa with basin configurations similar to that of the Cedar River are presented in graphs that compare the magnitude of the peaks to drainage area. Graphs showing flood magnitude and frequency along the mainstem of seven rivers are shown in figures 2 to 8. Maximum Floods ,~ Previous flooding in Iowa needs to be considered when plannins projects in flood plain areas where structural failures may be life threatening. Data for maximum flood and related information are shown 1n table 3. These data indicate maximum floods without reference to frequ8UC1 of occurence at some stations; the ratio of the maximum flood to the computed 100-year flood is listed instead of recurrence interval. The.. data are intended as a guide for making estimates of the discharge of rare floods at a given stream site. 7