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<br />Fortlfioatiod Creek Report
<br />Rev1eed 21 March 1949
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<br />outside of the channel had been included. Additional infcrmation on floods
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<br />ie given in Appendix 1: B1drology, and Appendix 4: Benefits from Improve_
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<br />Mont.
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<br />30. Flood cbaracterist1ce.-_Moet major floods on Fortification Creek
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<br />rssul. t from spring thaws that rapidly melt Q heavy snow cover and causs
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<br />the ics to break up and then to Jam in the stream channel. Ocoasionally,
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<br />r~off from rainfall may augment the snow-melt r~off. Near-peak flows
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<br />extend over a period of several days; the peak flow is estimated at 25
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<br />peroent.greater than the maximum mean daily flow. Disoharges in excess
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<br />of 700 cubic fest psr second at Highway U S 40 bridge ars coneidered
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<br />damaging under normal oonditions of flow. HOwever, even smaller flows in
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<br />the early spring result in ice jams that cause the water.to overflow the.
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<br />banks and damage adjacent property,
<br />31. Flood freQuonciee.__ReoQrds of psak flow in the Fortifioation
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<br />Creek Basin and peak-flow estimates based on data available for the years
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<br />of stream-flow record, 1905-6, 1909-18, and 1943-48, wars ussd in prsparing
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<br />a discharge-frequency curve. Although reoords of flood occurrences are
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<br />available for other years. satisfactory quantitative estimates are not
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<br />available for these years and Were therefore not included. According to
<br />the curve, the standard project flood (2,600 oubic feet per aecond at
<br />Highway U S .40) would occur once in 100 years. Detailed information on
<br />flood frequenciee in the basin 1e given in Appendix 4: Benefits from
<br />Improvement. The estimated frequenoies of floods of various magnitudes
<br />at Highway U 5 40 ars listsd in ths following table:
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