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<br />Flows ofa. large number of. the streams are swi,ft, and. cause extensive <br />d~ageduring flood periods. Due .to surroUnding topography, lII8II7 of <br />the roads parallel the stre.s.msto takeadvant~ge of ,the flatter, gentler <br />slopes. During the June flood.many sections or road. were destroyed and <br />parts of the area were isolated. <br /> <br />15. EXISTING IMPROVEMENTS <br /> <br />\ <br /> <br />Existing water resource pro~ects in the area consist of the Tiber <br />DSllland Reservoir on ,the Marias River;. Gibson' Dam and Reservoir loc.ated <br />in the headwater area of the, Sun fliver; Canyon Ferry Dsmand Reservoir <br />on the MissouriRi~r near Helena; a.number .01' relatively s~all reser- <br />voirs throughout'the are!l- used primarily for irrigation purposes; and <br />some channel .improvements.. .01' these 'improvements ,: only the. Tiber and <br />Canyon Ferry,' projects were effective during the 1964,flood.. The Tiber <br />DSJIl and Reservoir, constr1l!:ted b7the llureau of ReClamation, has 400,000 <br />acre-feet of exclusive flood control storage, and regulation.during the <br />flood pe,riod. virtually elill1inated all flood damage along the. Marias River <br />below the dSJll. Canyon Ferry, also constructed by the llureau 01' ..Reclama,- <br />tion, does not have any exclusive flood control storage. However, since <br />the reservoir was not filled at the time of the flood, it too was regulated <br />in such a manner as to reduce downstreSJll stages and discharges by signi- <br />ficantSJllo\!llts. . The Gibson proJect", completed by the llureau of Reclamation <br />in 1929, is exclusively for irrigation, and at the time of flood, the <br />reservoir was r~lled. No 'significant: flow reductions were achieved since <br />the inflow' could not 1:le,contained and with the spillway operating the <br />dam was overtPpped by about 3. feet,' and. flpws accordingly passed down- <br />'stream undiminished. The numerous small irrigation reservoirs were of <br />l~i;tle value during this flood period and ,in some instances failure of <br />some;of,these' structures. added to.. flooding "'., .for the larger structures <br />the results ,were a disaster in the downstreSJll areas. The limited channel <br />imprOVements ha<;lno significant effect. , <br /> <br />IV. DESCRIPTION OF JUNE 1964 FLOOD, <br /> <br />];6. SliMl'IARY <br /> <br />a. General. The direct cause of the June 1964 flood in ,Montana <br />was heavy. rainfall creating a large volume of runoff into streSJllchannels <br />that were already carrying l!eavy runoff from lllQuntainsnowmelt. Snow <br />surveys made . on 1 May 1964, indicated that thesnowpack in the mountains <br />was generally 150 to 200 percent of normal. Precipitation during the <br />month of May in the area was well above normal and conditions were <br />favorable for runoff at elevations below the snowpack. . During early <br />June, reservoirs in the area were filled or rapidly filling, mountain <br />snowmelt runoff. was in progress, and streams were high-- but not flooding. <br />Then on 7 and 8 June 1964, ,a major rainstorm, one of the largest and <br />most. intense for which there is any record, OCCurred over the northwestern <br />portion of Montana. The storm extended generally from Lewistown, Montana, <br /> <br />6 <br />