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N III , l{ r.,-C---- GGI; 2'76 water Y6ars ending September 30, 1943, were found to have the smallest dlrrerencea from the average discharges for th~ entire periods of record. Table 1 contains figures that show the comparison of average discharges for the periods of record with tho average dlscnarges for the 6-year period ending September 30, 194~. Discharge for the 6-~ear pertod ~a5 somewhat below the averBge for the entire perIod of record for most of the stations in the northern pa:rt of the Missouri River Baslr_ and was slightly above the average for the period of record for most of the stBtlorls in the southern PSI't of tho basin. An IncJ'sase in use of water for irrigation during rec~nt years probably decreased appreciably th~ dis- charge at several of tho stations in the northern part of the basin during the 6,year period. For example, the discharge was de- C~~ased at the gaging station on Bull Lake Croek by storage begun in September 1936 in a reservoir just upstream from the station. Precipitation During Base Period Average annual precipitation by StQte~ or parts of Stat~s is listed in table 2 for different periodS of calendar years. The first period listed for each area is that Table 2.--Average precipitation in Missouri Basin States by areas and periOds Average annual Area Calendar :rears precipitation (inches) Central Montana 1895-1944 14.79 1908-44 14.86 1920-44 14.11 1938-43 14.98 Eaatern Montana 1895-1944: 13.B6 1938-43 14.82 'lIeatern 'North 1890-1944 15.36 Dakota 1938-43 16.90 Western South 1892-1944 17.94 Dakota. 1938-43 17.1.4 Wyoming 1892-1944 14.12 1901-5,1911-44 14.22 191.1-44 14.40 1919-44 14.11 1938-43 14.32 Colorado 1888-1944 16.50 1897_99,1901, 17.01 1905-44 .. 1938-43 17.04 Nebraska 1876-1944 22.56 1938-43 20.44 Iowa 1873-1944 31. 61 1938-43 32.13 Kansas 1887-1944 26.86 1898-1903 27.24 1920-44 1903-5,1920-44 27.30 1918-44 26.83 1920-44 26.B5 1938-43 27.95 Missouri 18BB-1944 40.21 1938-1943 J9.63 ~ for which average annual precipitation was published by the Weather Bureau.lI The oth~r periods correspond approximately to periodS for which average discharge from one or more drainage basins in the areB was com- puted. All the figures of average annual precipitation were copied from reports ot the Weather Bureau and presumably had been computed for each area as an arithmetical average of records from all the Weather Bureau precipitation stations In that area. The rigur63 of average precipitation were computed on the basis of calendar tears, but these figures should differ only slightly from those based on water years, which begin and end 3 months earlier than calendar years. The figures given in tabl~ 2 are not exactly correct, but they are accurate enough to show that the precipitation throughout the Missouri River Basin during the base periOd 1938-43 was reasonably close to the average precipitation based on longer periods of precipitation records. Map Showin~ Avera~e Dischar~e The average discharge during the 6- year period ending S~ptember 30, 1943, is shown by width of symbolic lines that follow the stream channels on the map of figure 1. This map waS based principally on average discharge at gaging stations for which re- cords covering the base period have been published by Federal and State agencies. Records for several other stations were com- pleted for the base period by estimating the flow for missing periods of record. Average discharge at man~ points where no discharge records had been obtained during the ba8e period was estimated from s12e of drainage a~eB and probabl~ de~ths of runoff. The width of 11ne on f1gure 1 1ndlcBte~ average discharge; no adjustment for di- versions, storage, or water losses from res~rvolrB bas been made. However, it should be noted that storage WBS begun 1n three large new reservoirs during the 6- year periOd and that at the end of the per- iod the storage in them was as rollo.s: 1 . i 1 -./ .......1 'I I .... Totftl cOJ:lt.nt:~ R~8el"Vo1r R1 ...er ston;,ge begAD se~:. 30, 19"3 . ae e_te8t' fort Peck lUe8<lur'l Hc'-.bo'l' 1.9~? 12.,020,000 Semi DO 81 :North Platte April 1939 392,500 .u:eConBush1 do~ FebZ'UI:I.ry 19011 M4..:500 :.;.:.<::.:1 ".' .' '. .~':. :/:y The flow of Missouri River originates in two general regions: the mountainous areas, particularly near the continental divide, and an area near the mouth of the river where precipitation nnd resultant runoff are rela.tively high. MonthlY Distribution of AV.ra~e Dischar~e Plate I shows the percentage dIstribu- tion b~ montbs of the average di~chBrge, un- adjusted for diversions and storager ror the 6-year period ending September 30, 943. As monthly distribution diagrams for only a few gsging stations could be shown on the plate, stations were selected that would ShOW reaso~ ably representative distribution or diSCharge. 1/ U. S. Weather Bur., Climatol. Data, 1944. - -.-.'. ":.;."