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9. CLIMATE The area east of the Continental Divide in Montana lies mostly within the. .s~aridzone, but. with marked climatic differences bet~een the plains and the moUntains; Summertime in the area generally is . quite pleasant , with cool nights , moderately warm and sunny days, and relatively low' hJJJDidi ty. Winters are usually cold. Cold waves, . blizzards, and the w.arm chinook are features of the winter season. Most summer rainfall occurs in showers or thundershowers, but steady rains may occur during late spring or early summer, During late fall, winter, and early spring, precipitation generally falls ,as snow. Snow cover over the plains area and lower valleys is seldom continuous thro"gh the winter" but, heavy snow; accumuJ.ations can. be expected over the highe;r mountain ranges., The average ,annual' total. precipitation varies widely throughout thll area, from. 11 to. 18 inches at lower eleva.,. tions tq 30 inches. or. more in the high mountains. No~lly about 70 percent of. the Yearly moisture falls during the pe;riod from A~ril through September,with May and. June peing the;wettest.months. 10. RUNOfF CHARACTERISTICS a. General.. The.runoff charactetistics of the tributary streams in 'the area are similar. The upper portions of the' basins are lOcated in mountainous areas with the middle and lower portions draining plains areas which are moderately to steeply rolling. The mountain slopes are in many cases.t:prested and the plains, areas ",re either under cuJ.tiva- tion or covered with wild grasseS., A considllrable amount of the plains area is irrigated, b. Characteristics. The normal sequence of, runoff from the area il> low winter nows, augmented in March and April with snowmelt rUnoff from. the plains area, Another increase, in runoff, genera,lly'larger, takes place duz:ing, May andJutle when the mountain snowpack is melting.. Flows, are. of'ten, augmented during this period as a resuJ.t of rainfall runoff. Late summer and fall will find.a return to low flows ,which will prevail until the cycle begins again in -the -sp-r:iag.- '. All the maj.{)l" floods on the Missouri River in this region have resuJ.ted from heavy rains during the month of June falling on a relatively high snowmelt base. The 1964 flood fits perfectly into this category since it too was the resuJ.t, of heavy June rainfall occurring on. a high snowmelt base. The degree to which this is typical of past years is evidenced from the 72 yel1-rs of record, at the Fort Benton USGS station which shows that the maximum annual discharge has occurred 47 times during June, ,20 times in May ,4 times in April and once in JuJ.y. ' 11. STREAMFLOW The U. S. Geological Survey has maintained stream gaging stations on the Missouri River and tributary 'streams at a number of locations in this area., The oldest record available is for the Missouri River at 4