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UUlon 1984 Snowmelt Season as of May 1 SPRING AND SUMt-lI::H IWNUfF VOLUMES WILL B~ AI:IUNDANT THIS YEAR UVEK A LAKGJ:: PUKTlUN UF !H.E l.,I!:STEfI.N V.S. !-ilGH F'U_l~S (!<!!TH. PI~IS5:!.:!!...:: ~L(}L\!:!.!'!(.;) !.!l!...!... UC!:!..'P. .".C.".P! !!-:!.S YE!l.P. 1"1 THE FuLLUIHNG \.IATEKSHEDS: COLORADO, KIU GRANDE. ARKANSAS, NURTH ANlJ SUUTH PLATT!::, MUST UF THE GREAT IlASIN, AND TRIBUTARiES TO THE SNAKJ::: KIVEH. IN SOUTH~KN lLJAI1U AND EASTEKN OREGON. WATEK USERS IN NORTHERN MUNTANA AND CENIkAL ARIZUNA, ON THE OTHER HAND, WILL HAVE TO RELY ON STKLAMFLOW AUGMENTATION FHOM THEIl{ KESERvulR SYSTJ::MS. General Outlook Cool, wet ....eather in April caused increases In the streamf low forecasts over a large portion of the "estern U.S. Significant increases are expected in portions of eastern Oregon, southern Idaho, northeastern Nevada, Utah, Southern Wyoming, Colorado, and northern New Hexico. Major ilouding is expected in the Nurth and South Platte Basins, and the Humboldt Babin. Predicted streamflow volumes have increased it} to :LO p\:'rcent over April I forecasts in the Arkansas Hasin, 15 to 2U percent in the Great Basin, and ;15 to l.U percent in the Colorado Basin. Significant increases are 31so expected from southern Idaho tributaries into the Snake H.iv\:'r. Hasins in northern Idaho, northern Montana, and Ari1:ona are still expected to produce much below norrual flow volumes. Streams in the Columbia ~asin and along the Sierras are expected to yield near normal volumes. The old saying "the rich get richer and the poor get poorer" describes the precipitation patterns ill April. Areas of the West that were already saturated and had heavy accumulations of snow, received above norwal precipitation; and areas where seasonal precipitation had been below normal saw a continuation of that trend. ES6entially, the central portion of the West, stretching diagoni'llly from Oregon to northern New M(lxico, received above normal amounts of precipitation. The Sierras, central Arizona, and northeastern Montana received below normal precipitation. One notable exceptIon to the trend was above average precipitation in western Montana, which will help to increase the water supply from this area. The above average precipitation and cool temperatures in April have combined with an already above average snowpack to push sever3l snow courses to record snow water contentS tor May 1. MaxImum water content records were set on snow courses in Colorado, northern New Mexico, Utah, southern Idaho, northern Nevada, and southeastern Oregon. Un the other hond, below normal precipitation in the Sierra.!;, northern Montana, and the Yellowstone Park area has left snowpacks below to much below normal; and snow is almost nonexistant in the central Arizona watersheds. Reservoir storage throughout the ,,"est is excellent, and in some cases may be over abundant, with all states reporting generaily above normal levels for Hay I. Some western reservoirs have been drawn down in antici.pation of the above normal runoff volumes. Basin by Basin Summary SAN JOAQUIN, SACRAMENTO AND NORTH COASTAL BASINS As this water year began, nature seemed intent on exceeding the recoro high precipitation and streamflows of last year. Fortunately for many water users, but to the detriment of others, a major shift in storm patterns produced a very dry !;He winter and spring over most of CalifornLa. April precipitation followed the same pattern as each succeeding month since January. The monthly precipitation ranged from 80 percent of normal in the Upper Sacramento River drainage through the ~lukelurmle River drainage to 50 p\:'rcent of normal in the Kern kiver drainage. Only the extreme north coastal basins receiv~d above normal April p~ecipitation. 120 percent of average. The expected water year streamflow volumes continued to drop as a resuit of the drier than normal precipitation pattern, but are expected to be close to the normal range throughout California. COLUMBIA BASIN The water supply outlook for the ColumbIa River olld Pacific Coastal B.3sins calls for normal runoff volumes this season. In general, water supplies should be adequate for most tlsers in the area. Above normal precipitation during April boosted the forecast for tile Columbia Basin as a whole roughly 5 percent. The most significant increase came in the centrai section of the Snake River Basin, where forecasts nolo' indicate volumes will be well above normal. Very little change Is seen for the Canadian portion of the basin, which is forecast to produce slightly below normal \/olu;';l€s. Forecasts continue to indicate well below normal runoff volumes from the Clark Fork, Flathead, Pend Oreilie, and Kootenay Hasins. Basins in the Idaho panhandle and those draining the east slopes of the Washington Cascades are also expl:'cted to generate below normal volumes this season. Mony basins in central and southeastern Oregon are expected to yield two to three times their normal volumes. Mild and wet weather persisted across the Columbia Basin during April to provide 2U wet days west of the Cascades and 10 to 15 such days for eastside stations. Aprii temperatures averaged near normal except for depart.ures of -+2. degrees Fahrenheit for Princeton and Revelstoke, B.C. and -'1. degrees F03hrenheit for southeastern Washington, eastern Uregon and southern Idaho. Precipitation for the month of April was Ill. percent of average for the Columbia Basin above Grand Coulee, 1U5 percent of average for the ~nake above lee Harbor, and IU~ percent of average for the Columbia Hdsin above The ualles. As of May I, snow course measurements throughout the Nurthwest indicated a slightly below normal snowpack for the Columbia Basin as a yhole. The majority of Canadian stations reported a mountain sno~pack of near aV8rage to slightiy below average except for thl.! Similkameen and upper reaches of the Kootenay, which WLr~ much below aver<lge. The Snake River tributaries in central Idaho and Wyoming reported near normal snuwp.:lcks. ~lany Snake Kiver tributary basins in southern Idaho have accumulated record snowpacks this season. The upp~r reaches of the Deschutes, John Day, and Grande Ronde all reported above average snowpacks. The snowpack in the Cascades is generally near normal. Thl:' January-July forecast for the Columbia River at The Dalles, Oregon, calls for 107 million acre-feet or IOU percent of the 1961-I':ltlu average. At Grand COulee, Washington, the January-July Columbia River flow is forecast to be 57.7 million acre-f~et or 89 percent of normal. The total contribution from the Snake i:(iver Bas.in, as measured at Lower Granite Project, is forecast to be 38.1 million acre-feet or lib percent of normal for the J.:lnuary-September period.