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Show than in tr»e upper, largely because of denser set- tlement. Even here, however, the less densely pop- ulated arenas such as the Ozarks or the eastern plains have blanks either due to insufficient gages or too short a record, or both. This situation is marked fu_rther west where reliable and long-time records, particularly of intensities, are lacking. Of special comcern is the lack of adequate data in the high mountain country which is the source of most of the flow in western sub-basins. Here data on stream flo^v may be a more reliable indicator of precipitation than estimates based on topography. Additional snow surveys are needed. These sur- veys do not obtain detailed information as to the total precipitation which falls, but taken peri- odically tlirough the winter and into the snow melt season, give on a sample basis information as to the amount of water in the snow. This in- formation then forms the basis for forecasts of stream flow. Stream flow.-In planning use has been made of all avaUable stream flow records, supplemented by precipitation and climatological data. Some stream gages have been in existence for a long time: Sioux City, Omaha, Kansas City, and Her- mann, Mo., have records extending back to 1870. Practically all of these older records were obtained from staff gages read more or less irregularly. All have disturbing omissions or lapses and contain considerable errors. "Adjustments"\ taking into account precipitation records and runoff data from other localities therefore have had to be used. Only in the past 20 years has the measurement program included recording gages and in some trib- utary streams the records as yet are insufficient for detailed planning. Data on the lower river reaches are also subject to error because of variation in sedi- ment movements. At Omaha, as much as 30 feet difference in the depth of the river has been found in measurements only a few days apart with almost no change in the water level. Extensive efforts are necessary to obtain frequent discharge measure- ments to meet such conditions. Few stream-flow records have been analyzed or made generally avail- able for public use. Additional stream gages are currently being in- stalled, but not at a rate sufficiently rapid to pro- vide the needed substantial data. The present 1,600 gages now in the watershed should be in- creased to about 3,000. Such records will be es- sential in later construction and in the operation of the reservoirs. Water quality.-Only in some tributaries have detailed records been collected and few of these on a systematic basis over a lengthy period. It has been necessary therefore to judge the quality of sur- face waters on the basis of soils and geology. Only limited information on the water quality of mineral- ized streams is available. Some, Kke the lower Yel- lowstone, are so highly mineralized that they must be used cautiously for irrigation. The quality situation will become more serious with greater irrigation use of water. The return flow carrying the leachings from irrigated lands in the upper basin will materially increase the min- eral concentration in downstream reaches, and may make some present suitable waters there unfit for irrigation use. Sanitary surveys have been made of reaches of some streams and most sources of pollution are known. There is, however, an undetermined amount of pollution coming from some 465 com- munities and numerous industries and from the in- creased number of recreationists in mountain areas. Information is reasonably complete on pollution conditions in the Kansas and Platte Rivers, and for the lower main Missouri River, but is incomplete elsewhere. There is need for a continuing program of assessing sanitary water quality throughout the basin. Data are especially needed in view of the limited amount of water available for dilution purposes and the extensive use made or to be made of surface supplies for various purposes. Within irrigation projects sanitary information will be required for canals and streams used for domestic purposes and for truck crops. A greatly expanded program of sanitary surveys throughout the basin is therefore required. Ground water.-The use of ground water in the basin is growing, especially in the subhumid area where surface irrigation is being questioned. Little is known as to the extent, availability, or replen- ishment of ground water supplies. Here and there in some of the heavy use areas, a considerable body of information has been collected, but there are immense areas for which ground water data are very meager. It is known that the Sand Hills area is a great reservoir feeding the ground waters of the Platte and Republican Basins. It is also known that the Dakota sandstone outcrops are important in re- plenishing the artesian basin of the eastern plains, and that the Yellowstone River bottoms play a significant part in ground water replenishment. The part that other sections play in this replenish- 195 |