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<br /><:> <br />c.v <br />c.v <br /> <br />BASIC Sl'UDIES <br /> <br />0'; <br /> <br />Considerable variation in the effects of irrigation return flow on <br />water quality is to be expected. Differences arise due to the size of the <br />irrigated areas, the number of times the return flow is reused, properties <br />of the soils and drainage area, number of years land has been irrigated, <br />nature of aquifers, rainfall, dilution, temperature, irrigation methods, <br />storage reservoirs, vegetation, and type of return flow channels. <br /> <br />Consumptive use and return flow studies are now being undertaken by <br />the Bureau of Reclamation on existing projects in small, closely con- <br />trolled areas. With a small additional expense, it has been possible to <br />obtain quality data and thereby determine the effect of irrigation on <br />water quality. The study areas are purposely being held small to achieve <br />better control, but they will be as representative as possible of exist- <br />ing projects. The results pertaining to the quantity of return flow will <br />be very helpful in estimating effects on water quality of return flows <br />from larger areas where measurement of inflow and outflow is not always <br />possible or practical. <br /> <br />Special studies in other areas in the basin will be undertaken from <br />time to time to determine water quality conditions, and studies of proj- <br />ects such as Florida and Eden will be repeated or continued in order to <br />evaluate changes with time. The more complex areas will be considered <br />for investigation at a later date when sufficient funds are available <br />to carry out meaningful studies. Projects in this category include the <br />Grand Valley and Uncompahgre projects in Colorado and possibly some di- <br />rect diversion projects along the Colorado River below Hoover Dam, such <br />as Palo Verde Valley and the Colorado River Indian Reservation. An im- <br />portant consideration in quality studies is measurement of return flow. <br />If the return flow can be measured and its quality determined, the water <br />and salt budgets can be computed because the inflow is nearly always <br />gaged and its quality easily determined. <br /> <br />One area now being monitored for irrigation effects is a block.of <br />about 700 acres of land within the Grand Valley project. This particular <br />area is partially isolated by natural washes and drained sufficiently to <br />permit sampling and measuring of return flows. Sampling began during the <br />1966 irrigation season, but the results are not significant enough to <br />indicate a trend. <br /> <br />3. Salinity Variation in the Colorado River below Lees Ferry <br /> <br />Continued utilization of the water of the lower Colorado River re- <br />quires that its salinity remain within limits tolerable for irrigation <br />of present crops. Under virgin conditions the salinity of Colorado River <br />water, like that of all rivers, varied daily, seasonally, and annually. <br />This natural primordial variation has been greatly changed as a conse- <br />quence of the works of man. Moreover, individual dams and diversions have <br />had substantially different effects on the river depending on their size <br />and location. Diversions within the basin have generally resulted in <br /> <br />27 <br /> <br />