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. 'I r. n;\ ,11.".. tw' v' near the mOWltains tended to dry up the lower river during the irriga- tion season, and development downstream was not attempted in the early period except where favorable conditions existed. In the late 1800's the return flows from irrigated lands in the upper valley began to increase and stabilize the summer flows, and laws authorizing the creation of irrigation districts were enacted by the State Legislature. These two factors caused rapid development in the lower South Platte Valley. Immediately following the turn of the centUI"J, several irrigation districts were formed \olhich brought about further develop- ment. Some of these districts contemplated irrigation of dry land; but most of the districts were organized to provide storage water for land already Wlder irrigation \olhich could not obtain enough water from their direct flow rights. The formation of these districts was the last major surface irrigation development in the South Platte Valley and all significant surface irrigation expansion had come to an end by the beginning of World War I. .i The history of irrigation in the South Platte River Valley has been one of almost constant overdevelopment. Since irrigable land has always been plentiful, the irrigation companies tried to spread the cost of irrigation works over as large an acreage as possible in order to attract a maximum number of land buyers and new settlers. As a result of this over-extension more acres were placed under irrigation projects than could be served adequately by the available water. This period of development in the late 1890's and early 1900's was one of abnormally high precipitation which encouraged over-exten- sion of the irrigated acreage. Thus, water shortages developed when precipitation and run-off from the eastern slope again returned to, or fell below, normal. The critical need for supplemental water for irrigated land in the South Platte River Valley and the drought which started in 1931 fostered development of pump irrigation as a means of alleviating water shortages. Where ground water is readily available, pump irrigation has become an important factor in the agricultural economy of the area and many irrigators have oecome as dependent on pumping, if not more so, as on surface supplies. This rapid develop- ment of ground water pumping continued on through the 1930's, 1940's and into the early 1950's. During the late 1950's expansion of well development nearly stopped but was resumed during the first half of the present decade. Even with the increased supply from ground water pumping which reached as high as 181,700 acre-feet in 1956, serious shortages still exist in the lower South Platte Valley when either ,the river flow or the precipitation in the area is oelow normal. . The irrigated lands within the Narro\ols Unit are served oy 33 ditch systems and 4 principal reservoirs. The irrigation systems range in size from very small ditches serving only 300 to 400 acres to very large complex systems such as the North Sterling System which ll+