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I G'f\"'J~) . J" i u. I, . J ~_ . L.! I of the pioneer settlers, westward railroad development, military outposts, and mining activities. I I I The subbasin's 1975 population was estimated to be 2.7 million persons. Eighty percent lived in urbanized areas; most of these in the three Standard Metropolitan statistical Areas along Colorado's front range. The rural component of the population is projected to decline further during the remainder of the century. I I I I I I I I I The subbasin supports a diverse economy, including agriculture, manufacturing, mining, forestry, and recreation. Denver, Colorado, is the dominant economic nucleus, supported in part by the vast agricultural activities and natural resources found throughout the basin. Typical of the entire Missouri River Basin, water availability is highly variable across the Platte-Niobrara Subbasin. Flows in the Platte River drainage are subject to extreme seasonal fluctuations caused by high spring runoff and extensive irrigation development. Flows in the Niobrara and its tributaries are more regular due to subsurface inflows from the Sand Hills, under which lie vast ground-water reservoirs. Outside of the Sand Hills, major sources of ground water are the Ogallala aquifer and stream alluvia. Extensive ground water pumping occurs along the Platte, South Platte, and NiObrara Rivers. Irrigation is the dominant water use in the subbasin; withdrawals totaled some 12.4 million acre-feet in 1975. Municipal water supply is another important consideration, especially I along Colorado's front range. A number of diversions from the Colorado River Basin supplement Platte River Basin sources. I I I Water quality in the subbasin's streams and underground aquifers is generally good. Seasonal degradation of some watercourses is caused by runoff from agricultural lands. I 7-3