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live in the basin and the well established legal and structural features of water <br />development that have occurred over the past 100 years and more, these legal and <br />structural features generally remain functional in managing, importing, storing, diverting, <br />and utilizing water resources throughout the basin. Surface and groundwater <br />management and use continue to affect the hydrology, geomorphology, and habitat in the <br />Platte River Basin. The effects of water resources and other development, regarding <br />hydrology and water quality, channel morphology, and habitat, are described in the <br />remainder of this report. <br />2.2 Flow History <br />The history of flow as it has occurred over time is reflected in data that have been <br />collected over time at various stream gages in the basin. Since the time frame from about <br />1895 to 1930 flow data have been collected and are available through the U.S. Geological <br />Survey (USGS). Prior to that time, information on flow can only be based on anecdotal <br />sources. In order to gain an initial understanding of the history and pattern of flow, <br />graphs of flow data are presented. The gages for which graphs have been prepared are: <br />North Platte at North Platte, South Platte at Julesburg, Platte at Overton, and Platte at <br />Grand Island. These stations were selected because they represent a geographic range <br />from upstream of the Big Bend reach, down through most of the Big Bend reach of the <br />Central Platte River. Two of the gages have some of the longest periods of record of any <br />gages in the basin so any long -term trends in flow may be more apparent. <br />2.2.1 Annual flow <br />Hydrographs showing the total annual flow in million acre -feet (maf) were prepared for <br />the selected stations (see Figures 2.10- 2.13). Starting on the North Platte, for the time <br />period up until about 1930, the total annual flow fluctuated about a value on the order of <br />approximately 2 maf per year. A dramatic decline in flow occurred starting about 1930 <br />with the total flow remaining very low (generally less than a half a maf) until about 1970. <br />This period of low flow can be attributed to a combination of several factors including: <br />the major droughts of the 1930's and the 1950's; as well as water resources development <br />including the construction of several large reservoirs, continued diversions, and <br />groundwater pumping. Thereafter, the flow ranged from as low as less than half a maf to <br />over 1.5 maf. A similar trend is apparent, although the period of record is shorter, at <br />Overton and Grand Island. The same trend is not evident in the Julesburg record. In -this <br />case, flows are generally lower in the early 1900's with some higher flows occurring near <br />the end of the record in the more recent years. There are several explanations for the lack <br />of a similar trend despite significant water resources development in the South Platte <br />Basin as described by Simons & Associates (1990e), <br />This lack of change in Julseburg streamflow is explained by several <br />factors. First, the reservoir storage capacity on the South Platte River is <br />substantially smaller than that on the North Platte River. Second, canal <br />construction started earlier in Colorado than in Wyoming and the effects <br />of this early water resources development are not reflected in the data at <br />Julesburg. Third, annual imports of water primarily from the Colorado <br />River Basin increase the available water supply of the South Platte River <br />15 <br />