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<br />46 <br /> <br />character has changed considerably since man began to withdraw water for <br />irrigation. It was previously a stream which carried a large flow for a <br />relatively short time during early spring. By mid-summer the river was <br />reduced to a trickle by the time it reached Greeley or perhaps even <br />before that. While river flow was substantial near the foothill s, the <br />streamflow larg,elydisappeared into the ground as it proceeded down the <br />basin. <br />As ditches were constructed to withdraw water from the river in the <br />upper basin, the condition of the. river began to change. More and more <br />the 'almost dry streambed in the lower basin began to contain water <br />throughout summer and fall seasons. <br />The result of man's development of irrigation was the filling up of <br />the underground reservoir.. Irrigation water not consumed by growing <br />crops entered this reservoir and became ~ supply source for surface flow <br />in the downstream river channel. Each, season, as irrigation added to <br />the underground reservoir, its surface level rose and forced groundwater <br />to flow into the streambed stream channel feeding the river flow. <br />Groundwater records show that the water table level toward the <br />outer edge of the aquifer rose as much as 50 feet in the first 20 years <br />of irrigation. Instead of a "losing" stream, the South Platte he came a <br />"gaining" stream due to groundwater return flow. This tended to make <br />additional water available in the lower river during late summer and <br />fall (compared to, pre-irrigati on times). Measurements of return flow <br />were estimated. as' early as 1889 by L.G.' Carpenter, a civil engineer at <br />Colorado State University (later Colorado State Engineer). By 1930 the <br />groundwa ter buil d-up throughout the all uvi al aquifer reached an "equ i- <br />