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differences between the data sets, mostly due to the additional data estimated to signify OFF <br />(release) dates for calls and corrections made to priority dates assigned to calls. Unfortunately, <br />there did not appear to be a consistent approach for setting release dates between consultants and, <br />in some cases, there did not appear to be a consistent approach within individual call data sets. <br />Therefore, the historical call data maintained by the Water Division 1 office was enhanced, with <br />Division 1 guidance, to develop a complete data set representing ON and OFF dates for South <br />Platte River main stem call records in the South Platte River Basin from 1950 to present. <br />After filling the historical call database, these data were analyzed to identify changes in calls <br />over time (e.g., becoming more or less frequent, becoming more junior or senior, et cetera). <br />These changes are discussed in the context of water development over time in the SPDSS Study <br />Area. <br />Background <br />River Calls <br />The Water Commissioner evaluates the natural flow of water in the river and the demand for that <br />water from senior water rights. Based on the demands from water right holders and available <br />natural flow of water, the Water Commissioner sets the location and priority date of a valid call. <br />Upstream water rights holders junior to the call are curtailed. <br />In the South Platte River basin, there are two basic types of calls -standard and bypass. When a <br />standard call is placed, any right junior to the senior, calling right and located upstream is <br />curtailed completely. Multiple calls can be active in the river basin at the same time; the <br />upstream calls are always more senior than the downstream calls. <br />A bypass call operates when an upstream junior water right can divert a portion of its water right <br />while bypassing a sufficient amount past its headgate to satisfy a downstream senior water right. <br />A bypass call operates when an upstream user with a junior water right can divert a portion of its <br />water right while bypassing a sufficient amount of water past its headgate to satisfy a <br />downstream senior water right. The priority date of the call at the downstream structure is the <br />priority date of the water right of the ditch passing a portion of the water right amount. All users <br />with rights junior to the call date upstream of the senior downstream ditch are called out. In other <br />water divisions in the state, the ditch passing a portion of its water is sometimes called the <br />"swing ditch." For example, the Cheesman Reservoir 6/27/1889 right bypassed to satisfy the <br />downstream Burlington Ditch direct 11/20/1885 water right is administered with the 6/27/1889 <br />priority at the Burlington Ditch headgate. Bypass calls allow more beneficial uses from river <br />diversions than might occur if only the senior downstream ditch was the calling water right and <br />upstream, junior water rights were completely curtailed. <br />Page 2 of 14 <br />