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[":;]:.1 i I I I j I J .1 I ~ I I I The South Platte River is the principal source of surface water for! lirrigation in the plains of northeastern Colorado. Between 1850 and 1900, jnumerous irrigation canal and ditch companies formed and constructed head- [gates along the river to divert water to the fields and to off-channel reser- lvoirs. leakage from these canals and reservoirs plus applications of water ~on the land in excess of consumptive use Justains a huge aquifer within the ialluvium adjacent to the river. Discharge from this aquifer has transformed Ithe South Platte River into a gaining stream throughout most of its course. AVAilABilITY OF SURFACE WATER StreamfJoh'Stati5tics , Data to determine the streamflow available for diversion have been col- !lected at the gaging stations in the Kersey-Balzac reach of the river by either the U.S. Geological Surveyor the Colorado Department of Natural Resources, Division of Water Resources, Office of the State Engineer. These stations (fig. 3) and dates for which data are avai lable are: Kersey (1905- 'present); Sublette (1927-55); Weldona (1952-present); and Balzac (1916- present). I I , The streamflow of the South Platte River is quite variable--within the year, from year to year, and from site to site. The flow-duration curves for Kersey and Balzac (fig. 4) show this temporal vari"bi 1 ity, and the different placement and shape of the two curves indicate the spati"l variability. In ~pite of the fact that the South Platte .River is an effluent stream (a gain- ~ng stream due to ground-water inflow), the many diversions cause the stream- flow to decrease downstream through this reach. From 1927 through 1955 the average flows were: Kersey, 625 ft3/s; Sublette, 336 ft3/s; and Balzac, 301 ft3/s. From 1952 through 1976 the average flows were: Kersey, 847 ft3/s; Weldona, 572 ft3/s; and Balzac, 439 ft3/s. I Water Rights To gain legal access to water for diversion dUI'ing the nonirrigation season, the project must conform with the water rights of existing reser- voirs. The reservoirs in the Kersey-Balzac reach of the river with senior ~Iater rights are Bijou No.2, Empire, Jackson, North Sterling, Pre,~itt, and Riverside (fig. 3). On the basis of records provided by the State Engineer's Office (Walter Knudsen, written commun., 1975), the apparent rights of these reservoirs are I isted in table 1 along with the basin rank, which is a prior- ity numbering system. I I Potential Diversions I The data used for determining the potential diversions for the proposed project came from an analysis by the U.S. Bureau of Reclamation (Roger Wiedlemann-,-,written commun. ,--1976).- -The "Bureau"computed-,the-streamfJow' that , .6