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1779 reservoir releases. In April, however, streamflow begins to increase as lower elevation snow begins to melt. The ~portance of this pre-snowmelt runoff or ''flushing" period on the water quality of the Arkansas River is discussed in the trace metal section of this report. During the snowmelt-runoff period, streamflow is at its annual maximum and increases substantially downstream bel:ause of reservoir releases and tributary inflow. Peak runoff typically occurs during the second week in June. The post-snowmelt runoff period includes the receding limb of the snowmelt hydrograph in July and releases of stored warer from Twin Lakes Reservoir in August and September. Additionally, during the post-snowmelt runoff period, streamflow can increase substantially over short periods of time following intense rainstorms and subsequent runoff from tributaries. Streamflow in the lower Arkansas River Basin is highly regulated by storage and release operations at Pueblo Reservoir and John Marrin Reservoir. Surface-water diversions and return flows downstream from Pueblo Reser- voir, also, have a pronounced effect on the streamflow of the Arkansas River. Large quantities of water are removed .. n.___ u_ _____ .. - --------- -- from the river by a system of irrigation diversion strUctures downstream from Pueblo Reservoir, thus, substantially _creasing the streamflow in the river. Irrigation return flows can make up a majority of the streamflow downstream from Manzanola during certain times of the year (Cain, 1985). During the study period, the mean-monthly stream- flow of the Arkansas River between Pueblo Reservoir and John Martin Reservoir ranged from 784 ft3/s at Avondale to 201 ft3/s at Las Animas. In the lower basin. the largest tributary flow to the river was Fountain Creek which con- tributed about 15 percent of the annual streamflow at Avondale. The mean-monthly streamflow near the Colorado- Kansas state line was approximately 130 ft3/s. Streamflow in the lower Arkansas River Basin also exhibits large seasonal variability and can be separated into three similar flow regimes: low flow (October-April), snowmelt runoff (May-June), and reservoir release (July- September). Streamflow is at a minimum during the low-flow period because of decreased inflow from the upper basin and water storage in the reservoirs. Water stored in Pueblo Reservoir during the winter months is subsequently released during the growing season to meet downstream irrigation needs. During the snowmelt-runoff penod, flow into and out of the reservoirs increases substantially due to snowmelt from the lippet.l>;;;;-,,;::Thi; ~~jfJiJ1 cr~~!;';.~; o~: .:." ;1',:;-'~J ~ :~~.;....;.i a c.. "-" _t;:c,....\.,;c:.M5LO ~hvision -.ro:ed by releases of water from the reservoirs that are matched by the inflow to the r."..;"...,ttS:"5 aciJl.~SIl= ~> ~;:; . iT QIJ u: Ul\ .,=...1">-- 9 .??J~O A ~rov~\ by Dircc~c,r, ".",!,...1i'l53 P. \ ;""\ S~r/e'l Ll.::>. Geo.cg.c... I