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~ " ~ I!: l"'" ,'~; " I , ' ,.' " ~ senior surface water right holders to meet their irrigation needs. Several methods are used to calculate this return flow. The method of calculation is important in that errors may result in no water in the river when it is needed. The State Engineers office gives credit for the recharge water that is returned to the river which in effect reduces or eliminates the diminishment caused by the pumping wells covered under the augmentation plan. An example of what the authors considered to be a well done augmentation plan is given in Appendix B of this report. Appendix B contains both the engineering report prepared for the plan of augmentation and the final decreed augmentation plan. 1.3 Hydrogeology The main source of groundwater in the South Platte River Basin is the Valley Fill aquifer, which consists of pleistocene and recent alluvium deposited by the South Platte River. The alluvium is made up of clay, sand, and gravel. Larger particles, such as pebbles, cobbles and boulders occur less frequently. Groundwater in the alluvium is in close hydrologic connection with surface water in the South Platte River. The thickness of the alluvium ranges from less than a foot at the edges of the valley to about 300 feet in the center near the river. The alluvium has a high hydraulic conductivity. Most irrigation wells tap these alluvial deposits as their source of water. The sand hills found along the edge of the valley are eolean deposits consisting of fine to medium sand. These deposits range from 1 foot to more than 100 feet in thickness. These areas provide good locations for recharge sites because they overlie the alluvium which is hydraulically connected to the ri ver. Loess mantles much of the alluvium, and is less permeable. It is mostly silt with some fine sand. The loess deposits are thickest along 5 ;;. 1.. :. - ,',k_ '~_ .k___._4L;,-~_~,.. ~.-~:ci~~