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. 'I . e e e "'OQkl~ OF TAMARACK'S WATER MANAGEMENT POTEN,.AL FOR STRBAM FLOW AUGMENTATION The past ten years have proven that groundwater recharge of excess river flows on the lower South Platte River is a cost effective water resource management strategy for reregulation and augmentation. Continued development of the groundwater resource can provide solutions for our increased concerns about state~line flows, well augmentation, native species issues in Colorado, endangered species issues in Nebraska, and water quality. This memorandum focuses on the Tamarack State Wildlife Area. Developing groundwater recharge projects at the Colorado Division of wildlife's (CDOW) Tamarack property can be an important component .of managing the groundwater resource on the lower South Platte River. An'example of another possible component is the utilization of the existing GASP canal wells for the Peterson and South Reservation Ditches for pumping to recharge basins during the winter months. Pursuing water management development at CDOW's Tamarack property is attractive because it is a large tract of land bordering the South Platte River under single ownership by the State of Colorado. Such a large tract of contiguous land reduces the concerns of recharge causing high water tables and seepage impacts on intervening landowners. With the State of Colorado'S ownership of Tamarack and the State's commmitment to a Recovery Program for endangered species, State funding will be forthcoming for water management development at Tamarack because such development would provide recovery program credit by eDhancing state-line flows through reregulation. State of Colorado dollars would provide a funding base which combined with private water user dollars could develop groundwater recharge projects at Tamarack. Dollar contributions from water users would entitle them to a proportion of the credits for well augmentation and/or instream flow augmentation in a Recovery Program. CDOW's Tamarack Ranch wildlife Area is located near Crook, Colorado and comprises more than 10,000 contiguous acres along the South Platte River 25 miles above the state.line. The property extends up to 4 miles south of the river into the sand dunes that rise as much as 100 feet above the valley meadows of the floodplain. Recharging in these sandhills could develop extensive ground water storage and subsequent return flows to the South Platte River. By utilizing groundwater wells (pumping plants) next to the river, water could be pumped during the winter months of no river or Compact calls to recharge basins south of the river and in the sandhills. The State Engineer considers the use of such supply wells as direct diversions at times of no call so that there are no depletion replacement requirements at later times. The distances of the recharge basins from the river would be selected such that the return flows or accretions would return to the river at beneficial times that would provide augmentation for irrigation wells and instream. flow credits. Preliminary analysis shows that the groundwater storage potential in the sandhill area of Tamarack south of the. river and north of highway 1-76 is 10,000 to 15,000 acre-feet while the area south of 1-76 could potentially store more than 30,000 acre-feet in a groundwater reservoir. Infiltration or seepage rates from basins would not be a limiting factor as shown by pond tests done by CDOW in March of 1985 which indicated infiltration rates of 4 to 5 feet per day in the sandhills. CDOW has drilled seven large capacity wells for the purpose of supplying ponds for wildlife habitat. These wells are not located near to the river but south of the river next to the ponds. This operation required the development of an augmentation plan (Case No. 85CW236; W-7713) co replace depletions because these wells cause out-of-priority depletions to the South Platte River. In order to hold water. the bottoms of the oonde have been sealed.