1977 by Morrison .Vtaierle Inc C^'bcssner. et al. 1981). 

 Ground%vater discharge into the river during this baseflow 

 period was about "5 cubic feet per second at Miles City 

 (see Figure 6). However, since imgadon return flows were 

 ignored, the natural groundwater discharge may be 

 slighdy less. 



Two gaged off-rescrvarion tributaries enter the Tongue 

 River downstream from or on the boundary of the reserva- 

 don: Hanging Woman CreekandOtterCreek. Both creeks 

 sustain irrigated agriculture (mosdy spring sub-irrigadon 

 and natural flooding), peak during the Februarv' to April 

 period, and average close to zero flows during the July to 

 August period. 



3. The Tongue River Reservoir 



The Tongue River Reservoir, in operarion since 1939, 

 regulates the streamflows for irrigadon purposes. It is owned 

 by the State of Montana and operated by the Tongue River 

 Water Users ,Associadon (TRWL'A). Recreadon is an im- 

 portant secondary use. The reservoir has a capacity of 

 69,000 acre-feet and can store about 19 percent of the 

 average annual inflow (Woessner, et al. 1981). Currendy 

 because of safety reasons, storage is limited to 40,000 acre- 

 feet, 32, 500 of which is obligated to downstream users other 

 than theTribe,mosdyassupplemental irrigadon water. The 

 average annual evaporadon from the reservoir is esdmated 

 to be around 10,000 acre-feet (Woessner, et al. 1981) 



In 1 978, the dam's spillway was severely damaged by a 

 7,000 eft flow, although it was theoredcally designed to 

 handle flows up to 98,000 cfe. To evaluate the dam failure 

 risk, the Department of Natural Resources and Conserva- 

 don (DNRQ has commissioned a number of studies; the 

 latest one calls for a labyrinth spillway construcuon and an 

 increase in storage capacity to 80,300 acre-feet (Anderson, 

 Bucher, 1 990). This increase in storage capacity is crucial to 

 satisfy the Northern Cheyenne reserved water right without 

 adversely aifecdng existing water uses below the dam. 



4. Groundwater 



Several distina aquifers underiie the Northern Chey- 

 enne Reservation (Woessner, et al. 1981). In the Tongue 

 River basin, only unconsolidated, quaternary alluvium and 

 the deep Madison aquifer have vields high enough (10 to 



700 and 70 to 2,000 gallons per minute respeoively) for 

 agriculniral or industrial purposes. Withdrawal of any water 

 from the alluvial aquifer would be reflected in the Tongue 

 River streamflows. The Madison aquifer is about 6,000 feet 

 below surface and its water has a high ion conccntradon 

 and a temperature of about 1 80 degrees F (Woessner, et 

 al. 1981.) 



The other aquifers, compxjsed primarily of cretaceous 

 and terdary sandstones, clinker, siltstones and shale, have 

 yields varying from 1 to 80 gallons per minute (Woessner, 

 et al 1981). This is adequate for domestic and stock water 

 use, but not enough for irrigadon. 



5. Water Ri^is and Existing Irrigation 



The estimates of irrigated acreage below the dam range 

 from 14,000 acres (DNRC 1985) through 21,000 acres 

 (DNRQ 1981) to 36,000 acres (USGS, 1988). Because 

 irrigation diversions, depletions and return flows on the 

 river are not measured, the Tongue River has not yet been 

 adjudicated through the SB 76 process, and water rights 

 claims have not been verified by the DNRC for the Water 

 Coun, quantification and timing of existing water use on 

 the Tongue River is an imprecise process. Estimates of use 

 can be based on monthly volume difl^erences between up- 

 stream and downstream gages but this does not distinguish 

 among diversions, depletions, return flows, evaporadon, 

 surface runoff and groundwater discharge. .-Mtemadvely, 

 diversions, depletions and return flows can be calculated 

 from known irrigated acreage, irrigation system efficiency, 

 irrigation timing and crop irrigadon requirements. The 

 second option entails detailed field investigation and aerial 

 photography interpretation. 



Water in the Tongue River was first apportioned in 

 1914 in the Miles City Decree, which recognized rights 

 totaling 419.17 cubic feet per second Since then, the 

 Tongue River Reservoir has inundated some of the decreed 

 acreage. If 4.56 cubic feet per second for land which was 

 flooded by the reservoir is subtraaed, the total decreed 

 amount comes to 125,368 acre-feet during the May to 

 September irrigation season. The irrigation season flows 

 exceeded 80 percent of the time at sate line are 106,619 

 acre-feet. Assuming high irrigation return flow reuse, the 

 total amount of the Miles Gey Decree usually can be satis- 

 fied from the direa flow of the Tongue River during May 

 and June. The river is ovcrappropriated during July and 



17 



