IV WATER RESOURCES 



A. Tongue River 



1 . Basin Characteristics 



The Tongue River headwaters originate in W»'oming's 

 Bighorn Mountains. Annual precipitation in the Bighorn 

 Mounuins at elevations around 1 3,000 feet averages over 

 25 inches and occurs primarilyas wintersnowfall and spring 

 rainfall. Flows peak in May and June — the time of major 

 snowmelt runofF(see figure 1 ). Litde of the discharge enters 

 the river in Montana. 



The Northern Cheyenne Reservation lies in Montana 

 about 60 miles downstream (northeast) from the base of the 

 Bighorn Mountains. The Tongue River forms the 

 Reservations 47-mile eastern boundary. Here, the river 

 dissects plateaus and benches up to 4,400 feet in elevation. 

 .Annual precipitation in theTongue River valley averages 12 

 to 14 inches. Seasonal and year-to-year variations are high. 

 Prior to the construction of the Tongue River Dam, the river 

 had summer flows near zero at its mouth on several occa- 

 sions (Woessner et aJ. 1981). In contrast, the largest flood 

 occurred at the end of May, 1978, with flows over 7,000 

 cubic feet per second The Tongue River enters the Yellow- 

 stone River near Miles City. 



The Tongue River Reservoir with a storage capacity of 

 about 69,000 acre-feet, lies 37 river miles upstream from the 

 Reservation. Since the 1978 flood, which damaged its spill- 

 way, it is operated at about 40,000 acre-feet storage capacity 

 for safety reasons. 



2. Streamflows 



The uses gage na 06306300, at die Montana/Wyo- 

 ming border, has been in operation since 1 961 . The river at 

 this point drains about 1,480 square miles with about 

 64,300 irrigated acres and 1 5,000 acre-feet combined vol- 

 ume of small reservoirs (USGS, 1988). To come up with a 

 representative set of data, synthetic streamflows for the 

 1 929- 1 960 period {Systems Technology, 1 984) were added 

 to the existi ng record. The flows exceeded 50 and 80 percent 

 of time during a water year are as follows (see also figure 1 ): 



Tongue River Percentile Flows 

 Month 50% 80% 



Annual 



287.485 af 



177,522 af 



On average, the flows leaving the dam (USGS gage no. 

 06307500) equal the flows at die state line during the period 

 from October to January, fiom February to June, water is 

 stored in the reservoir and released from July to September 

 to supply irrigation needs (see figure 2). 



On the stretch of the river between the dam and Miles 

 City (USGS gage no. 06308500), the flow is usually stable 

 from October to December. During January to April, flows 

 at Miles City exceed flows leaving the dam with biggest gains 

 in March. Irrigation ofabout 21,000 acres (DNRC, 1981) 

 and natural evapo transpiration account for net losses during 

 the May to September period (see figure 3). 



In general, flows at Miles City are lower than the dam 

 releases during the May to September irrigation period and 

 higher from October to April (see figure 4). The highest 

 measured volume deficit (flows at Miles City minus flows 

 leaving the dam) peaked at about 72,000 acre-feet during the 

 1959 and 1988 May to September irrigation seasons. On a 

 probability basis, during the 1 947- 1 988 period, 80 percent 

 of the time the volume deficit did not exceed 56,000 acre- 

 feet, and 8 percent of the time the flows at Miles City 

 exceeded those leaving the dam during the irrigation season 

 (figure 5). 



A comprehensive seepage run was conducted on the 

 Tongue River between the dam and Miles City in November 



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