Streamflow assessment of Birch Creek, southern boundary of the Blackfeet Indian Reservation, Pondera County, northwestern Montana—Synoptic and continuous streamflow measurements through 2022
The U.S. Geological Survey, in cooperation with the Blackfeet Water Department, performed this study to assist water-resource managers in understanding and quantifying irrigation diversions, instream flows, and the spatial and temporal streamflow dynamics of Birch Creek in northwestern Montana. Birch Creek forms the southernmost boundary of the Blackfeet Indian Reservation and is highly valued culturally as an aquatic life and wildlife resource and as a principal source of water for irrigation. The study area included Birch Creek from Swift Dam to 53.3 miles downstream just above its confluence with the Two Medicine River. The study period was calendar year 2022, but selected historical information was also included in the study. Birch Creek streamflow originates through releases from Swift Reservoir, with the largest releases typically occurring during the irrigation season (May–October) to primarily meet the needs of downstream irrigators. This study included the compilation and collection of (1) continuous streamflow measured at streamgages installed for this study, (2) instantaneous streamflow measurements made during four synoptic events, and (3) reported daily values of Swift Reservoir outflow and irrigation diversions. Continuous streamflow data were available from two U.S. Geological Survey streamgages: Birch Creek below Heart Butte Road, near Heart Butte, Montana (upper streamgage; station 06094900), at river mile 12.9 and Birch Creek at Bullhead Bridge near Valier, Mont. (lower streamgage; station 06098120), at river mile 53.3. The upper streamgage (station 06094900) was primarily affected by the regulated releases from Swift Dam with daily mean streamflow typically slightly higher than the reported Swift Reservoir outflows. Daily mean streamflow ranged from 9.87 cubic feet per second (ft 3 /s) (November 18, 2022) to 640 ft 3 /s (June 22, 2022) during September 16, 2021–December 31, 2022. The mean annual streamflow was 122 ft 3 /s in water year 2022 (a water year is the 12-month period from October 1 to September 30 and is designated by the year in which it ends), about 13 percent greater than the mean annual reported Swift Reservoir outflows. The lower streamgage (station 06098120) was downstream from all major tributaries and diversions within the Birch Creek drainage basin. Daily mean streamflow ranged from 6.02 ft 3 /s (September 8, 2022) to 95.0 ft 3 /s (June 5, 2021) during August 26, 2020–December 31, 2022. Mean annual streamflow was 27.8 ft 3 /s for water year 2022. Flows at the lower streamgage (station 06098120) were typically lower than at the upper streamgage (station 06094900) from late April to early October, whereas flows from early October to late spring were similar to or greater than those at the upper streamgage. The upper streamgage had a total streamflow volume of 87,960 acre-feet (acre-ft) for water year 2022, whereas the lower streamgage had a total streamflow of 20,357 acre-ft. A net volume of about 67,603 acre-ft was removed from Birch Creek through diversions, losses to groundwater, and evapotranspiration along that reach during water year 2022. Most of the changes in main-stem streamflow could be accounted for by the tributary inflows or diversion outflows. The most dynamic streamflow gains and losses occurred along the upper few miles of Birch Creek. Streamflow was consistently lost to the groundwater system immediately below Swift Dam and then returned to levels that slightly exceeded the outflow at the dam near river mile 11.0. The overall net gains for the entire 53.3-mile study reach ranged from 15.5 to 42.6 ft 3 /s among the synoptic events. On several occasions, streamflow fell below minimum instream flow conditions specified in the Blackfeet Compact (water rights compact among the Blackfeet Tribe, State of Montana, and United States), which are subject to a 15-year deferral period (until June 12, 2033). Notably, a section of Birch Creek immediately downstream from Swift Dam was completely dry during the October 18, 2022, synoptic event.