Birch Creek assessment records low flows and a dry reach

What you need to know

• In water year 2022, the upper streamgage recorded 87,960 acre-feet and the lower streamgage recorded 20,357 acre-feet. The assessment estimates a net reduction of 67,603 acre-feet along the reach. 1
• That estimated reduction is not attributed to irrigation diversions alone. The report includes diversions, groundwater losses and evapotranspiration among the contributing factors, and does not establish how much each accounts for. 12
• The assessment reports several occasions when flows fell below the compact’s specified instream-flow conditions. It also records that a section immediately below Swift Dam was dry during a synoptic measurement on 18 October 2022. 1
• Montana’s summary of the Blackfeet Compact lists Birch Creek instream-flow rights of 25 cubic feet per second from April to September and 15 cubic feet per second from October to March. The report’s low-flow observations do not, on their own, establish a compact violation. 13
• Montana’s DNRC describes a 15-year deferral of new development of the Tribe’s Birch Creek water rights. This is distinct from the assessment’s measurements of flow and does not show whether conditions have changed since the observation period. 31

Why this matters

The assessment provides measured evidence about flow variation and water-volume reduction along a creek where instream flows are set out in the compact. Because the estimated reduction combines several possible causes, understanding their separate contributions remains important for water-resource management. 13

What happens next

The reporting leaves open what flows have been like since 2022, how much of the estimated reduction is due to each contributing factor, and whether low flows affected aquatic life. The assessment does not report that these conditions have been remedied. 12

What we know

• The assessment was carried out by the USGS with the Blackfeet Water Department to clarify streamflow patterns, irrigation withdrawals and instream flows. 14
• The reported upper and lower streamgage totals for water year 2022 differ by 67,603 acre-feet, an amount the report characterises as a net reduction along the reach. 1
• The report documents flows below compact instream-flow conditions on several occasions and a dry section below Swift Dam during one measurement in October 2022. 1
• Daily data for Swift Reservoir outflow and selected Birch Creek diversions are available for January 2020 through December 2022. 2

What we don't know

• The supplied reporting does not establish Birch Creek’s streamflow conditions after the observation period ending in 2022. 15
• It does not quantify how much of the estimated stream-volume reduction is due to irrigation diversions, groundwater losses or evapotranspiration individually. 12
• The reporting does not establish what biological effects, if any, the low-flow and dry-reach conditions had on aquatic life. 1
• The supplied sources do not show whether the low-flow conditions have since been remedied. 13

Read the reporting

This WildNews briefing synthesises and summarises reporting from the sources below. WildNews did not produce the original reporting. Learn more about how we use AI