A reality based independent journal of observation & analysis, serving the Flathead Valley & Montana since 2006. © James Conner.
Instream flows protect fish and aquatic habitat. For the Flathead Water Compact, they are set by negotiation based on input from scientists, water managers, and other experts. On the Swan River, as displayed in the graph below, the Minimum Enforceable Flow value is set between the historical minimum and median for the river. The MEF numerical values are published Appendix 26 of the Flathead Water Compact.
The enforceable levels of this water right take the form of static distributions of unique daily flow values, one each for every day of the year. Call may be initiated on the day following a 24-hour period where average daily river flows fall below their respective daily enforceable value.
If the Swan’s daily streamflow falls to or below the MEF for at least a day, the owner of the instream flows is empowered to make a call on the holders of junior rights within the Swan basin. Being empowered to make a call does not, however, mean a call will be made. Whether a call is made will depend on a variety factors, among them how far the streamflow is below the MEF, whether shutting off holders of junior water rights will do any good, and weather and runoff forecasts. I seriously doubt that a call on junior rights would be made if the streamflow only dropped below the MEF by a percent or few. The MEFs are set high enough to give water managers some flexibility in handling the authorization for a call.
The irrigation season is April through September. I compared the USGS’s published daily streamflows for the USGS gage on the Swan River with the MEF values to see how often daily flows have been at or below the MEF values for the 1923–2014 irrigation seasons, and by how much. The histogram below displays the results.
Download an Excel spreadsheet of the daily streamflow and MEF values for the Swan River. These data are organized in the federal water year format, which runs from 1 October to 30 September (fall, winter, spring, summer). The spreadsheet omits February 29 so that all years have 365 days.
