There is a common misconception of what does and does not constitute a Montana Flume. For some, a Montana flume is a Parshall flume with the discharge removed. For others, it is a Parshall flume with the throat and discharge removed.
The US Department of the Interior Bureau of Reclamation’s Water Measurement Manual (2001), states (emphasis added):
The portion of the (Parshall) flume downstream from the end of the converging section need not be constructed if the flume has been set for free-flow where it is not expected to operate above submergence limit. This truncated version of the Parshall flume is sometimes referred to as the Montana flume.
Further, Gerald L. Westesen, Professor of Civil and Agricultural Engineering, Agricultural Experiment Station, Montana State University issued two technical publications on the subject:
- MT 9127 (AG) Montana (Short Parshall) Flume (Part 1)
- MT 9128 (AG) Montana (Short Parshall) Flume (Part 2)
MT 9127 and MT 9128 both clearly show the Montana flume as lacking the extended throat and discharge sections of the more widely known Parshall.
However, confusion persists even today. As recently as 2010, some published research and theses (e.g. Effects of Submergence in Montana Flumes) included the throat as part of a Montana flume.
A Parshall flume with the throat section but no discharge section is more properly termed a Modified Parshall Flume in recognition of the same titled research into the configuration conducted By A.I. Johnson of the US Department of the Interior Geological Survey at its Hydrological Laboratory in Denver, Colorado.
Johnson’s work was an extension of the design work by H.C. Troxell and C.A. Taylor (1931). At no time in work of either party was the throated flume described as a Montana flume.
This Modified Parshall flume is now more commonly know as a (USGS) Portable Parshall as offered by the Hydrological Instrumentation Facility of the USGS.