I would start by contacting the regional folks at USACE: Omaha District > Missions > Regulatory Program > Montana
You may also contact:
Also, the various Survey Bulletins covering the periods in question are a good place to start
Thank you for contacting History Hub with your inquiry.
A search of National Archives records through the NARA online catalog reveals records from the U.S. Army Corps of Engineers pertaining to Montana may be found at several NARA facilities. Those that may be most pertinent to research, though, may be the facilities in Seattle and Kansas City. The search found series related to Civil Works Project Files, 1948-1995 in Seattle, as well as project files for the years 1900-2012, 1881-1995, and 1900-1995 in Kansas City. The catalog descriptions for these series indicate that surveys are among the documents included in them. In addition, the Records of the U.S. Geological Survey includes a series title Field Notes of Surveys, 1882-1980 which lists the Corps as one of the contributors. Montana is listed as one of the states within this series. The National Archives at Denver also has some RG 77 Records for Montana, although they predominately cover Fort Peck.
A secondary issue to consider when dealing with water in the West is to always look at the Bureau of Reclamation (RG 115) records to see if there is any related information to the Corps records as they often worked hand in glove. The NARA facility in Denver holds the majority of all the Bureau of Reclamation's records.
The NARA catalog (https://catalog.archives.gov) listed over 1,000 results when the terms “U.S. Army Corps of Engineers Montana” were searched, so you might want to also peruse these results as well. You can also contact the NARA offices at Denver (firstname.lastname@example.org), Kansas City (email@example.com), and Seattle (firstname.lastname@example.org) for more information on their holdings. The best of luck in your research, and thank you again for contacting History Hub!