Tools for water right administration include:
Montana water law is based upon the theory of "prior appropriation". Under this theory water rights and uses are rank against one another. That ranking, and in Montana the only ranking between water right is it date of development or priority date. Thus priority between water rights and water uses is solely determined as "first in right is first in time".
Under this system each water right is to be limited to the level of use and period of use as of the time that the water right was first developed. Every water right holder has a right to expect the conditions on the stream, the source of supply, to be the same as they were the day their water right was created. Thus in times of shortage a water user can require all water users to a) limit their use to the level on the date it was developed and b) to have all junior (water rights developed after theirs) reduce or shut-off there diversion until the senior water rights are satisfied.
Instream flow water rights are administered, like all other water rights, under their respective priorities.