Montana exempt well rules

The thresholds

Outside a stream depletion zone, an appropriation is exempt from the permit requirement when it is 35 gallons per minute or less and does not exceed 10 acre-feet per year.

Inside a stream depletion zone the thresholds are considerably tighter: 20 gallons per minute or less, and no more than 2 acre-feet per year. Stream depletion zones exist because groundwater pumping near a hydraulically connected stream reduces streamflow, which affects surface-water right holders.

An important limitation of any online tool, including ours: we could not find an authoritative public map of stream depletion zone boundaries. So while we can tell you whether a property falls inside a controlled groundwater area or a closed basin, we cannot tell you which exempt threshold applies to your site. DNRC can. We show both thresholds and say so plainly rather than guessing.

The combined appropriation trap

This is the provision that catches developers and subdivision buyers. Even when each individual well stays under the flow threshold, a combined appropriation from the same source by two or more wells that together exceed the annual volume limit requires a permit — regardless of each well's individual flow rate.

In a subdivision served by individual domestic wells, the combined limit can therefore become the binding constraint rather than the per-well flow rate. If you are evaluating a parcel in a platted subdivision, this is worth a direct conversation with DNRC before you assume an exempt well is available to you.

The Notice of Intent — the 2023 change

Montana used to treat exempt wells as a purely after-the-fact filing: you drilled, then filed a Notice of Completion. That is no longer how it works, and this is the single most commonly misunderstood point.

Since legislation enacted in 2023 and amended in 2025, a person must file a Notice of Intent to appropriate groundwater with DNRC on the department's form before appropriating under the exempt provisions. DNRC notifies the filer of any defects within 10 business days, and a notice not corrected within 60 days of a defect notice terminates. Within 10 business days of the notice being correct and complete, DNRC reviews it for compliance and either authorizes or denies it — and if it denies, it must give reasons.

Once authorized, the appropriation must be completed within five years, with one possible five-year extension. A Notice of Completion is then due within 60 days of completing the well and putting water to beneficial use.

The practical shift is significant: this converted a post-hoc filing system into a pre-authorization review. DNRC can say no before you drill.

Where the exemption does not apply at all

Inside a controlled groundwater area, the ordinary exempt-well exception does not apply. Groundwater may generally be appropriated only under a permit, or according to the rules that established the area. Montana currently has 21 controlled groundwater areas, ranging from small contamination-site boundaries to the very large Powder River Basin designation.

Closed basins carry their own restrictions on new appropriations, and those cover a substantial share of Montana's desirable residential land — including the Upper Missouri, the Jefferson and Madison basins, the Bitterroot, and the Upper Clark Fork. Restrictions vary by closure, and new groundwater development in a closed basin commonly requires additional analysis and sometimes mitigation for stream depletion.

This is not legal advice

The thresholds and procedures above are quoted from Montana Code Annotated 85-2-306 as it reads currently. Statutes are amended, rules are adopted, and application to a specific property depends on facts no database can see. Confirm requirements with Montana DNRC and, where the stakes justify it, a water-right professional before drilling or relying on a right.