Last Updated 08/24/2026
Montana Stormwater Regulations
The state of Montana operates under the Montana Pollutant Discharge Elimination system (MPDES) program. Permits for the MS4 Stormwater Program were originally issued in January 2005 and reissued in January 2010, and are regulated under an NPDES Permit (MTRO40000) through the State-wide Administrative Rules of Montana (ARM), Title 17, Chapter 30, Subchapters 11 and 13.
Standards apply in areas with regulated MS4s and where development projects distrub a land area of one acre or greater including as part of a larger common plan. Post-construction requirements includde on-sitre retention and volume control.
Excerpts from Montana’s Post-Construction Storm Water BMP Design Guidance Manual
Montana’s MS4 municipalities include: Billings, Bozeman, Butte, Great Falls, Helena, Kalispell, and Missoula.
1.3 MS4 General Permit
MCM5 in the General Permit requires MS4s to develop, implement, and enforce a program to address stormwater runoff from new development and redevelopment projects that disturb greater than or equal to 1 acre, including projects less than 1 acre that are part of a larger common plan of development or sale and that discharge into a permitted small MS4. This program must ensure that controls are in place that would prevent or minimize water quality impacts.
MCM5 has multiple General Permit requirements; however, this manual primarily focuses on the performance standard for new and redevelopment projects.
1.3.1 Post-Construction Performance Standard
For new and redevelopment projects regulated by the General Permit, the Post-Construction Performance Standard presented in Part II.A.5.b.iii of the General Permit—which was issued on November 30, 2016, and became effective on January 1, 2017—is as follows:
Require that all regulated projects implement post-construction storm water management controls that are designed to infiltrate, evapotranspire, and/or capture for reuse the post-construction runoff generated from the first 0.5 inches of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation. For projects that cannot meet 100% of the runoff reduction requirement, the remainder of the runoff from the first 0.5 inches of rainfall must be either:
- Treated onsite using post-construction storm water management control(s) expected to remove 80 percent total suspended solids (TSS);
- Managed offsite within the same sub-watershed using post-construction storm water management control(s) that are designed to infiltrate, evapotranspire, and/or capture for reuse; or
- Treated offsite within the same subwatershed using post-construction storm water management control(s) expected to remove 80 percent TSS Permittees allowing offsite treatment shall do the following:
- Develop and apply criteria for determining the circumstances under which offsite treatment may be allowed.
The criteria must be based on multiple factors, including but not limited to:
- Technical or logistic infeasibility
- High groundwater
- Groundwater contamination
- Poorly infiltrating soils
- Shallow bedrock
- Prohibitive costs
- A land use that is inconsistent with capture and reuse or infiltration of storm water)
Determinations may not be based solely on the difficulty and/or cost of implementation. The permittee must develop a formal review and approval process for determining projects eligible for offsite treatment. The offsite treatment option is to be used only after all onsite options have been evaluated and documented through the permittee’s developed formal review and approval process.
4.3.5 Maintenance
Routine and proper maintenance is essential for long-term effectiveness of all BMPs. Even when BMPs are correctly designed and installed, they will likely become eyesores and cease to function if not properly maintained. Because maintenance requirements vary for different BMPs, maintenance must be considered during the BMP design and selection process. A brief summary of maintenance items that should be considered during the BMP selection process is discussed below; however, additional details specific to each BMP discussed in this manual are addressed in Sections 5.2 to 5.9(10).
Compatibility with the Project Owner’s Maintenance Capabilities
Each BMP requires certain equipment and skills to conduct proper maintenance. For instance, sediment removal will be a common maintenance requirement for BMPs which have a sediment forebay. When selecting a BMP, it is important to consider who will conduct the long-term maintenance on the BMP and assess whether the project owner/operator has the correct maintenance equipment, necessary skills, and is in agreement with the required maintenance schedule.
Vegetated BMPs
Vegetated BMPs such as bioretention and biofiltration swales require special care to maintain the functionality of the BMP. When planning to use vegetated BMPs, the maintenance frequency and need for specialized training often varies depending on the type(s) of vegetation selected for use. For example, the designer should consider whether vegetation will require supplemental irrigation throughout the growing season and verify that the project owner/operator will irrigate and maintain the vegetation.
Accessibility
Access must be considered because it is critical that all BMPs be accessible for inspections and maintenance. When selecting and siting a BMP, consider what type of equipment will be needed to conduct the required maintenance activities and the frequency at which the maintenance will be conducted. Large BMPs such as extended detention basins will require both regular access for equipment such as lawnmowers and less frequent access for large equipment to remove accumulated sediment from the main treatment cell. Difficult access situations, including those with safety concerns, must also be considered. These include BMPs close to buildings and high traffic areas (13). Considering community screening factors such as safety and aesthetics helps the designer consider whether a BMP is well-suited for the general project area. This section describes two community factors that should be considered during the BMP selection process (13).
Recommended Maintenance Activities
Routine and proper maintenance is essential for long-term effectiveness of all BMPs. Even when BMPs are correctly designed and installed, they will likely become eyesores and cease to function if not properly maintained. Because maintenance requirements vary for different BMPs, maintenance must be considered during the BMP design and selection process. A brief summary of maintenance items that should be considered during the BMP selection process is discussed below; however, additional details specific to each BMP discussed in this manual are addressed in Sections 5.2 to 5.9(10).
Compatibility with the Project Owner’s Maintenance Capabilities
Each BMP requires certain equipment and skills to conduct proper maintenance. For instance, sediment removal will be a common maintenance requirement for BMPs which have a sediment forebay. When selecting a BMP, it is important to consider who will conduct the long-term maintenance on the BMP and assess whether the project owner/operator has the correct maintenance equipment, necessary skills, and is in agreement with the required maintenance schedule.
Vegetated BMPs
Vegetated BMPs such as bioretention and biofiltration swales require special care to maintain the functionality of the BMP. When planning to use vegetated BMPs, the maintenance frequency and need for specialized training often varies depending on the type(s) of vegetation selected for use. For example, the designer should consider whether vegetation will require supplemental irrigation throughout the growing season and verify that the project owner/operator will irrigate and maintain the vegetation.
Accessibility
Access must be considered because it is critical that all BMPs be accessible for inspections and maintenance. When selecting and siting a BMP, consider what type of equipment will be needed to conduct the required maintenance activities and the frequency at which the maintenance will be conducted. Large BMPs such as extended detention basins will require both regular access for equipment such as lawnmowers and less frequent access for large equipment to remove accumulated sediment from the main treatment cell. Difficult access situations, including those with safety concerns, must also be considered. These include BMPs close to buildings and high traffic areas (13). Considering community screening factors such as safety and aesthetics helps the designer consider whether a BMP is well-suited for the general project area. This section describes two community factors that should be considered during the BMP selection process (13).
Recommended Maintenance Activities
Infiltration Basin:
Annually
- Inspection of all components of the infiltration basin in accordance with an approved inspection form in accordance with local jurisdiction requirements
- Removal of sediment from inlets, pretreatment facilities, diversion structures, and overflow structures
Semiannually
- Remove all green waste
- Trimming of vegetatino for aesthetics to prevent establishment of vegetation that may clog the facility
As needed
- Remove litter and debris from all components of the infiltration basin
- Repair structural components including inlets, diversion structures, and outlet structure
- Inspect the basin for signs of erosion and repir eroded areas accordingly
- Perform spot resseding if necessary
- Observe drain time following rainfall events to determine if the facility is clogged
- Regularly manage all vegetation associated with the infiltration basin and remove all clippings
- Repair and maintain maintenance access routes
Bioretention Areas:
Upon establishment
- Inspect the bioretention area and contributing drainage area following rainfall events
- Conduct any needed repairs or stabilization, one-time spot fertilization may be needed for initial planting
- Follow the watering schedule provided by the designer
- Remove and replace dead plants
Once every 2 to 3 years
- Remove sediment in pretreatment facility and inflow points
- Remove and replace the mulch layer and the top 2 to 3 inches of the bioretention soil media
Annually
- Inspect all components of the bioretention area in accordance with an approved inspection form in accordance with local requirements
- Supplement mulch where needed to maintain a 2 to 3 inch layer
- Cut back plants to maintain the nutrient mass removal
Semiannually
- Perform spot weeding, trash removal, and mulch raking
As needed
- Add reinforcement planting to maintain the desired vegetation density
- Manage all vegetation associated with the bioretention area
- Remove sediment from inflow points, pretreatment facilities, diversion structures, and overflow structures
- Remove any dead or diseased plants and invasive plants using recommended control methods
- Stabilize the contributing drainage area to prevent erosion
- Observe drain time following rainfall events to determine if the facility if clogged
Permeable Surfaces:
Annually
- Inspect the PICP facility
- Replenish aggregate in joints if more than half an inch of space exists between aggregate and chamber bottoms on the paver surface
- Inspect and repair all paver surface deformations exceeding half an inch
- Repair all pavers offset by more than a quarter inch above/below adjacent pavers or curbs, inlets, etc.
- Replace cracked pavers
- Check underdrain system and outfalls for free flow of water and outflow from the observation well after a major rainfall event
- Flush the underdrain system to check for clogging
- Inspect all components of the PIP facility in accordance with an approved inspection form in accordance with local jurisdiction.
Semiannually
- Vacuum sweep the surface with equipment such as a regenerative air vacuum sweeper
- Adjust the vacuum settings to remove visible sediment without uptake of aggregate from paver openings
As needed
- Observe the system during and following rainfall events to determine if the facility is clogged
- Stabilize the contributing drainage area to prevent erosion
- Regularly manage all vegetation around the permeable pavers and remove all clippings
- Keep the pavers free of trash, debris and sediment
Dispersion Areas:
Upon establishment
- Apply irrigation until vegetation has been established
- Inspect the dispersion area for signs of erosion and immediately stabilize eroded areas with grass cover
Annually
- Maintain or restore the level spreader so that flows are spread evenly over the entire area
- Remove sediment deposits and re-level so lateral slopes are even and flows pass evenly through the dispersion area
- Reseed as needed during fall seeding season to maintain 90% turf grass cover
- Inspect all components of the dispersion area in accordance with an approved inspection form
As needed
- Remove trash and debris from the dispersion area
- Regularly manage all vegetation in accordance with the designer’s recommendations
- For locations where the grass is mowed, remove all clippings
Biofiltration Swale:
Upon establishment
- Make sure full coverage of turf grass or erosion control fabric is achieved
- Inspect the swale during and after runoff events.
Annually
- Reseed as needed during fall seeding season to maintain 90% turf grass cover
- Remove any accumulated sand or sediment deposits behind check dams
- Examine channel bottom for evidence of erosion, braiding, excessive ponding or dead grass
- Check inflow points for clogging and remove any sediment
- Inspect side slopes and filter strips for evidence of erosion
- Inspect all components of the biofiltration swale in accordance with an approved inspection form according to local jurisdiction
Quarterly
- Ensure that the contributing drainage area is clear of debris and stabilized and perform spot-reseeding if or where necessary
- Repair undercut, and eroded areas as needed at swale inflow and outflow structures
- If applicable, inspect upstream and downstream of check dams for evidence of undercutting or erosion
- Remove trash or blockages at weepholes
As needed
- Remove litter/debris
- Manage vegetation and remove all clippings
Wet Detention Basin:
10-20 years
- Remove accumulated sediment from the bottom of the wetpool cell to maintain volume and deter algae growth
1-4 years
- Remove sediment from the forebay before if becomes a significant source of pollutants
Annually
- Inspect all components of the WDB in accordance with an approved inspection form according to local jurisdiction
Semiannually
- Trim vegetation for aesthetics and mosquito control, prevent excessive growth
- Evaluate the health of vegetation, remove and replace dying plants
As needed
- Remove litter/debris from all components of the WDB
- Repair basin inlets, outlets, and all other structural components required for the basin to operate as intended
- Repair and revegetate eroded areas, regularly manage all vegetation along maintenance right of ways and the embankment
- Remove all clippings, repair maintenance access routes, inspect the WDB for signs of mosquito larvae during summer months and provide treatment when breeding is found
- When necessary drain the WDB during dry periods to prevent the release of untreated water
5.9.5 Maintenance
Maintenance is required on all BMPs because clogging of devices can hinder pollutant removal capabilities and create drainage problems. Specific maintenance tasks and schedules vary for different devices. A maintenance schedule should be provided that considers the manufacturer’s recommended maintenance activities. The volume of pollutants draining to a device dictates the rate at which it reaches its capacity; therefore, the characteristics of the contributing drainage area are often a primary factor in establishing the frequency of maintenance activities. Frequent inspections throughout the first year of installation are recommended to understand how often maintenance is needed.
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Frequently Asked Questions
What stormwater performance standard applies to regulated projects in Montana?
Regulated new and redevelopment projects must implement post-construction controls designed to infiltrate, evapotranspire, or capture for reuse the runoff generated from the first half inch of rainfall in a 24-hour storm preceded by 48 hours without measurable precipitation. If a project can’t fully meet that runoff reduction target, the remaining portion must be treated on-site or off-site using controls expected to remove 80% of total suspended solids.
When can a Montana project use off-site stormwater treatment instead of managing runoff on-site?
Only after all on-site options have been evaluated and documented through a formal review and approval process. Qualifying factors include technical or logistic infeasibility, high groundwater, groundwater contamination, poorly infiltrating soils, shallow bedrock, prohibitive costs, or a land use inconsistent with capturing or infiltrating stormwater. A determination can’t be based solely on difficulty or cost of implementation.
Which Montana cities operate their own MS4 stormwater programs?
Montana’s MS4 municipalities include Billings, Bozeman, Butte, Great Falls, Helena, Kalispell, and Missoula. These communities are required to develop, implement, and enforce a program addressing runoff from new development and redevelopment projects disturbing 1 acre or more, including smaller projects that are part of a larger common plan of development or sale.
How often does a bioretention area need maintenance in Montana?
Sediment in the pretreatment facility and inflow points should be removed every two to three years, along with replacing the mulch layer and the top two to three inches of bioretention soil media. Annual tasks include a full component inspection and cutting back plants to maintain nutrient removal, while spot weeding, trash removal, and mulch raking should happen twice a year.
How often does a wet detention basin need sediment removal in Montana?
Sediment should be removed from the forebay every one to four years, before it becomes a significant pollutant source, while the main wetpool cell typically only needs full sediment removal every 10 to 20 years to maintain volume and deter algae growth. Annual inspection is required regardless, and vegetation should be trimmed twice a year for aesthetics and mosquito control.
What should a property owner consider before choosing a stormwater BMP for a Montana site?
Maintenance compatibility matters as much as design performance. The manual recommends confirming the project owner has the right equipment, skills, and willingness to follow the required maintenance schedule before selecting a BMP, since vegetated systems like bioretention and biofiltration swales need specialized care and access for equipment ranging from lawnmowers to large sediment-removal machinery must be planned into the site design from the start.
The information on this page is provided for general informational purposes only and is not legal advice. Regulations change frequently — AQUALIS makes no warranty as to the accuracy or completeness of this content, and any reliance on it is at your own risk. Consult a qualified professional or your local regulatory authority to confirm requirements applicable to your specific property.
