MS4 Program

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Municipal Separate Storm Sewer System


Missoula County is working to reduce pollution caused by storm water runoff. The County is permitted under a storm water general permit, formally known as the MS4 permit. The Environmental Protection Agency and Montana Department of Environmental Quality regulate Municipal Separate Storm Sewer Systems (MS4s). This permit requires urbanized areas (populations of 10,000 people or more) to establish six specific methods of improving storm water quality:

1. Public education and outreach (view 2022 outreach plan)

2. Public involvement/participation

3. Illicit discharge detection and elimination

4. Construction site runoff control

5. Post construction runoff control

6. Pollution prevention/good housekeeping 



Related Events:




Click the links below to see documents related to the MS4 program:



View this presentation to learn more about the MS4 program. 

Only Rain Down the Drain - Download and print for jobsite

MS4 Boundary Area


What is illicit discharge?

Illicit discharge is any discharge into a storm drain system that isn’t entirely composed of storm water. Essentially, anything other than water falling from the sky is illicit discharge. 

Examples include a leaking sewage pipe, a fuel or oil spill or any other pollutant that flows into the storm system when conditions are dry (no precipitation). There are some exceptions of course, like when groundwater or drinking water enters a storm system. Illicit discharge can often be prevented by best management practices such as storing chemicals safely, conducting inspections, installing dry-well protection and education. Prevention is much cheaper than cleaning polluted water!

2022 Illicit Discharge Investigation and Corrective Action Plan

To anonymously report illicit discharge, fill out this online form or contact:
Missoula Valley Water Quality District
8 a.m. to 5 p.m. Monday through Friday
Messages can be left 24 hours a day


It is a critical interest of the County to manage its storm water. From its impacts on public health and safety, surface water quality, wildlife habitat and future development, the effectiveness and efficiency of storm water management is crucial. Missoula County’s Storm Water Management Plan can be found here. This plan outlines how Missoula County complies with permit requirements and works to keep our water ways clean.

2022 Storm Water Sampling Plan



Storm Water

Missoula County's storm water requirements are on page 46, Section 12.2, of the Public Works Manual 

MS4 Annual Reports: 

For annual report attachments or questions about the MS4 program, please contact Missoula County Public Works at 406-258-3701. The County coordinates with the Missoula Valley Water Quality District to implement program. To report inappropriate use of the storm water system, fill out this online form or contact the Missoula Valley Water Quality District at 406-258-4890.