Cascade County's Storm Water Management Program

Municipal Separate Storm Sewer System (MS4)

The Municipal Separate Storm Sewer System, referred to as MS4, is a conveyance or a system of conveyances designed to capture stormwater and discharge it to surface waters, such as: roads with drainage systems, streets, catch basins, curbs, gutters, ditches, storm drains, and other human-made channels.

Storm water is rain and snow melt that runs off surfaces such as rooftops, paved streets and parking lots that does not percolate into the soil. As water runs off these surfaces, it can pick up pollutants such as animal waste, fertilizers, oil, pesticides, oil, trash, etc.

Storm water can discharge into local streams, creeks, rivers and lakes. In the urban areas of Montana, storm water may go into a storm drain and continue through a storm sewer collection system until it is discharged into a local waterway. Storm water is discharged into water bodies without treatment.

Small Municipal Separate Storm Sewer System (MS4) Permit

The Montana Department of Environmental Quality (DEQ) regulates discharges of storm water through Montana Pollutant Discharge Elimination System (MPDES) permits.  Cascade County is a Small MS4 subject to permitting requirements under DEQ's MPDES General Permit for Storm Water Discharges Associated with Small MS4s.

Storm Water Management Program (SWMP)

To protect Montana's surface water resources, Cascade County implements a Storm Water Management Program (SWMP).  Cascade County's SWMP describes the program elements and associated best management practices, referred to as BMPs, to comply with applicable storm water regulations.  The purpose of this program is to reduce the discharge of pollutants to local surface waters.  There are six (6) minimum control measures of this program:

  1. Public Education & Outreach

  2. Public Involvement & Participation

  3. Illicit Discharge & Elimination

  4. Construction Site Storm Water Management

  5. Post-Construction Site Storm Water Management in New & Redevelopment

  6. Pollution Prevention/Good Housekeeping for Operations

The Cascade County SWMP is the document containing strategies Cascade County is taking to control, reduce and/or eliminate storm water runoff pollution into Montana's waterbodies.

Storm Water Contamination & Illicit Discharges

An illicit discharge is any discharge to a municipal separate storm sewer that is not composed entirely of storm water. Illicit discharges typically refer to storm drain flows during dry periods which contain pollutants and/or pathogens and can be caused by a number of factors such as a sewage disposal system interacting with the storm drain system, accidental spills/dumping, outdoor washing of fueling areas, and non-target landscape irrigation.

Illicit discharges have significant impacts on the natural environment and receiving water body and can contribute to high levels of pollutants to receiving waterbodies, including: 

  • Bacteria
  • Heavy Metals
  • Nutrients
  • Oil and Grease
  • Solvents
  • Toxins
  • Viruses

Pollutant levels from these illicit discharges have been shown in EPA studies to be high enough to significantly degrade receiving water quality and threaten aquatic, wildlife, and human health.

If you observe an illicit discharge or suspect discharge or have general permit requirement questions please send an email to notify us. Cascade County will review all suspect discharges immediately.

Construction Related Storm Drainage

The Montana DEQ regulates discharges of storm water from construction activity through the Montana Pollutant Discharge Elimination System (MPDES) General permit. An authorization under the General Permit for Storm Water Discharges Associated with Construction Activity is required for construction activities that include clearing, excavating, grading, grubbing, or placement/removal of earth material with a total area of one or more acres. Construction projects that disturb an acre or more are required to develop a Storm Water Pollution Prevention Plan (SWPPP) whereby BMPs are implemented in order to reduce and/or eliminate sediment erosion and pollution caused by storm water and associated with the construction activities.

Cascade County's SWMP Public Feedback

Public input is used to improve the function of the program and further protect water quality.  If you have areas of concern regarding storm water discharge and/or suggestions to improve other areas of the program, please complete the Stormwater Management Feedback Form and email it to [email protected] or mail it to: 

MS4 Coordinator

Cascade County Planning & GIS Department

121 4th Street North, STE 2H/I

Great Falls, MT 59401