Larimer County Municipal Separate Storm Sewer System (MS4) Program

Protected curb inlet

Larimer County has a long history of protecting our natural resources. When dealing with stormwater,  the County, as well as business owners, residents, and land developers all play an important role in protecting the quality of the stormwater that leaves their property. Water quality is important for the whole community and Larimer County is committed to protecting this valuable resource through the various programs it implements. 

What is a MS4?

A MS4 is defined as a conveyance or system of conveyances that carry stormwater. Conveyances like roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, or storm drains all carry stormwater. The term “Separate” indicates that the system is separate from the sanitary sewer system, which carries sewer effluent to a water treatment facility. It’s important to realize that the stormwater system carries stormwater, untreated, to its eventual discharge point, which is typically a lake, river or stream. But it can also be a pond where the water infiltrates into the ground.


MS4 is also a term that describes the owner/operator of the stormwater conveyance system. Usually this is a municipality or a county, but it can also be a school or even CDOT.  Each MS4 is permitted by the State of Colorado to discharge stormwater to a water of the state like a lake, river or stream if they implement water quality controls to reduce the amount of pollution in the discharged stormwater.
 

Larimer County is a Phase II Municipal Separate Storm Sewer System (MS4) Community. This means that the County is permitted by the State of Colorado to discharge stormwater to State Waters. Under the regulation of the permit, Larimer County is required to have a Stormwater Quality Management Plan that has the goal to protect stormwater quality by controlling and/or eliminating pollutant discharges into its stormwater conveyance systems.

As part of the Stormwater Quality Management Plan, Larimer County maintains a Program Description Document (PDD) that describes how the County’s program works. This PDD is available, upon request, for review and comment. Other aspects of the Plan include control measures that Larimer County implements within its MS4 permit coverage area. 

To view the County MS4 Permit coverage area, use the Land Information Map (LIL). Once on the LIL map:

  • click "Flood" in the Layer Themes box
  • click "Layer List"
  • open "Stormwater" by clicking the + icon
  • scroll down and select "MS4 Permit Area"
  • in "Feature Layers" make sure "Incorporated Areas" option is selected
  • zoom the map into your area of interest, or
  • type your address in the "Search" field, hit enter
  1. Public Education and Outreach:

    Larimer County educates residents, developers and members of the business community to help reduce water quality impacts associated with pollutants in stormwater runoff.  To learn how you can help protect our waterways visit our other webpages and our videos and links pages.
     
  2. Illicit Discharge Detection and Elimination (IDDE):

    Larimer County  implements and enforces a program to detect and eliminate spills, illegal dumping and other non-stormwater discharges into the County's storm sewer system. The IDDE program is authorized through the Larimer County Stormwater Quality Ordinance. 

    You can help the IDDE program by recognizing what is an illicit discharge (i.e., a pollutant spill). Some things to look for include:
     
    • An unusual color of the water or a stain on storm drain
    • An unusual or foul odor of the water or storm drain
    • An oil sheen on the water
    • Suds or puddles in the gutter when it hasn’t recently rained
    • Illegal dumping of waste near storm drains (e.g., stains and/or containers of household chemicals, automobile fluids, paints, etc.)
    • Many dead or dying animals including fish, crayfish, insects, waterfowl or other animals near a waterbody
    • Please visit our Stormwater Videos page to watch the IDDE videos to learn more about Illicit discharges and how to detect them.
  3. Construction Site Pollutant Control:

    Land disturbance and other activities associated with construction projects can be a major source of pollutant discharges. When it rains, stormwater can carry sediment off a construction site and into a waterway where it negatively impacts aquatic habitats and wildlife. 

    Larimer County Engineering implements a program to review development plans for stormwater and drainage impacts and an inspection program for construction activities greater than 1 acre within the MS4 permit area. As part of the inspection process we ensure that construction sites implement control measure to keep sediment and other pollutants from leaving the project boundaries.
     
  4. Post-Construction for New Development and Redevelopment:

    Preventing flooding and ensuring stormwater quality are the goals of the Engineering Department’s Post-Construction Program. Ponds, basins, swales and ditches not only serve as flood and drainage control but when constructed and maintained properly these facilities settle out sediment and debris and remove other pollutants carried by stormwater before discharging that stormwater to waterways.
     
  5. Preventing Stormwater Pollution from County Facilities and Operations:

    Larimer County protects water quality when its departments implement good housekeeping practices. Department staff follow standard operating procedures (SOPs) for pollution prevention and staff participate in training for good housekeeping practices, illicit discharge detection and elimination, and stormwater management during maintenance and construction activities. 

    The Engineering Department keeps records of the staff training and works with departments to ensure the SOPs they implement are up to date and relevant to the work they do.

Larimer County Stormwater Quality Ordinance serves as one of the County’s regulatory mechanisms to implement and enforce requirements to prohibit non-stormwater discharges into the storm drainage systems.