Last Updated 08/11/2026
Excerpts from The Catalog of Stormwater Best Management Practices
Section 1 – Introduction
The Catalog of Stormwater Best Management Practices for Idaho Cities and Counties provides technical guidance for construction site design and the selection of stormwater best management practices (BMPs). The catalog is a guidance document containing voluntary controls that could be formally adopted by a jurisdiction to establish standards, if desired. Measures, such as those described and other recognized equivalents, should be used to manage the quantity and quality of stormwater runoff from land development.
Technical guidance regarding storm water management is necessary for several reasons:
Idaho’s fast growing population has led to expanded land development, which is a recognized source of nonpoint source pollution called polluted runoff. This catalog
includes structural and nonstructural BMPs to prevent discharge of pollutants from developing areas, both during the construction phase and for the life of the development.
BMPs can also be used to reduce polluted runoff from existing land uses.
Many water bodies throughout the state are not in compliance with state water quality standards. Beneficial uses such as domestic water supply, fishing, swimming, boating,
and agricultural water supply may be impaired due to excessive pollutants such as those that come from storm water runoff. The catalog identifies various controls to reduce
conventional pollutants with special consideration for phosphorus and sediment, both common pollutants in Idaho.
The National Pollutant Discharge Elimination System (NPDES) storm water regulations mandate that some communities develop and implement storm water management
programs to ensure that pollutants in storm water runoff are controlled to the maximum extent practicable. Because polluted runoff can potentially contribute to the degradation
of receiving waters, improved implementation of storm water management programs at the local level is important for attaining and maintaining high water quality standards.
3.5.1.2 Runoff Minimization
Impervious areas directly connected to the storm drain system are the greatest contributor of nonpoint source pollution in storm water runoff. Streets and other transport-related structures
typically comprise between 60% and 70% of total impervious area, and unlike rooftops, streets are almost always directly connected to a storm water system. Literature on the impacts of
development indicates that the level of imperviousness (or directly connected imperviousness) in a watershed is a factor that negatively influences the structural integrity of a stream or the health
of its aquatic species.
Methods for minimizing runoff include disconnecting impervious surfaces and using permeable pavements when a stabilized surface is needed to minimize the impact. Development plans can
optimize positive attributes of the site such as soil groups, vegetation, and landform. Open space design, conservation development, and cluster development refer to site-planning techniques that
concentrate development on one or more portions of a site, and conversely preserve more of the site as open space. Reconfigured development forms offer an attractive alternative to
conventional subdivision design and are applicable to most forms of residential development, with special opportunities in rapidly growing rural settings and within suburban fringes of
growing communities.
BMPs that help minimize runoff are as follows:
- BMP 3: Minimize Disturbance of Steep Slopes
- MP 4: Manage Impervious Surfaces—Disconnect and Reduce
- BMP 5: Reduce Roadway and Sidewalk Surface Areas
- BMP 6: Green Streets and Parking
- BMP 19: Porous Pavement
3.5.1.3 Rainwater Capture
Rain and snow fall as naturally distilled water that can be collected on site and consumed as a resource for many beneficial uses such as irrigation, water for recreational lakes, ground water
recharge, industrial cooling and process water, and other nonpotable domestic uses. Water harvesting collects, captures, and stores rainwater from roofs, paved surfaces, and
landscaped areas. Water harvesting uses tanks, cisterns, or sealed wells to collect storm water runoff that can be used for irrigation or other nonpotable domestic uses. The result is a reduction
in the total volume of runoff from the site because the impervious surface from the roof no longer generates runoff.
Green roofs are another tool for capturing storm water and mimicking a variety of hydrologic processes normally associated with open space. Plants in green roofs capture rainwater on their
foliage and absorb it in their root zone, encouraging evapotranspiration and reducing the amount of storm water runoff. Storm water can also be captured and infiltrated into subsurface storage
associated with green space on the site.
BMPs that focus on rainwater capture and reuse are as follows:
- BMP 20: Vegetated Roofs
- BMP 21: Storm Water Planters
- BMP 27: Rainwater Harvesting and Reuse
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Frequently Asked Questions
Is Idaho's stormwater BMP catalog legally required, or just guidance?
It’s voluntary. The Catalog of Stormwater Best Management Practices for Idaho Cities and Counties is a guidance document that individual jurisdictions can formally adopt to set their own standards if they choose to. That said, communities covered under NPDES stormwater regulations still have to develop and implement programs that control pollutants to the maximum extent practicable, so a city or county’s actual binding requirements come from whichever local ordinance adopts these BMPs, not from the catalog itself.
Why does Idaho's guidance focus so heavily on phosphorus and sediment?
Because many of the state’s water bodies don’t meet water quality standards, and those two pollutants are identified as especially common contributors to that impairment. Beneficial uses like domestic water supply, fishing, swimming and agricultural water access can all be compromised when stormwater runoff carries excess phosphorus and sediment into a receiving water, which is part of why the catalog gives special consideration to controlling them.
What's the biggest source of polluted runoff on a typical Idaho development site?
Impervious surfaces that connect directly to the storm drain system, and streets in particular. Roads and other transportation-related structures typically account for 60% to 70% of a site’s total impervious area, and unlike rooftops, streets are almost always directly tied into the storm system. That level of direct connection is a documented factor in degraded stream health and aquatic habitat, which is why the guidance singles out roadway and parking design as a priority area for BMPs.
What are some practical ways to reduce runoff on an Idaho property?
The catalog points to several approaches: disconnecting impervious surfaces so they don’t drain straight into the storm system, using porous pavement where a stabilized surface is still needed, minimizing disturbance on steep slopes, and reducing the overall footprint of roadways and sidewalks. Site design techniques like cluster development and open space design, which concentrate building on part of a site while preserving the rest, are also recommended as alternatives to conventional subdivision layouts.
Can rainwater capture reduce a property's stormwater footprint in Idaho?
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.
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