The Impervious Cover Model

The Impervious Cover Model (ICM) is a foundational framework used in watershed management to predict how development impacts stream quality. The model establishes a direct relationship between the percentage of impervious cover in a subwatershed and the health of its streams. Impervious cover—surfaces like rooftops, roads, and parking lots that prevent rainfall from soaking into the ground—serves as a reliable and easily measurable indicator of the cumulative effects of urbanization.

As impervious cover increases, two primary changes occur in the hydrologic cycle. First, the volume of stormwater runoff increases significantly. Second, the speed at which this runoff reaches the stream is much faster. This altered hydrology, combined with the pollutants washed off impervious surfaces, triggers a cascade of negative impacts, including channel erosion, habitat loss, and degradation of water quality. The ICM provides a simple yet powerful tool for planners and engineers to classify streams, set realistic management goals, and guide land use decisions to protect or restore aquatic resources.

The Three Stream Categories

The Impervious Cover Model organizes streams into three broad categories based on the percentage of impervious cover within their contributing subwatershed. These categories—Sensitive, Impacted, and Non-Supporting—correlate with a predictable range of physical, chemical, and biological characteristics. While the thresholds are best viewed as transition zones rather than absolute lines, they represent points where the rate of degradation often accelerates.

The table below summarizes the three stream categories, their associated impervious cover ranges, and their expected characteristics.

The Research Behind the Thresholds

The thresholds defined in the Impervious Cover Model are not arbitrary; they are derived from decades of urban stream research conducted across North America. While individual studies show variability, the collective evidence consistently points to significant degradation beginning at relatively low levels of development. The urban stream research literature provides evidence across three major areas: channel stability, biological indicators, and water quality.

Channel Stability

One of the most immediate and visible impacts of increased impervious cover is the physical alteration of the stream channel. Higher and more frequent peak storm flows exert immense erosive force on stream banks and beds. Research has documented that urban stream channels often enlarge their cross-sectional area by a factor of two to five compared to their pre-development state. Studies in the Pacific Northwest found that channel stability and fish habitat quality began to decline rapidly after subwatershed impervious cover exceeded 10%. Further research has shown that a majority of the sediment delivered to urban streams often comes not from construction site runoff, but from the erosion of the stream channel itself, a process initiated by altered hydrology.

Biological Indicators

Aquatic organisms are excellent indicators of long-term stream health because they integrate the effects of pollution and habitat degradation over time. A large body of research demonstrates a strong negative relationship between impervious cover and the health of biological communities.

• Aquatic Insects: Numerous studies, particularly in the Mid-Atlantic region, have documented a sharp decline in the diversity and abundance of sensitive macroinvertebrates (such as mayflies, stoneflies, and caddisflies) once impervious cover surpasses 8-12%. Streams in urbanized areas consistently show lower insect diversity compared to forested or rural reference sites.
• Fish: Fish communities also show a predictable response to urbanization. As imperviousness rises above 10%, studies have noted a decline in spawning success for both resident and anadromous fish. Communities tend to shift away from sensitive, specialist species (like salmon and trout) toward more tolerant, generalist species (like creek chubs and sunfish).
• Habitat and Wetlands: The physical habitat structure required to support diverse life also degrades. Research in Washington state documented a decrease in large woody debris, a critical habitat component, in streams with impervious cover around 10%. Nearby urban wetlands also show impacts, with declines in plant and amphibian diversity linked to the dramatic water level fluctuations caused by flashy urban runoff.

Water Quality Findings

Impervious surfaces accumulate pollutants from vehicles, atmospheric deposition, and other urban sources. During storm events, these pollutants are washed directly into streams. The landmark Nationwide Urban Runoff Program (NURP) conducted by the U.S. EPA in the early 1980s established that annual pollutant loads for nutrients (phosphorus, nitrogen), metals, and organic compounds increased in direct proportion to watershed imperviousness. Other research has shown that stream temperatures rise as impervious cover increases, due to both heated runoff from pavement and the loss of shading from riparian vegetation. This thermal pollution can be lethal to cold-water species like trout.

How to Apply the ICM

The Impervious Cover Model is more than a descriptive tool; it is a practical framework for watershed planning, management, and regulation.

Measuring and Forecasting Impervious Cover

The first step in applying the ICM is to determine the current impervious cover for each subwatershed in a jurisdiction. This is typically accomplished using geographic information systems (GIS) to analyze aerial or satellite imagery. Once a baseline is established, planners can forecast future impervious cover based on zoning maps and development projections. This allows a community to understand the potential future condition of its streams if no changes are made to land use policy. Runoff from projected impervious areas can be estimated using tools like the Simple Method runoff calculator.

Setting Realistic Management Expectations

The model helps managers set achievable goals for streams based on their current or projected category.

• For Sensitive streams (0-10% impervious), the primary goal is protection. Management strategies should focus on preventing future degradation through land preservation, low-density zoning, and stringent site design standards.
• For Impacted streams (11-25% impervious), the goal is often mitigation and restoration. This may involve retrofitting existing development with stormwater controls, restoring riparian buffers, and implementing stream stabilization projects to prevent further degradation and, where possible, recover some ecological function.
• For Non-Supporting streams (>25% impervious), the most realistic goal is typically stabilization and pollutant load reduction. While restoring a diverse biological community is often infeasible, management can focus on reducing downstream impacts by stabilizing eroding channels and aggressively treating stormwater to remove pollutants.

This tiered approach is a core concept of the broader Watershed Treatment Model.

Informing Land Use Planning

Perhaps the most powerful application of the ICM is in proactive land use planning. By understanding the link between impervious cover and stream health, communities can create regulations designed to keep future development from pushing healthy watersheds past the critical 10% threshold. This is the foundation of watershed-based zoning, which adjusts allowable land use density based on the management category of the receiving stream. For example, a community might create a “sensitive stream overlay zone” that restricts impervious cover to less than 10%. Such regulations are often codified in model ordinances that also incorporate principles of Better Site Design to minimize imperviousness at the site level.

Limitations and Caveats

While the Impervious Cover Model is a robust and valuable tool, users must understand its assumptions and limitations to apply it correctly.

• It is a screening model. The ICM predicts the average or potential condition of a stream, not the precise state of a specific reach. Local factors, such as a wastewater discharge, a well-preserved riparian forest, or a legacy pollution source, can cause a stream to be better or worse than the model predicts. The thresholds are transition zones, not sharp cliffs.
• Scale is critical. The model is most accurate when applied at the subwatershed scale (roughly 0.2 to 10 square miles) for first- to third-order streams. In large river basins, other factors like dams, large point-source discharges, and agricultural runoff can overwhelm the signal from urban imperviousness.
• Regional variation exists. Much of the foundational research was conducted in the temperate, humid climates of the Mid-Atlantic and Pacific Northwest. The specific percentage thresholds may require local calibration in other ecoregions, such as arid, semi-arid, or mountainous areas with different natural hydrologic regimes.
• BMPs have a modest effect on thresholds. Research has shown that conventional stormwater best management practices (BMPs), such as detention ponds, have a limited ability to fully reverse the impacts of impervious cover. While essential for water quality treatment, they often fail to replicate natural hydrology well enough to restore a diverse biological community in streams with high impervious cover.
• Riparian buffers are important. An intact, forested riparian zone can mitigate some of the effects of urbanization and may modestly extend the impervious cover thresholds. Protecting and restoring stream buffers is a critical complementary strategy to managing watershed-wide imperviousness.
• Pervious surfaces are not all equal. The model implicitly assumes that pervious areas are ecologically functional. However, highly compacted turfgrass lawns that receive significant fertilizer and pesticide inputs do not function like a natural forest floor. Managing the quality of pervious areas is an important part of a comprehensive watershed strategy.

Frequently Asked Questions