Numerous studies in different regions of the country have found that there is a relationship between the amount of urbanization in watersheds and the quality of the water and aquatic habitat in streams, lakes, estuaries, and aquifers. This relationship is largely the result of the conversion of farmland, forests, wetlands, and meadows to relatively impervious surfaces such as rooftops, roads, driveways, parking lots, and sidewalks. Where impervious surfaces have replaced undisturbed vegetation and soil, the rainfall and snowmelt flows across the surface as runoff rather than seeping into the ground. Even lawns and playing fields, which have underlying soils that have been compacted by construction equipment, foot traffic, and yard maintenance equipment, are relatively impermeable compared to the forest floor. The increased amount of runoff results in larger flows in streams and rivers. Stream and river channels that are enlarged by the erosive effects of the larger flows experience a decline in water quality and habitat.

The Libby River Watershed is an urbanizing watershed and, as the amount of impervious area increases in the watershed, it is expected that water quality and habitat in the streams, river, and estuary will decline. Research conducted in other urbanizing watersheds indicates that there are thresholds for impervious area which, if exceeded, results in an observable decline in water quality and habitat. Therefore, in conjunction with the watershed survey, the Friends considered it appropriate to determine whether the amount of impervious area in the headwater stream subwatersheds and the overall Libby River watershed exceed these thresholds.

A model has been developed to estimate stream quality based on the percentage of impervious area in stream watersheds (Center for Watershed Protection, 1998). The model classifies streams into three categories; sensitive, impacted, and non-supporting. A description and the ranges of percent impervious area for each category are presented in the following table:

Table 2 - Stream Categories

The percent impervious area can be calculated by adding up the area covered by impervious surfaces, dividing the impervious area by the total watershed area, and multiplying the result by 100. Fortunately, there are accepted values for percent impervious area for various land uses, so it is only necessary to determine the areas occupied by each land use in a watershed and combine them to develop an overall estimate of the percent impervious area in a watershed (Center for Watershed Protection, 1998). The following table presents the percent impervious area for the land uses that are characteristic of the Libby River Watershed:

Percent impervious area was calculated for six subwatersheds within the Libby River watershed. Based on the topography and layout of the storm sewer systems in various neighborhoods, subwatershed boundaries for the headwater streams were approximated as shown in Figure 9. Using Geographic Information System (GIS) software, the area of every land use within a subwatershed was calculated and multiplied by the percent impervious area according to the information contained in Table 3. The products of the area of land use and percent impervious area were added together, divided by subwatershed area, and multiplied by 100 to obtain an overall percent impervious area for the subwatershed. The overall percent impervious area for each of the Libby River subwatersheds is presented in Table 4.

By comparing the results in Table 4 with the ranges given in Table 2, all but one of the headwater streams fall within the "sensitive" stream category. The stream in Subwatershed 2 falls within the "impacted" stream category. This result is consistent with field observations of this stream as it does show evidence of channel erosion and stream enlargement downstream of Mast Road. This stream has also been severely affected by channel straightening and ditching that occurred upstream of Mast Road when this area was developed.

Also of interest are the potential effects of proposed development in the subwatersheds of the Libby River. For instance, a 113-acre, 50 lot development along Fogg Road in Subwatershed 1 is in the planning stages. The conversion of forested land in this area into a residential community is estimated to bring the percent impervious surface for Subwatershed 1 to approximately 9 percent. Although the stream in this watershed would still be categorized as a "sensitive stream", it would be on the verge of becoming an "impacted" stream.

The percent impervious area was also calculated for the entire Libby River watershed. Including the six subwatersheds and all of the remaining developed and undeveloped land in the watershed, the estimated percent impervious area is approximately 6 percent. Although the relationship between percent impervious area and estuarine water quality and habitat has not been as well researched as it has for streams, research conducted to date indicates that the threshold at which water quality and habitat in estuaries noticeably declines is approximately 10 percent. This may be good news for the estuary but it is important to recognize that the ecological well-being of the estuary is directly linked to the condition of its headwater streams, and that preventing the deterioration of headwater streams is vital to preserving water quality and habitat in the estuary.