Trends in Watershed Imperviousness in the Christina Basin, 1995-2002

Kevin J. Vonck and Gerald J. Kauffman
University of Delaware, Water Resources Agency

The University of Delaware, Water Resources Agency recently updated projections of watershed imperviousness to the 2002 timeline for the Christina Basin in Delaware and Pennsylvania.  Due to proximity to urban job centers in Wilmington and Philadelphia and a pastoral quality that makes the Christina Basin a pleasant place to live, more people are moving into the basin which drives the development of open land (Figure 1).  One of the prime indicators of development and the effect on watershed health is impervious cover — the amount of pavement and roof area in a watershed.

Figure 1.  Comparison of land uses in the Christina Basin 1995 and 2002

Numerous scientific studies indicate that watershed health suffers when the impervious cover exceeds a threshold of 10-20%.  Too much impervious cover in the wrong places can have the following negative impacts on watershed health:

• Hydrologic — decreased recharge and base flows and increased peak flows exacerbate drought and flooding.
• Chemical — increased presence of bacteria, nutrients, pathogens, and sediment limit the viability of drinking water and recreational activities.
• Physical — decreases in streambank stability, the amount of large woody debris, and channel roughness lower the quality of habitat available for biological species.
• Biological — species diversity declines, trophic structures (nutrient components present in the water) shift, and pollution-tolerant organisms are more prevalent.

Table 1 summarizes the increase in impervious cover in the Christina Basin since 1995.  In the Pennsylvania portion of the basin the impervious cover increased from 9% in 1995 to 11% in 2000.   In the Delaware portion the impervious cover increased from 26% in 1995 to 29% in 2002.  On a watershed scale, streams like the Brandywine, Red Clay, White Clay and Christina in Delaware all appreciably exceed the 20 % good health threshold.  The rural watersheds in Pennsylvania have healthier impervious cover indices, coming in under less than 11%.

Table 1.  Impervious cover in the Christina Basin

On a subwatershed basis, there are a half dozen streams in the Delaware portion of the Christina Basin that have impervious cover thresholds of less than 20 % (Table 2 and Figure 2).  These subwatersheds of reasonable good health are recommended for increased protection as water resource protection areas by the New Castle County Unified Development Code (which would set a maximum threshold of development of 20% impervious) and include:

Figure 2. Watershed Imperviousness in the Christina Basin 1995 and 2002

Figure 3. Change in Watershed Imperviousness 1995 and 2002

(Adapted from K. J. Vonck. Trends in Watershed Imperviousness. Presented at University of Delaware Water Policy Forum. The Historic Christina Basin: Delaware’s First Watershed. University of Delaware Clayton Hall. October 13, 2004)