MEASURING THE IMPACT OF URBANIZATION ON TWO WATERSHEDS USING HISTORIC AERIAL PHOTOGRAPHS: IMPLICATIONS FOR RESTORATION EFFORTS

A watershed's land use exerts a strong influence on the characteristics of its trunk channel. While more studies are examining stream restoration at the watershed scale, this is often done by comparing multiple watersheds to determine the effects of different watershed characteristics on a river. However, there are inherent problems of scale and complexity that limit the usefulness of comparative studies. One watershed variable, land use change, does act on time scales short enough to measure its effects directly using the aerial photograph record. High-resolution topographic surveys comparing current channel morphologies are not able to differentiate between the Little Lehigh Creek and Sacony Creek in east-central Pennsylvania, two rivers with different amount of urban land use within each watershed. However, from historic aerial photographs of Little Lehigh Creek a majority of the measured widths (67 of 85) were statistically wider in 1999 than in 1947. The measured widths from Sacony Creek are more evenly distributed among those that widened (18), narrowed (28), and those that were statistically unchanged (6) from 1946 to 1999. From 1946 to 1999 the only section of Sacony Creek with consistently wider channels in 1999 corresponds exactly with the only sizable urban area in the watershed. The 1999 land use in the Sacony Creek watershed resembles that of 1946, while the urban area within the Little Lehigh Creek watershed has more than tripled from 1947 to 1999. It is proposed that the increase in urban areas that subsequently increases peak discharges is the mechanism behind the widening that occurred in the Little Lehigh Creek. These wider channels can affect restoration efforts by impacting water quality, aquatic habitat, suspended sediment loads, and river aesthetics.