Effects of development configuration and subsurface connectivity on urban streams

Background/Question/Methods

Urbanization is a changing force, erasing and altering ecosystems across the globe. Streams in developed areas undergo a suite of fairly predictable changes, including incision, stormflow heat pulses, and more flashy hydrographs, described as the “urban stream syndrome.” These alterations are typically attributed to the coarse metric of percent of development in the watershed. This does not take into account the unique characteristics of development and the mechanisms through which they influence stream ecosystems. One attempt to address this problem has been to use surface-connected development as a more nuanced metric of watershed urbanization. However, these variables have failed to explain more variation than total development in the watershed. This is likely due to the lack of attention to subsurface infrastructure in calculating connectivity of development to streams and omission of explicit metrics of development patch size. Subsequently, we asked:

What is the relative importance of the spatial aggregation of development and the connectivity produced by stormwater sewer systems to urban stream ecosystems?

We have calculated these nuanced landscape metrics for 25 streams in the Piedmont Region of North Carolina to understand the variation in these characteristics across a landscape.

Results/Conclusions

Across 25 urban streams, maximum stormflow temperature change ranged from 0.1 to 4 degrees Celsius, with a mean of about 1 degree Celsius. The percent developed land cover in the watershed described 16% of this variation, while only 4% was described by the surface-connected development in the watershed. Stormwater pipe density in the watershed explained 26% of the variation, showing that subsurface connectivity better captures the mechanisms by which development influences stream ecosystems. We are currently obtaining data for additional watersheds to further explore these relationships. These findings emphasize the importance of using development configuration and subsurface connectivity to better understand how urban watersheds function and to provide guidance to watershed managers about ameliorating the effects of development on streams.