Urban streams with green walls

Note: This post is a collaborative effort by Anne and guest blogger Will Dalen Rice, a graduate student in the Department of Geography and Earth Sciences at UNC Charlotte. He had the misfortune of taking a couple of courses from Anne this semester and has become a certified stream junkie, going out on rainy nights to see how high Charlotte’s urban streams are running.
Most cities were started around the idea of available surface water resources. Development and misuse of our streams (ex: “dilution is the solution to pollution”) has resulted in the modern urban stream. These streams are straight and good at carrying storm water, full of sediment and pollutants, and they lack good habitat for plants and animals. Now that we are beginning to notice how degraded and trashed these city waterways are though, scientists and engineers are beginning to attempt to address the form and function of these waterways to hopefully return them to a more “natural” (or at least aesthetically pleasing) state. While there are many stream restoration techniques, they often involve mechanical manipulation of the stream channel and banks and the planting of riparian plants along the stream corridor. As the streamside ecosystem redevelops, the idea is that health of the stream will also improve (leave it to nature to clean up our messes, given the chance).
For large urban streams, the standard practices in stream and habitat restoration are sometimes not possible, often because decades of infrastructure development have pinned the stream into a narrow corridor. So other approaches need to be considered, and Robert Francis and Simon Hoggart of King’s College London discuss ways that existing artificial structures can be put to work to mitigate some of the ecological impacts of urbanization. In the specific case of the River Thames in England, habitat development has been observed on man-made structures, and furthermore, certain types of man-made structures grow life better than others. Francis and Hoggart show that indeed plants (and therefore animals) can develop in a riparian zone better when brick and wood and rougher materials are used over concrete and steel. If concrete and steel already exist, adding brick and wood can further trap sediment for habitat growth (like gluing a cup of dirt to a steel wall). They suggest that this should become standard practice when thinking of restoration efforts in large, urban waterways.
The NOAA’s Northwest Fisheries Science Center says Thornton Creek in downtown Seattle exemplifies “the challenges facing rehabilitating urban streams.” But a look at the NOAA picture below shows that this stream is also emblematic of a riparian ecosystem that has developed within the constraints of the existing structures and maybe even a spontaneous model for the sort of restoration that Francis and Hoggart envision.

Francis, R., & Hoggart, S. (2008). Waste Not, Want Not: The Need to Utilize Existing Artificial Structures for Habitat Improvement Along Urban Rivers Restoration Ecology, 16 (3), 373-381 DOI: 10.1111/j.1526-100X.2008.00434.x