Urban Waters: New Study Highlights the Impacts of Urban Development on Streams


Flint River, GA

Flint River, GA | Alan Cressler

As a child, I had the opportunity to grow up in an idyllic setting in a small southern town on the Flint River. Back then, fishing and swimming with my Grandfather in the Flint, or its nearby tributaries, consumed the summers, along with homemade desserts and ice cream.  I had no idea that people lived in places where streams weren’t fishable and swimmable, let alone buried and put into pipes. Oh, to be young and naïve.

As I grew older, heading off to college and then to Washington, D.C., I came to realize just how rare the opportunity to safely swim in the river in your backyard.

Currently, 80% of the U.S. population lives in urbanized areas. In other words, 80% of the population lives in an area where urban streams exist. It is quite likely this 80% cannot safely fish or swim in their backyard, even if these urban streams were visible, mainly due to pollution from urban development and the resulting stormwater runoff.

A multitude of pollutants including oil, gas, bacteria, trash, sediment, and nutrients all wash into urban streams with every rainfall event. The combined effect of surfaces impervious to rainwater, like roads and rooftops, along with piped stormwater drains leads to increased flashiness of streams and stream bank erosion.

The U.S. Geological Survey’s (USGS) National Water Quality Assessment just released a study on the impact of urban development on stream ecosystems across the U.S. The study is unique in that it accessed nine metropolitan areas with 30 watersheds in each area.

One of the main findings is that there is no safe level of urban development when it comes to stream life.  Essentially, even low levels of development impact stream life (fish, aquatic insects, and algae).  The biology living within streams are a great indicator of stream health.  Aquatic insects, in particular, are often used as biological monitors (like the canary in the coal mine).  Depending on the type of aquatic insects present within a stream, researchers can determine its health. 

When assessing stream health, researchers look for a wide diversity of aquatic insects as a good indication of streams with little or no pollution; however, if researchers only find one insect taxa present (low diversity) this signals poor, highly polluted streams. The report found aquatic life responded differently to urbanization depending on geographic location and whether the land use previous to urban development was forest or agriculture.  This result is especially important to city and water district managers, as they are charged with maintaining clean water for cities and its citizens and may provide insight for management strategies.

Because of the varying response of stream life to urban development, monitoring fish, aquatic insects, and algae will provide more evidence of stream impacts to land use change.  Sensitive organisms are always the first to be lost in streams, shifting to a more pollution tolerant community. 

For example, a forested, cool flowing headwater stream may support a thriving trout community. However, if development occurs along the streamside (riparian) zone, increased erosion can cause sediment to accumulate in the stream and clog the trout’s gills. As trees are removed, stream temperature also increases, and trout can no longer survive the high temperature.  The stream will no longer support a sensitive trout community, but shifts to support communities that can tolerate increased sediment and temperature.

While the finding that no level of development can protect biological communities is of concern, the report includes a note of optimism. Streams which are highly developed (near 80% surrounding urban development) did not become so damaged that fish, aquatic insects, and algae were no longer capable of bouncing back.  This potential ability to bounce back means efforts to improve stream health in urban areas can help restore stream ecosystems.

Today, as I live and work in a highly urbanized environment much different from my hometown, this finding highlights a bright spot in the future health and resiliency of our urban streams.