New Study Examines the Effects of Development Intensity on Stream Health
The health of a waterway is entirely dependent on the status of its riparian zone, the area of land from which storm water flows. In urban environments, which are typically found along a waterway, that land is altered, and the flow of water to the stream or river is affected. These changes can have a devastating effect on the populations of aquatic life that reside there.
As development increases, impervious surfaces like roads, buildings, and parking lots cover more of the riparian zone. During a storm, the water is unable to penetrate the ground surface to recharge the aquifer. Instead it rapidly flows over the surface in the direction gravity takes it. Storm water drains aid in its precipitous movement to the nearby stream or river.
This massive influx of storm water into river systems can spell trouble for its native species. According to Tom Cuffney, biologist for the United States Geological Survey (USGS), when the area of impervious cover reaches ten percent, one can expect many types of pollution-sensitive aquatic insects to decline by up to one third. He says, "We learned that there is no 'safezone,' meaning that even minimal or early stages of development can negatively affect aquatic life in urban streams."
There are several reasons why excess storm water runoff is detrimental to riverine ecosystems. First of all, in urban environments, storm water is a major source of water pollution. Human activities result in the deposition of environmental contaminants onto the ground in the riparian zone. Fertilizer is used widespread on lawns, parks, and golf courses. While the fertilizer helps the growth of grass, its chemicals can be toxic when ingested by other organisms. Another human source is ice melt, which is strewn intentionally over impervious surfaces, often in liberal quantities. The rock salt and calcium chloride pellets can change the chemical composition of water systems, which can affect sensitive species. Another source that comes from our roadways is spilled gasoline and leaks of other automotive fluids. In a storm, these petroleum chemicals are whisked directly to the nearest stream. This all does not even include litter and other industrial pollutants that make their way into storm drains.
Secondly, stream/river systems are damaged by the amount of water itself. The rapid rise and fall in the water level can alter the temperature of the water and completely alter the natural flow regime. A steady flow regime is critical for the spawning, juvenile development, and migration of aquatic species. The increase in water flow can also lead to increased erosion of the stream/river banks and encourage the spread of invasive weeds.
The USGS studied the effects of urbanization of nine metropolitan areas. They looked at the effects on algae, aquatic insects, fish, habitat, and chemistry. After comparison, they found that not all the streams react the same way to urbanization. A lot of this depends on the surrounding land use prior to urbanization, whether it was forested or agricultural. Forested streams were degraded more than agricultural streams. The reason for this is that the streams in agricultural riparian zones were already degraded from fertilizer runoff. The differences also stem from a number of other factors such as population density, soils, hydrology, and climate.
The management of storm water has become a priority for many state and local officials like Tom Schueler, coordinator for the Chesapeake Stormwater Network. Schueler says of the USGS study, "The information has been useful in helping us to predict and manage the future impacts of urban development on streams and reinforces the importance of having green infrastructure to control storm water runoff and protect aquatic life."
There are several methods used for storm water management. Retention ponds can be built near impervious surfaces to capture the runoff and gradually let it evaporate or infiltrate the ground. Systems can be constructed that remove contaminants before they enter the surface water such as drain filters and skimmers. Wetlands, ponds, or swales can be built using existing drainage structures like pipes and concrete channels. Source control can be implemented to prevent the release of hazardous substances from being released in the first place. And of course, community education about pollution and improving water quality is important.
The USGS studies are helpful in meeting the challenge of urban storm water runoff because they represent an integrated approach. They take into account urbanization's effects on the physical, chemical, and biological characteristics of river/stream systems. This information is useful to develop strategies for stream protection in varying geographic locations, and can be used to evaluate the effectiveness of those strategies down the road.
For more information from USGS: http://water.usgs.gov/nawqa/
For more information from EPA: http://cfpub.epa.gov/npdes/home.cfm?program_id=6