Thermal pollution, or overheated water, is often associated with industrial discharges, but also includes the more pervasive problem of stream overheating due to inadequate stormwater management in developed areas. When rainfall generates surface runoff in the summertime, it becomes heated due to its flow over hot surfaces (think of summer thunderstorms falling on asphalt-paved parking lots.)
If such water flows into streams, they will become much hotter than they were prior to development. Other effects of inadequate stormwater management are excessive streambank erosion which causes loss of tree cover and allows the sun to heat streams, and the loss of groundwater recharge between rainstorms, reducing flow and magnifying the effects of solar heating. Higher water temperatures are directly harmful to most organisms, and also drive off dissolved oxygen, which remains deficient even when the water cools.
The Severn watershed contains a unique stream habitat that is threatened by thermal pollution. Jabez Branch , a major tributary of Severn Run near Interstate 97, contains a naturally reproducing population of native brook trout, and is classified as a Class III stream under the Clean Water Act. It is the only stream in Maryland's coastal plain to sustain this highly desirable game fish, although other streams in the Piedmont and west also do so. Brook trout are intolerant of water temperatures above 69oF and prefer water temperatures in the 50s, making them less tolerant of warm water than the commonly stocked rainbow or brown trout.
Although much of the Jabez Branch watershed remains forested, development is increasing and two major highway projects have cut through the watershed. In the 1980s road construction without proper stormwater controls led to thermal pollution and a near extirpation of the Jabez brook trout population. After construction of improved stormwater handling facilities along the highway, the Department of Natural Resources restocked the stream with brook trout in 1989, and established a program of monitoring water temperature and other stream conditions. The threat to the Jabez brook trout population is described in more detail elsewhere.
Response: The Severn River Commission has strongly advocated measures to prevent further damage to this unique freshwater habitat. This area of Anne Arundel County is being developed at an increasing rate, and more impervious surfaces will generate more heated runoff. The SRC has advocated a special environmental overlay zoning district with stringent stormwater controls to protect the Jabez watershed from thermal pollution, and the Odenton Small Area Planning Committee has recently adopted this concept.