Protecting cold water streams in urbanizing watersheds

Excerpt

In 2001, among the rolling green hills of south-central Wisconsin, Dane County passed an Erosion Control and Stormwater Management Ordinance. It requires temperature reduction practices for most development projects that drain to a cold-water stream.

The purpose of the ordinance stems from the fact that the thermal impact of urbanization can be detrimental to aquatic life in cold-water streams. Streams in urbanized watersheds generally tend to show increased diurnal and seasonal stream temperature fluctuations compared to streams in less developed areas (Pluhowski, 1970; Klein, 1979). Heated runoff from impervious surfaces has some responsibility for this effect. Other factors associated with urbanization that contribute to increased stream temperature fluctuations include the removal of riparian vegetation, the widening of stream channels, and the reduction in groundwater inflow to streams.

Impervious surfaces, such as concrete, typically absorb and store solar radiation better than natural surfaces, so they usually are warmer, especially during the summer months. Summer afternoon thunderstorms can cause a direct thermal loading on urban streams because the impervious surfaces transfer some of their heat to precipitation. This heated water then moves quickly over the impervious surfaces and through drainageways to nearby streams, giving it little chance to cool …