STORMWATER HYDROLOGY AND QUALITY - Low Impact Development and Sustainable Stormwater Management - page 40

Figure 2-5 Reduced base flow has a major water quality impact, destroying the aquatic
environment.
measures of the diversity of fish species in rivers clearly shows that as urbaniza-
tion increases (using impervious cover as a metric), the fish community degrades.
When the upland runoff increases in volume with every rainfall runoff period,
it affects the aquatic habitat in several ways. The increased sediment eroded
from the upland surface and the additional load eroded from the stream channels
covers the stream bottom, smothering the habitat for the macroinvertebrate com-
munity that provides the food source for all of the higher organisms, including
finfish communities. In addition, the sequence of pools and riffles that support the
entire food chain in a natural stream is disrupted. Where the vegetation has been
cleared along the channel banks and contiguous floodplain by human activity,
usually land cultivation or development, the exposure to direct sunlight alters
the aquatic habitat and further degrades the ability of many species of organisms
to reproduce. The warming of surface runoff conveyed from upland impervious
surfaces (Figure 2-6) also adds to the warming of natural streams, and can alter
the habitat for sensitive fish species, such as brown trout, to the point that they
cannot survive.
Where stormwater discharges into a lake, the influx of phosphorus-laden sed-
iment can enrich and quickly change the trophic state of any impoundment or
estuary. An extensive body of literature documents how this beneficial fertilizer
on the land becomes a major pollutant in the water, where it does exactly the
same thing; it makes plants grow. Unfortunately, in a natural lake or human-
made impoundment, this nutrient excess leads to a plant excess, which in turn
decomposes within the reservoir late in the summer, depleting the oxygen con-
centration and resulting in significant fish kills. The decomposition process also
produces severe odors and floating mats of detached periphyton, rendering the
use of the lake for recreational purposes impractical. It is estimated that 93%