Aquatic and terrestrial habitats are increasingly degraded
and lost as greater percentages of watershed areas are made impervious. The
following sections focus upon these impacts which include habitat degradation,
habitat loss, and habitat fragmentation.
Habitat degradation is the diminishment of habitat quality
and its ability to support biological communities. It stems from the
adverse effects of urban development, such as increases in impervious
surfaces within watersheds. Its adverse effects can be immediate or
cumulative.
Habitat loss is the outright destruction of habitat, such
as filling a wetland or channelizing a section of stream. Its impacts
upon biological communities are immediate and catastrophic.
Habitat fragmentation is the piecemeal disassembly of terrestrial
habitats into discontinuous, oftentimes isolated, patches as a consequence
of development. Its adverse effects are cumulative and not immediately
noticeable. Habitat fragmentation stems from habitat loss.
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Effects on aquatic organisms and streams
Aquatic and bay habitats are adversely impacted by the nonpoint pollutants
and the higher volumes of runoff issuing from urbanized lands. Observable
declines in the biological integrity of streams and the quality of stream
habitats occur when watershed imperviousness reaches 10% to 15%.
Source: NEMO after Schueler, 1994
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Observable indicators of stream health degradation
include:
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population shifts to aquatic organisms tolerant
of poor water quality and poor habitat,
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less riparian vegetation,
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reduced macroinvertebrate, fish, and amphibian
diversity,
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lower plant and amphibian density,
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increased sediment and stormwater runoff,
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fewer snags in channels to dissipate energy,
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channel instability.
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The biological degradation of streams is manifested more quickly
than physical degradation. There is a time lag, often of several years, between
both increased runoff and the sedimentation that results from urban development
and subsequent stream channel readjustment. Channel readjustments may include
bank or bed erosion and channel scour or deposition, depending upon climate
and other factors. In contrast, aquatic organisms respond quickly to poor
water quality, more frequent peak flows, higher flow velocities and loss of
resting areas, habitat loss, higher sediment loads, and the reduced availability
of food, all of which are associated with the urbanization of streams.
Stream bank erosion due to rapid stormwater runoff.
Source: Jonson, Baltimore County DEPRM.
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