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CADDIS Volume 2: Sources, Stressors & Responses

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Thresholds of imperviousness

Relationships between impervious cover and measures of stream condition, defined by either physical, chemical, or biological parameters, can take several forms (Fig 19). When the relationship is linear, any increase in imperviousness results in a decrease in condition (Fig 19, yellow and Fig 20); in other cases, there may be threshold values of impervious cover above which condition either decreases rapidly (Fig 19, green) or remains consistently low (Fig 19, blue and Fig 21).

Figure 19. Example relationships between stream condition and impervious cover: a linear decline in condition (yellow); an upper threshold switching to a lower threshold (green); a linear decline to a lower threshold (blue).
Modified from Walsh et al. (2005a).

Example thresholds or critical levels of imperviousness reported in the literature include:

  • Consistent channel instability when EIA > 10% [Booth & Jackson 1997]
  • Different geomorphic response patterns (e.g., in terms of depth diversity, maximum pool depth) across sites with < 13% vs. > 24% TIA [Cianfrani et al. 2006]
  • Consistently higher conductivity, dissolved organic carbon, and filterable reactive phosphorus when EIA > 5%, 4%, and 1%, respectively [Walsh et al. 2005b]
  • Uniformly low summer baseflow when TIA > 40% [Finkenbine et al. 2000]
  • Consistently high algal biomass when EIA > 5%, low diatom index value when EIA > 2% [Walsh et al. 2005b]
  • Sharp declines in macroinvertebrate diversity and richness when TIA between 8-12% [Stepenuck et al. 2002]
  • Invertebrate taxa sensitive to impervious cover lost when TIA between 2.5-15% in Piedmont streams and between 4-23% in Coastal Plain streams [Utz et al. 2009]
  • Brook trout absent when TIA > 4% [Stranko et al. 2008]
  • Occurrence probability of three sensitive fish species approaches zero when EIA between 2-4% [Wenger et al. 2008]
  • Sharp declines in fish IBI score and trout abundance when EIA between 6-11%, consistently low values when EIA > 11% [Wang et al. 2003]

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Figure 20. Relationship between total macroinvertebrate richness and % impervious surface cover in 29 headwater Maryland streams sampled in 2001. Taxa richness declined linearly with increasing impervious cover.
From Moore AA & Palmer MA. 2005. Invertebrate biodiversity in agricultural and urban headwater streams: implications for conservation and management. Ecological Applications 15(4):1169-1177. Reprinted with permission.
Figure 21. SIGNAL scores (a biotic index) for macroinvertebrates in edge habitats vs. (A) effective imperviousness (EI) and (B) total imperviousness (TI). Solid lines are piecewise regressions, dashed lines are linear regressions; the piecewise regression for EI provided the best fit. Note that the threshold value was 0.07 for EI, approximately half the threshold value for TI.
From Walsh CJ et al. 2005. Stream restoration in urban catchments through redesigning stormwater systems: looking to the catchment to save the stream. Journal of the North American Benthological Society 24(3):690-705. Reprinted with permission.

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