CADDIS Volume 2: Sources, Stressors & Responses
Biotic responses to urban flows
Hydrologic changes associated with urbanization can directly and indirectly affect stream biota in many ways. Effects may include:
Direct scour and dislodgement from benthic surfaces due to increased peak flows
Altered physical habitat
- changes in in-stream hydraulic conditions (e.g., water velocity, wetted channel area and duration)
- changes in channel geomorphology
Life cycle disruption due to changes in timing of flows
Other flow-associated alterations (e.g., increased sediment, nutrient and contaminant delivery, changes in food resources)
For example, Booth et al. (2004) examined how benthic index of biological integrity (B-IBI) scores were related to two flow metrics associated with urbanization:
- TQmean = the fraction of a year that mean daily discharge exceeds annual mean discharge
- T0.5 yr = the fraction of a multi-year period that a channel is exposed to flows greater than the 0.5-year flood
For both TQmean and T0.5 yr, low values indicate the prevalence high discharge peaks that both rise and dissipate sharply—that is, increased flashiness and flow variability.
Booth et al. (2004) found that:
- TQmean and T0.5 yr decreased as % total impervious area increased, indicating that urban streams experienced flashier hydrographs.
- B-IBI scores increased as TQmean and T0.5 yr increased, indicating that macroinvertebrate biotic condition was reduced in flashier streams (Fig 37a,b).
- Sites with ≥ 54% urban land cover fall below the main trendline, indicating that macroinvertebrate biotic condition was poorer than predicted by hydrologic conditions alone (Fig 37c).
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