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CADDIS Volume 2: Sources, Stressors & Responses

energy sources physical habitat hydrology temperature water and sediment quality stormwater runoff wastewater inputs riparian and channel alteration urbanization

Baseflow in urban streams

Urbanization generally results in increased magnitude and frequency of peak flows, but baseflow effects typically are more variable, with studies showing a range of responses in urban streams [Lerner 2002, Brandes et al. 2005, Meyer 2005, Roy et al. 2005 (Fig 35), Poff et al. 2006].

Decreases in baseflow may result from:

  • ↓ infiltration due to ↑ impervious surfaces
  • ↑ water withdrawals (surface or ground)

These decreases may be offset, however, by increases in baseflow resulting from:

  • ↑ imported water supplies (i.e., interbasin transfers)
  • ↑ leakage from sewers and septic systems
  • ↑ leakage from water supply infrastructure
  • ↑ irrigation (e.g., lawn watering)
  • ↑ discharge of wastewater effluents
  • ↑ infiltration due to water collection in recharge areas
  • ↓ evapotranspiration due to ↓ vegetative cover

Urban-related increases in baseflow can be especially evident in effluent-dominated systems, or streams and rivers in which wastewater effluents comprise a significant portion of baseflow volumes. For example:

  • Discharge from two wastewater treatment plants accounted for at least 70% of river flow in the Bush River, SC in Summer 2002 (Andersen et al. 2004).
  • Average effluent flow in the South Platte River, CO is 41% total streamflow; during low flow conditions, this can increase to 90% (Woodling et al. 2006).

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Figure 35. Linear regression models for baseflow variables showing highest correlations with subcatchment imperviousness: (A) minimum daily stage/mean daily stage during late spring; (B) maximum duration of low stage < 25th percentile during autumn. Of the nine baseflow variables tested across five seasons, only these two variables showed relationships with r2 > 0.25, and only in (B) was this relationship significant.
From Roy AH et al. 2005. Investigating hydrologic alteration as a mechanism of fish assemblage shifts in urbanizing streams. Journal of the North American Benthological Society 24(3):656-678. Reprinted with permission.

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