Relationship between stream temperature and ecosystem health modeled for effluent discharge studies

SCCWRP and its partners have successfully used modeling tools to estimate how changes in stream temperature can influence the ecological health of Southern California streams – the latest step in a pair of ongoing studies investigating how changes to the volume of treated wastewater effluent discharges into the Los Angeles and upper Santa Clara Rivers, respectively, affect stream health.

The modeling analyses, completed in December, involved developing a series of curves that explain the relationship between stream temperature changes and the biological condition of stream benthic communities, as measured by stream bioassessment index scoring tools. Little field-collected temperature data exist for Southern California stream sites, necessitating the use of a modeling approach.

The study is motivated by a new generation of wastewater discharge permits that have lowered the maximum temperature at which receiving water is required to be maintained from 86 to 80 degrees Fahrenheit; the permits also prohibit effluent discharges from triggering more than a 5-degree increase in water temperature.

The lower allowable temperatures are more difficult to reach because wastewater effluent is typically discharged into streams above the streams’ ambient temperature; streams are subject to seasonal heating associated with warm weather and low canopy cover (i.e., minimal shading).