Empirical Flow-Ecology Relationships from Ohio

The Nature Conservancy (in Ohio) in conjunction with our contractor Midwest Biodiversity Institute developed an empirically derived approach for determining Great Lakes Compact Implementation low-flow regulatory permit thresholds for streams in Ohio's Lake Erie Basin.  During 2009-2010, researchers utilized elements of the recently published ELOHA (Ecological Limits of Hydrologic Alteration) framework to analyze more than 3,100, 1977-2010 fish population samples and September mean daily flows (including some regression derived flow values).  Flow alteration-sensitive fish species response curves were developed for stream segments classified by tiered combinations of catchment size and aquatic life use (coldwater, exceptional warm water, warmwater, and modified warmwater).  Proposed regulatory permit application thresholds for the tiered stream segment types were then developed based on differing loss rates of flow sensitive species to consumptive withdrawal quantities.

The methodology utilized in this project incorporates a recognition that fish assemblages and their resiliency are shaped by various levels of past environmental alteration (including factors beside flow alteration) while effectively determining a threshold relationship at which there is a high probability that flow alteration is the controlling variable for sensitive fish diversity.  Although the current application was developed to prevent future ecological degradation it should be possible to also determine appropriate hydrologic conditions for sensitive fish species restoration.  We also believe that this methodology is an example of how flow protections can eventually be incorporated into state water quality protection programs.