Ecological Limits of Hydrologic Alteration (ELOHA): Past, Present and Future

Environmental flows are the seasonally variable streamflows and water levels needed to maintain healthy freshwater and estuarine ecosystems while providing for hydropower generation, flood management, water withdrawals, and land-use changes.  Over the past decade, many states, provinces, and countries have adopted policies requiring the provision of environmental flows.  However, expensive, time-consuming methods for quantifying environmental flows one river at a time have delayed implementation of those policies.  Ecological Limits of Hydrologic Alteration (ELOHA) is a flexible scientific framework for assessing and managing environmental flows across large regions, when lack of time and resources preclude studying individual rivers.  Thus, ELOHA enables the scaling-up from site-by-site efforts to the state, provincial, or national policy realm.

When leading river scientists from 10 international organizations first proposed ELOHA in 2006, they challenged the water management community to apply their framework in a variety of contexts.  Five years later, several jurisdictions have stepped up to the challenge.  This presentation reflects on how the ELOHA concept is playing out in practice by comparing several case studies.

In addition to the obvious social, political, and physical differences, these case studies run the gamut of scientific approaches, as well.  In Colombia’s Magdalena-Cauca River basin, the extreme diversity of habitats complicates the classification of rivers into distinct types for analysis.  In the Susquehanna River basin, hypotheses and initial flow recommendations are based on literature and facilitated expert consensus; graphics and new tools have been developed to track changes, visualize data, determine relationships, and conduct scenario-tests; and flow recommendations are expressed as departures from natural flow duration curves.  In the Connecticut River basin, scientists are using hydrologic modeling to synthesize daily flows at ungaged sites and optimization modeling to achieve multiple objectives within a multi-dam system.  In the middle Potomac River basin, which lacks sophisticated hydrologic models, an analytical approach fills the gap.  In Colorado, flow-ecology relationships based on literature and expert peer review are incorporated into a user-friendly tool for stakeholders.  In Ohio, the highly modified condition of streams and other factors confounds the quantification of flow dependencies.  In Massachusetts, scientists have developed new modeling approaches for synthesizing daily streamflow hydrographs.

The identification of common successful attributes of these and other ELOHA case studies is paving the way toward full protection of environmental flows worldwide.