Applications of the Ecological Limits of Hydrologic Alteration (eloha) in the United States

The Ecological Limits of Hydrologic Alteration (ELOHA) is a new framework for broadly assessing environmental flow needs when time or resource constraints preclude in-depth studies for all rivers in a region. The main steps of ELOHA are to: (1) build a hydrologic foundation, (2) characterize river types according to their flow regimes and geomorphic features, (3) compute present-day degrees of flow alteration, (4) define flow alteration-ecological response relationships, and (5) use flow alteration – ecological response relationships to manage environmental flows through an informed social process. Several projects within the United States are currently applying elements of ELOHA to accelerate the integration of environmental flows into regional water resource planning and management. By outlining some of these applications, this paper illustrates the use and flexibility of ELOHA. In Pennsylvania, daily streamflow data from 136 relatively unimpaired gauge sites were used to categorize streams throughout the 117,348-km state into 5 types, based on 71 hydrologic statistics. A pilot study of the 71,250-km Susquehanna basin in Pennsylvania identified linear relationships between aquatic invertebrate metrics and proportion of water withdrawn from 298 sites. In Tennessee, scientists identified functional relationships between insectivorous fish metrics and three hydrologic metrics -constancy, frequency of moderate flooding, and streamflow recession rate -based on data from 39 streamflow sites in the 106,200-km Tennessee River valley. In Michigan, scientists and stakeholders developed an online computer program, which allows prospective water users to determine whether their proposed ground-water or surface-water withdrawals would adversely impact river resources, defined as a percent of fish guild reduction. The tool links ground-water, surface-water, and fish-community models for 11,000 stream segments classified into 11 stream types in the 253,793-km state. In Arizona, researchers conceptualized flow alteration – ecological response relationships for the 8,000-km Verde River basin in a collaborative expert process. The relationships are helping water managers interpret separate ground-water modeling results in terms of the ecological impacts of proposed ground-water pumping scenarios. In Colorado, an expert team developed preliminary flow-ecology relationships for high-gradient, subalpine streams in the 770-km Fraser River basin, which supplies water to the City of Denver, to guide decisions regarding the timing of diversions from different locations. In Washington, researchers have completed a Bayesian classification of flow regime types, for which flow-ecology relationships are being established as a basis for setting environmental flow targets for rivers throughout the 176,477-km state.