Flow-Ecology Relationships are the key link connecting the hydrologic, ecological, and social processes of ELOHA.
Flow-ecology relationships generalize the tradeoffs between flow alteration and ecological condition for different types of rivers. ELOHA synthesizes existing hydrologic and ecological databases from many rivers within a region to generate flow alteration-ecological response relationships (or flow-ecology relationships, for short) for different types of rivers. These relationships correlate measures of ecological condition, which can be difficult to manage directly, to streamflow conditions, which can be managed through water-use strategies and policies. Detailed site-specific hydrologic and biological information need not be obtained for each individual river. As the flow chart shows, flow-ecology relationships connect the hydrologic, ecological, and social processes of ELOHA.
Successful projects follow a progression from hypothesis development to data assembly and analysis to build these relationships, and involve stakeholders in selecting ecological indicators.
Starting with flow-ecology hypotheses ensures that these relationships are mechanistic and not simply empirical, and subsequent data compilation is systematic. In its role as technical lead on regional e-flows development for the Great Lakes basin in New York and the Ohio, Susquehanna and Delaware River basins in Pennsylvania, The Nature Conservancy systematically searched for, weighted, organized, and presented flow-ecology relationships cited in literature. This approach benefits from facilitated expert input, and culminates with facilitated expert recommendations for environmental flow criteria. Each recommendation is supported by flow-ecology hypotheses that can be tested if and when appropriate hydrologic and ecological data become available. In the meantime, well-documented scientific consensus provides sufficient precautionary criteria to move forward with policy implementation.
The next step is to assemble existing ecological and related data to quantify the relationships. Ecological data used to develop the flow-ecology relationships - for example, aquatic invertebrate species richness, riparian vegetation flow response guilds (Merritt et al 2010), or life-history traits of fish - ideally are sensitive to existing or proposed flow alterations, can be validated with monitoring data, and are valued by society. Poff et al (2009) list several examples. Finn et al (2011) emphasize the unique suite of metrics needed to capture Indigenous values and uses of river resources.
Several case studies used statistical analyses to isolate the influence of flow alteration from that of other environmental stressors, and then to identify the flow and ecological metrics that best describe ecological response to flow alteration. However, although strictly exploratory data analysis may result in robust statistical relationships, if the metrics used do not resonate with biologists and water managers, then the results may be ineffective in supporting environmental flow policy.
Because biota respond to many factors in addition to hydrology, streamflow may not be the sole explanatory variable (McManamay et al 2013) but, instead, acts as a limiting factor on biota. Limiting factors can be analyzed using quantile regression (Cade and Noon, 2003; Konrad et al 2008).
For some river types, ample ecological data are available. In most places, however, data are scarce.
In places with limited data, scientists are nonetheless advancing ELOHA through facilitated expert workshops, modeling and statistical analysis.
Even where large biological databases exist, rarely will quantitative flow-ecology analyses represent all flow-dependent taxonomic groups and ecological processes. A holistic approach requires a combination of quantitative and qualitative relationships to represent the ecosystems and natural flow regimes across a state or basin.
Structured ecological literature review, historical streamflow analysis, and facilitated expert workshops can build scientific consensus around qualitative relationships of sufficient rigor to quantify flow criteria.
All stakeholders need to understand the process and uncertainties involved in developing the flow alteration-ecological response relationships that will be used as the basis for implementing policies.
Useful articles and reports on flow-ecology relationships
Armanini et al (2010) developed an index for assessing ecological impacts of hydrologic alteration, based on the sensitivity of macroinvertebrates to river flow. Intended as a tool for improving river management and restoration efforts, the Canadian Ecological Flow Index (CEFI) is an easily calculated metric that can be applied in many places across Canada without requiring collection of new data.
Armstrong et al (2010) determined relations between fish-community characteristics and anthropogenic alteration, including flow alteration and impervious cover, relative to the effects of physical basin and land-cover (environmental) characteristics. Fish data were obtained for 756 fish-sampling sites from the Massachusetts Division of Fisheries and Wildlife fish-community database.
Armstrong et al (2011) assessed factors that influence riverine fish assemblages in Massachusetts, including land use and flow alteration.
Cooper and Merritt (2012) discuss wetland and riparian classification, characteristics and ecology; surface and groundwater hydrology; plant physiology and population and community ecology; and techniques for linking attributes of vegetation to patterns of surface and groundwater and soil moisture. Several case studies are also presented. This USDA-Forest Service report is intended to assist water managers in determining environmental flow needs.
Dewson et al (2007) found that leaf breakdown and primary production were not good indicators of flow alteration, but coarse particulate organic matter retention did correlate well with streamflow reduction in small New Zealand streams.
DePhilip and Moberg (2010) systematically compile qualitative flow-ecology relationships for all ecosystem components of all river types in the Pennsylvania's Susquehanna River basin. This approach has also been applied to Pennsylvania's Delaware and Ohio River basins and New York's Great Lakes basin, resulting in precautionary environmental flow criteria to protect inter-annual and intra-annual flow regimes throughout these large watersheds.
Freeman and Marcinek (2006) investigated fish assemblage responses to water withdrawals and water supply reservoirs in Piedmont streams in Georgia, USA. Although never intended for this purpose, the state of Connecticut used these research results as the basis for its streamflow standard.
North American Benthological Society (2009). Special Session on Developing Flow-Ecology Response Relations to Support Regional Streamflow Management
Haney et al (2008) used a collaborative expert workshop to develop flow-ecology hypotheses for a ground-water dependent desert river in Arizona, USA.
Kendy et al. (2012) explain how scientists in 5 states and river basins (Michigan, Massachusetts, Colorado, Ohio, and the middle Potomac River basin) quantified flow-ecology relationships for water withdrawal permitting and planning.
Kanno and Vokoun (2010) evaluated the ecological effects of water withdrawals and impoundments on fish assemblage structure using electric fishing data collected at 33 wadeable streams in Connecticut, USA.
Kennen and Ayers (2002) examined population data for 43 fish species, 170 invertebrates species, and 103 algae species in their analysis of urbanization effects on aquatic health in New Jersey, USA
Kennen et al (2007) used non-metric multidimensional scaling (NMS) to evaluate variation in aquatic-invertebrate assemblage structure and built a series of multiple linear regression (MLR) models that identify the most important environmental and hydrologic variables driving the differences in aquatic-invertebrate assemblages across a disturbance gradient.
Kennen et al (2010) related streamflow patterns to aquatic macroinvertebrate assemblages in 67 small-to-medium upland streams in the northeastern United States, and found negative affects of hydrologic alteration on biotic integrity. Mean April flow and duration of high flows were particularly indicative of assemblage variability.
Kennen and Riskin (2010) evaluated structural and functional responses of fish and aquatic invertebrate assemblages to increased water extraction from aquatic ecosystems of the New Jersey (USA) Pinelands, using basin size as a surrogate for water availability. Forty-three 100-meter-long stream reaches were sampled during high- and low-flow periods across a designed hydrologic gradient.
Konrad et al (2008) used a nonparametric screening procedure to identify different forms of streamflow-invertebrate associations for streams across the western United States. Selected ceiling and floors that represent conditional responses of invertebrates to streamflow were analyzed using quantile regression. "Although this approach cannot distinguish the effects of streamflow on invertebrates from those of confounding factors that are correlated with streamflow ... [it] is an assessment of the potential for a particular type streamflow pattern, such as frequency of high flows, to limit a characteristic of benthic invertebrate assemblages."
Leigh et al. (2012) explored potential effects of a flow-regulation scenario on macroinvertebrate assemblage composition and diversity in two river systems in Australia's relatively undeveloped wet-dry tropics.
Merritt and Bateman (2012) linked streamflow and groundwater hydrology to avian habitat in a desert riparian system in the Sonoran desert, USA.
Michigan Groundwater Conservation Advisory Council (2007) and Zorn et al (2012) explain how relationships between fish community assemblage and median August streamflow reduction were developed, vetted, used to set environmental flow standards, and incorporated into Michigan's Water Withdrawal Assessment Tool (WWAT).
Mims and Olden (2012) tested ecological theory predicting directional relationships between major dimensions of the flow regime and life history composition of fish assemblages in perennial free-flowing rivers throughout the continental United States. Their results provide empirical evidence illustrating the value of using life history theory to understand both the patterns and processes by which fish assemblage structure is shaped by adaptation to natural regimes of variability, predictability, and seasonality of critical flow events over broad biogeographic scales.
Peake et al (2011) catalog flooding requirements for a long list of flood-dependent plant species in Australia's Murray River floodplain.
Phelan et al. (2015) applied ELOHA to northeastern Montana wetlands, representing a new application for the framework and highlighting an approach to the task of establishing hydrology-ecology relationships that elucidate the connection between water regimes and the needs of healthy ecosystems. The primary goal of this work was to identify precautionary limits to hydrologic alteration at the level of hydrologic features and at the broad landscape scale.
Propst et al (2008) describe how the natural flooding regime in a desert river was a primary factor in shaping fish assemblages. Figure 6 indicates an expected ecological response to increased diversions from the Gila River basin, New Mexico, USA
Rehn (2008) examined benthic macroinvertebrates as indicators of biological condition below hydropower dams on west slope Sierra Nevada streams, California, USA. Figure 5 shows the relationship between IBI scores and constancy/predictability index and May low flows.
Roy et al (2008) investigated the relationship between hydrologic alteration and fish assemblage in urbanizing streams in the Etowah River catchment, Georgia, USA. Overall, hydrologic variables explained 22 to 66% of the variation in fish assemblage richness and abundance.
Stromberg et al (2007) examined the influence of high and low streamflow durations; flood frequency; depth, magnitude, and rate of ground-water decline; and other hydrologic conditions on phreatic vegetation along rivers in the arid southwestern United States.
Taylor et al (2008) analyzed changes to flow regime and fish assemblage after construction of the Tennessee-Tombigbee Waterway in northeastern Mississippi, USA, based on contemporary comparisons to historical fish collections and discharge data.
Van Sickel et al (2006) used statistical methods to estimate both flow and biological conditions (fish and invertebrate status) for second- to fourth-order streams throughout the Willamette River basin in Oregon, USA.
Webb et al (2010) demonstrate the use of Bayesian hierarchical models to detect ecological responses to flow variation in data-poor situations such as large-scale, disperse environmental flow monitoring programs.
Wilding and Poff (2008) mined diverse data from 149 sources, including journal articles, technical reports and theses, to quantify relationships between streamflow conditions and riparian vegetation, fish, and aquatic macroinvertebrates for the three types of streams that exist in Colorado, USA. Comparison of measured ecosystem parameters across a range of flow conditions (varying levels of modification) allowed patterns to emerge that provided a basis for quantifying ecosystem response. Where ecosystem complexities precluded a simple monotonic response to flow change, the best-available flow-ecology relationship was inferred as the ceiling of the scattered data, as defined by quantile regression (Cade and Noon 2003). The flow-ecology relationships are embedded in Colorado's new Watershed Flow Evaluation Tool (WFET)
Zorn et al (2012) document the modeling approach used to quantify relationships between fish community structure and alteration of median streamflow during the driest month of the year. These curves are now incorporated into Michigan's water withdrawal permitting program.
