ELOHA extends the use of limited ecological data by systematically translating information between multiple rivers within hydro-ecologically similar River Types.
River type classification extrapolates understanding of ecohydrologic conditions at sites that have been studied to similar sites that have not. The first reason to classify river types is to strengthen the statistical significance of flow-ecology relationships by combining available information from many rivers. The second reason is to extend those flow-ecology relationships to ungaged rivers of the same type in order to define their environmental flow needs. A third reason is to direct future monitoring efforts to improve the strength of initial flow-ecology relationships or to extrapolate site-specific monitoring results. Apse et al (2008) covers classification for environmental flow standards in some detail.
Poff et al (2010) envisioned river classification based initially on ecologically-relevant streamflow characteristics. They further envisioned subclassification based on other factors that influence how biota respond to hydrologic alteration. Non-hydrologic factors could include water quality (e.g., water temperature; Olden and Naiman 2010), geomorphology (e.g., channel form and materials), or biologic conditions (e.g., community composition or endemic species).
Classification of river types for ELOHA is distinguished from regional classification systems such ecoregions (Omernick, 1987) and hydrologic landscape regions (Wolock, 2003) in that ELOHA river types are not necessarily geographically contiguous, as illustrated below for river types in lower Michigan, USA (source: Paul Seelbach, Michigan Dept of Natural Resources). For example, headwater tributaries and a mainstem river in the same basin would likely be classified as different river types.
River type classification is further distinguished from river condition goal classification, which is part of the policy implementation process. River types are based on natural conditions, whereas river condition classes are based on current or future conditions.
… ELOHA river types are not necessarily geographically contiguous …
Recent practice has demonstrated that river type classification is not always needed for setting scientifically-defensible environmental flow standards. In Massachusetts, for example, a statewide regression relationship links relative abundance of fluvial fish to watershed characteristics (area, gradient, etc.), obviating the need to classify aquatic system types according to those characteristics. In the Connecticut River basin, the project team decided that small differences between rivers within the project area did not warrant their being subdivided by type. The Middle Potomac project team found that segregating rivers by type did not significantly strengthen their statistical relationships, and in fact could weaken them by reducing the number of data points per analysis. Classifying watersheds may help reduce variability, but classification also reduces sample size, which increases uncertainty.
… river type classification is not always needed for setting scientifically-defensible environmental flow standards.
Other researchers have found river type classification to be useful. In New Zealand, Snelder et al. (2011) report that flow-ecology relationships (represented by habitat availability) vary among major river types defined by morphology and flow regime. In Michigan, classifying rivers according to water temperature and catchment size protects the fish communities that are most sensitive to streamflow depletion.
Poff et al. (2010) envisioned rather sophisticated, time-intensive river classification systems being fully developed for ELOHA before being tested by flow-ecology analysis. For example, Reidy Liermann et al. (2011) used Bayesian-mixture modeling, a recursive partitioning algorithm, random forests, and a geomorphic classification to create a 14-tier hydrogeomorphic classification for Washington, in preparation for flow-ecology analysis.
In practice, river type classification for ELOHA tends to be iterative or to use pre-existing classes. Based on 9 case studies, Kendy et al (2012) strongly recommend developing a river classification system in concert with flow-ecology analysis, rather than a priori, and note that starting with a simple, existing classification system helps build trust and speed the overall process. Moreover, incorporating watershed and macrohabitat variables from the onset is useful for assigning river types to ungaged sites.
The iterative analytical approach is well-illustrated in the Middle Potomac River basin project. The first iteration, based on hydrologic analysis and habitat type, classified river reaches according to watershed size and karst geology. This informed the flow-ecology analyses, which in turn informed re-classification. In the end, biological and hydrologic metrics were normalized so that data from all sites could be combined, thereby maximizing the size of the datasets used to quantify flow-ecology relationships.
The Colorado project used a pre-existing classification. Rivers were classified by ecoregion, using a classification system that was already established. Literature review and flow-ecology analyses confirmed that this simple typology sufficiently captures eco-hydrologic variability of Colorado’s river systems, especially considering the very limited databases with which the analysts had to work.
Using an existing classification system not only saves time, but also may help link streamflow management to regulatory programs that already are in place. By adopting Aquatic Life Use classes from an existing water quality program, the Ohio project team deflected water users’ concerns that biological flow criteria would create another layer of regulation. Moreover, the Ohio researchers were able to use extensive biological databases associated with the existing water quality program to develop flow-ecology relationships. Two of the river types for the Susquehanna River basin also borrowed from a water quality regulatory program.
More River Classification Examples
Water temperature is also a key component of environmental flows and is strongly influenced by ground-and surface-water hydrology. In Michigan, USA, 11 river types have been delineated, based on hydrology, temperature, and catchment size (Brenden et al 2008; Michigan Groundwater Conservation Advisory Council 2007).
Geomorphology is an important mediator of biological response to flow alteration. For example, in a homogenous stream reach, extensive dewatering could cause a stressful habitat bottleneck that induces a threshold-type reduction in fish populations; but if the river has deep pools, then these refuges could make possible a more gradual and continuous (linear) ecological response. Therefore, it is useful to subgroup river types according to geomorphic setting. For example,
Omernick, J.M. 1987. Ecoregions of the conterminous United States, Annals of the Association of American Geographers 77: 334-340.
Thompson et al (2001) developed a procedure that evaluates and links instream habitat and geomorphology at four different scales, and applied the procedure to the Manning catchment in northern New South Wales, Australia.
Wolock, D. 2003, Hydrologic landscape regions of the United States, US Geological Survey Open File Report 03-145, geospatial data available at http://water.usgs.gov/lookup/getspatial?hlrus
More articles and reports on characterizing river types
Apse et al (2008)explain options for classifying rivers in Pennsylvania, USA, and apply USGS HIP to all Pennsylvania rivers.
Brenden et al (2008) describes Michigan's river classification system for environmental flow standards.
Elliot and Jacobson (2006) used a multiscale classification to identify a hierarchy of naturally occurring clusters of reach-scale geomorphic characteristics for segments of the Missouri National Recreational River in South Dakota and Nebraska, USA.
Henriksen et al (2006) is the user's manual for USGS HIP.
Hersh and Maidment (2007), Texas
Hoffman and Rancan (2007), New Jersey
Kennen et al (2007) applied USGS HIP in New Jersey, USA.
Kennen et al. (2009), Missouri
Kennard et al. (2010), Australia
McManamay et al (2011) developed a regional classification framework for southeastern USA rivers based on daily streamflow data.
Michigan Groundwater Conservation Advisory Council (2007)
classified rivers according to temperature, hydrology, and size.
Monk et al. (2011), Canada
Olden et al (2011) reviewed hydrologic classification methodologies and applications in ecohydrology.
Olden and Poff (2003) provide a framework for identifying ecologically relevant hydrologic indices that adequately characterize flow regimes in a non-redundant manner.
Pusey et al. (2009)
Sanborn and Bledsoe (2005) present a methodology for stratifying streamflow regimes of gauged locations, classifying the regimes of ungauged streams, and developing models for predicting a suite of ecologically pertinent streamflow metrics for streams in Colorado, Washington, and Oregon, USA.
Snelder et al (2005) classified New Zealand's rivers according to topography and climate. Data and instructions can be found here. A primary components analysis indicates that the resulting classes have distinct flow regimes.
