Step 2: River Classification

Step 2: River Classification

What does the model discuss?

Classification is the process of organising objects, based on their characteristics, into groups so that objects within individual groups are similar in many respects but distinctly different from the objects within other groups. That is, like is grouped with like. Classification has great value in environmental management and assessment because of three main features:

  1. it allows the complexity to be reduced sufficiently for generalizations to be made about how classification types differ from one another and how they are distinguished (i.e. understanding);
  2. it allows generalisations about what and how many classification types are present in a given area (i.e. inventory); and
  3. it provides the capacity to assess the response of individual classification types to current and future impacts (e.g. increased water use, pollution etc.), and identify which river types are most at risk (i.e. prediction) (Fig. 2.1).

These features foster the transfer of specialist knowledge between knowledge providers from different disciplines (e.g. scientists), from knowledge providers to policy makers (e.g. government), and from policy makers to those tasked with implementing policy (e.g. managers and on-ground practitioners) (Fig. 2.1). Thus, high level management objectives for individual classes within a classification may be specified as can the means of achieving those objectives. Furthermore, classifications help in determining how outcomes of management decisions can be monitored and assessed.

The utility of a classification scheme is largely determined by the balance between the need for simplicity to foster understanding and inventory and the need for representational accuracy required for prediction and assessment. The utility of classification schemes to policy making and management may be further enhanced when classifications are based on a combination of purely descriptive or function/process approaches. Thus, a classification can extend beyond just a catalogue of features and include information on why those features exist and how they might change in the future. That is, classifications can become theories about the basis of natural order and thus provide predictive capacity.

River classification within the ELOHA framework

River classification serves two important purposes in the ELOHA framework. First, by assigning rivers or river segments to a particular type, relationships between ecological metrics and flow alteration can developed based on data obtained from a limited set of similar rivers or river reaches within a river basin (i.e. the prediction component of Figure 2.1). The baseline or reference condition against which ecological responses to alteration are measured can thus be established by deriving relationships between ecological attributes along a gradient of hydrologic alteration. Hydrologic alteration due to proposed water resource development can then be compared with the natural range of variation expected for rivers within a particular flow regime class. Large changes might result in the system no longer remaining with the flow regime class to which it was attributed with associated major changes in ecology occurring as a result. The extent of ecological impact associated with small levels of hydrologic change could be predicted using flow ecology relationships developed for that flow regime type.

Second, combining the regional hydrologic modelling with a river typology facilitates efficient biological monitoring and research design making it possible to strategically place monitoring sites throughout a region to capture the range of ecological responses across a gradient of hydrologic alteration for different river types. This is particularly valuable in regions with sparse pre-existing biological data or where monitoring and research resources are limited. 

Hydrologic classification

There are two main approaches to hydrologic classification (deductive and inductive) (Fig. 2.2). Deductive classifications are often constructed when hydrological data are limited. They make use of environmental data assumed to be important in influencing stream flow and/or extend existing flow data to predict stream behaviour in ungauged catchments. The greatest benefit of deductive classifications is that they can be used to construct spatially explicit predictions of stream flow over large spatial domains (e.g. regions) able to be incorporated in Geographic Information Systems.

Inductive classifications, in contrast, are based on summary information (i.e. hydrologic metrics) derived from existing flow data. Extension of inductive classifications to ungauged catchments is possible if environmental data can be used to reliably predict hydrologic class membership. Olden et al. (2011[1]) provides a thorough summary of these approaches, the methods used to derive them and their application in river management.

In the ELOHA framework, river flow regime assumes a dominant role as an ecosystem driver. It is recommended that, within the ELOHA framework, hydrologic classification be based on baseline (i.e. natural) hydrographs. The metrics used should: 1) should fully describe the range of hydrologic variability across the six key facets of the flow regime; 2) be ecologically relevant; and 3) be amenable to management. There are many considerations in generating a hydrologic classification (e.g. Kennard et al.

(2009[2]) and Olden et al. (2011[1]) provide a thorough review of approaches, methods and applications of hydrologic classification.

Kennard et al. (2010[3]) provide a classification of Australian streamflow regimes based on 120 flow metrics for 830 gauges located across continental Australia. A total of 12 distinct flow regime classes were recognised, six of which occur in northern Australia. Access to the outcomes of this classification is described in Step 1 above. This hydrologic classification of Kennard et al. (2010[3]) is a very broad scale classification (i.e. all of Australia). The metrics provided allow the user to perform additional classifications if this is required. Additional classifications could be limited to particular regions or drainages, or to a particular set of low metrics (e.g. low flow conditions only) for example.

We recommend that an additional classification restricted to northern Australia only is not required [see Related Resources and Projects section) and that the resolution provided by the existing classification is adequate. None-the-less, user access to the flow metrics allows great flexibility to explore spatial patterns of flow regime variability in detail.

Geomorphic sub-classification

The ELOHA approach also recognises that environmental factors other than hydrology play an important role in freshwater ecology. Moreover, environmental factors such as topography and channel form (e.g. geomorphology) play an important role in determining how changes in hydrology translate into changes in the physical environment (e.g. the hydraulic environment) and hence how they impact on freshwater organisms. They provide the context for interpreting ecological responses to flow alteration.

As with hydrologic classification, there are different types of geomorphic classification. Some are static classifications that divide rivers up into a series of reaches that conform to a set of previously identified river types or styles. Erskine et al. (2005[4]) describe such a classification scheme for northern rivers. Similarly, Ward et al. (2010[5]) applied a classification scheme in which aquatic systems, not just river types, formed the basis of an assessment of conservation value of aquatic systems across northern Australia. Under this classification scheme, aquatic systems can be classified broadly into different types of large ‘organising entities’ designed to represent the variety of aquatic ecosystem types (e.g. rivers, lakes, wetlands). This classification built upon the recently developed Australian National Aquatic Ecosystem (ANAE) Classification Scheme (Auricht 2010[6]) (Table 2.1). A finer-scale classification of aquatic systems can also be undertaken to distinguish various ecologically-relevant types of rivers, lakes or wetlands. These are often referred to as ecotopes (the smallest ecologically‐distinct features in a landscape classification scheme. Note that the Australian National Aquatic Ecosystem (ANAE) classification scheme (Auricht, 2010[6])now refers to ecotopes as ‘habitats’. 


  1. Olden, J.D., Kennard, M.J. & Pusey, B.J. A framework for hydrologic classification with a review of methodologies and applications in ecohydrology. Ecohydrology (2011).doi: 10.1002/eco.251
  2. Kennard, M.J., Mackay, S.J., Pusey, B.J., Olden, J.D. & Marsh, N. Quantifying uncertainty in estimation of hydrologic metrics for ecohydrological studies. River Research and Applications 26, 137-156 (2010).
  3. Kennard, M.J., et al. Classification of natural flow regimes in Australia to support environmental flow management. Freshwater Biology 55, 171-193 (2010).
  4. Erskine, W.D., Saynor, M.J., Erskine, L., Evans, K.G. & Moliere, D.R. A preliminary typology of Australian tropical rivers and implications for fish community ecology. Marine and Freshwater Research 56, 253-267 (2005).
  5. Ward, D.P., Stein, J.L., Carew, R. & Kennard, M.J. Hydrosystem delineation, environmental attribution and classification. Indentifying high conservation value aquatic ecosystems in northern Australia pp. 19-47 (2010).
  6. Auricht, C.M. Towards an Australian National Aquatic Ecosystem classification: initial report on an attribute based classification scheme. (Canberra, 2010).


The relationship between classification, policy and management.

Figure 2.1. The relationship between classification, policy and management. Arrows represent the flow of information between the various levels. Double headed arrows indicate that the flow of information is bidirectional as the outcomes of management actions are communicated back up to policy makers.

Different approaches to hydrologic classification

Fig 2.2: Different approaches to hydrologic classification (Olden et al. 2011).

Table 2.1: Australian aquatic systems and their associated ecosystem types within the ANAE classification system.


Aquatic system Description and representative Ecotopes

1. Marine

Oceans, seas and embayments

2.Near-shore coastal

Rocky shores, sand and mud flats

3. Estuaries

Semi‐enclosed embayments receiving sea water and fresh water inputs, mangrove forests, saltmarshes, saltflats, intertidal flats.

4. Riverine

Rivers and streams (but not including the hyporheic zone)

5. Lacustrine

Lakes formed by previous river channel migration (i.e. oxbow lakes, lagoons and billabongs), volcanic activity, subsidence, damming by glacial moraines or mass wasting

6. Palustrine

Floodplains and vegetated wetlands such as marshes, bogs and swamps, and including small, shallow, permanent or intermittent water bodies.

7. Niveal

Alpine snow fed

8. Subterranean

Groundwater environments including the hyporheic zone and underground streams, lakes and water‐filled voids

9. Artificial

Dams and impoundments


Related projects & resources

Classification systems in general

The many ways in which classification can be used in river management, including environmental water management, are discussed in the following documents

- Ch 7 River classification and its application to environmental flow management (

- A framework for hydrologic classification with a review of methodologies and applications in ecohydrology - Olden et al (2011[1]) (

Hydrological classification

The Australian Ecohydrological Classification (Kennard et al 2010[2]) is a very broad scale classification (i.e. continental). Classifications undertaken at finer spatial scales have the potential to reveal fine scale differentiation in attributes (e.g. flow regime). Pusey et al. (2010[2]) assessed the need for a flow regime classification restricted only to northern Australia (. They found that a separate classification essentially mirrored the classification group structure of the continental classification albeit with further differentiation based largely on catchment size. Accordingly, they recommended that a separate classification was not needed. None-the-less, the material contained within Pusey et al. (2010[2]) provides useful information on the spatial variation in flow regime for the region.

Moliere et al. (2008[3]) produced a classification of northern flow regimes based on an analysis of flow data from the Fitzroy, Daly and Flinders Rivers as part of the Tropical Rivers Inventory and Assessment Project (TRIAP). It can be accessed at

Geomorphic classification

AURICL is a web-based river classification system produced by TRaCK that allows the user to classify stream reaches according to a range of geomorphological variables. Such classifications allow stream reaches that respond differently to changes in flow regime to be identified. This may for example, allow appropriate monitoring sites to be identified and established. This important tool is hosted by GeoSciences Australia and can be accessed at This tool allows river reaches to classified according to Terrain, Modelled Hydrology,Landuse, Geology, Climate, Socio- economic factors or a combination of variables within these factors.

Other geomorphic classifications also exist. For example, information concerning the tropical rivers classification, produced as part of the Tropical Rivers Inventory and Assessment Project, can be accessed at Information on estuaries produced during this project can be accessed at

Ward et al (2010[4]) performed an ecotope-level classification of aquatic systems in northern Australia by attributing each aquatic system with ecologically relevant data (lithology, climate and vegetation for example) and then statistically dividing each aquatic system type into a number of defined subclasses (ecotopes). Full description of this process is available in Kennard et al. (2011[1]



  1. Olden, J.D., Kennard, M.J. & Pusey, B.J. A framework for hydrologic classification with a review of methodologies and applications in ecohydrology. Ecohydrology (2011).doi: 10.1002/eco.251
  2. Kennard, M.J., et al. Classification of natural flow regimes in Australia to support environmental flow management. Freshwater Biology 55, 171-193 (2010).
  3. Moliere, D.R., Lowry, J.B.C. & Humphrey, C.L. Classifying the flow regime of data-limited streams of the wet-dry tropical region of Australia. Journal of Hydrology 367, 1-13 (2008).
  4. Ward, D.P., Stein, J.L., Carew, R. & Kennard, M.J. Hydrosystem delineation, environmental attribution and classification. Indentifying high conservation value aquatic ecosystems in northern Australia pp. 19-47 (2010).


Environmental water terminology

A top down environmental water assessment method used in Queensland in which ecological condition is assessed against known deviations from the natural or pre-development flow regime whilst also taking into account the impacts of water infrastructure on ecological condition.
Bottom–up methods
Reconstructing an altered flow regime by sequentially adding water needed for specific functions i.e. adding a flushing flow designed to move refine sediment or a maintenance flow designed to provide a minimum amount of wetted area.
Cultural flows
Water required to meet the cultural and spiritual needs of Indigenous people. Environmental flows A term that supplanted the term instream flows in recognition that water was needed for more than just the maintenance of habitat quality and quality. Water is needed to provide cues for biota to move and to reproduce, to provide areas for food production, for refuge from temperature extremes, for maintenance of channel form and substrate composition, to create and maintain new habitats such as floodplains and tributaries, and many other needs.
Environmental water
A term that supplants the term environmental flows in recognition that flowing water is not the only water critical to the maintenance of ecosystem function. Hyporheic water (water held under the stream bed) and groundwater are also critical compartments of environmental water and groundwater dependent aquatic habitats may never be connected to the riverine environment.
Holistic flow management
A conceptual framework first described in 1992 in which water needs are considered more broadly than just those relating to in-stream or in-channel needs e.g. estuaries and the near shore marine environment are dependent on freshwater inputs as are riparian forests and off-channel wetlands.
Hyporheic water
(water held under the stream bed) and groundwater are also critical compartments of environmental water and groundwater dependent aquatic habitats may never be connected to the riverine environment.
Instream flow incremental methodology: a computer driven means of assessing changes in in-channel habitat quantity and quality.
Instream flows
The original term for environmental flow management, principally concerned with the maintenance of habitat quantity and quality defined by depth, water velocity and substrate composition. Typically, instream flow investigations of were undertaken at small spatial scales – i.e. at the reach scale.
Top-down methods
Environmental water assessment methods in which occurs the simulated sequential removal of volumes of water until an impact of nominated severity occurs, thus defining the limit below which this aspect of the flow regime can be altered.

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