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United Nations Environment Programme
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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


C. Catchment units and river typology

Each stream type possesses a set of inherent and presumably predictable attributes (e.g., channel pattern, dimensions and profile, biogeochemical signature, resistance and response to change, and biotic productivity), which reflect the local climate, geology, landform, and degree of disturbance (ultimate factors). According to the WFD (2000) and Rapid Bioassessment Protocols (U.S. Environmental Protection Agency 1999), river typologies should be applicable within a given ecological region (or ecoregion). These typologies are based upon a combination of criteria that directly define the biotic structure of a stream:

  • Size, as defined by stream order, catchment area, distance from source, etc.
  • Energy, as a function of gradient, discharge rates, etc.
  • Geology, based upon geochemical categories: siliceous, calcareous, mixed, organic, etc.
  • Geographical location (latitude/longitude),
  • Altitude, as defined by the elevation of the source, the elevation of the reach being assessed, etc.
  • Hydrological regime, as quantified by the range and frequency of flow variations, etc.

European rivers are classified either on the basis of a set of obligatory descriptors specifying the geographic location by ecoregion (System A), or by using an equivalent system involving both obligatory factors and a set of optional factors (System B). A "reference network" is used to establish hydromorphological quality values that equate to the "highest status" for each river type.

Figure. 9.2.
The schematic, distributional pattern of studied sites, according to the DCCA ordination diagram, reflecting the significant role of landscape and/or reach scale factors: (1) DCCA ordination diagram for axes 1 and 2 with the main environmental variables and sampling stream reaches; and, (2) DCCA ordination diagram for axes 1 and 2 with groups of Trichoptera larvae (from Bis et al. 1992)

D. Reference conditions

The identification of reference conditions and sites - defined as the best example(s) of a given river type with a minimal degree of modification from an undisturbed (e.g., pristine) state - is an essential pre-requisite for assessing the hydromorphological quality of stream systems across the full spectrum of water bodies. The variation in stream types should be considered in the description of type-specific reference biocenoses. For this reason, an hydromorphological quality assessment is also critical to the interpretation of biological status.

There are two primary approaches for selecting or determining reference conditions at surveyed sites:

  • Selection of reference sites based on an a priori definition of reference site criteria: This approach is used when a sufficient number of pristine or unimpacted rivers and/or reaches exist (the level of impact must be minimal relative to freshwater ecosystems within the region). Reference sites are initially selected using expert local knowledge on candidate sites, mapping information, and existing data bases;
  • Determination of reference conditions a posteriori based on the best conditions found in a representative sample of freshwater ecosystems within a class: This approach is used when few reference sites exist or are not suitably defined. A number of systems within the class are surveyed, and the best conditions for each class are determined from within the entire sample of river ecosystems.

E. River reach unit

The concept of stream reach identifies one of the most critical levels within the landscape hierarchy with respect to river habitat variables (both ultimate and proximate controls). This level exhibits the influence of the major 'inputs' of water, sediment, nutrients, and organic matter to the stream. The basic set of defining features for the identification of a stream reach for use within a classification system are:

  • channel gradient, defined as by the change in water surface elevation over a given distance expressed as a percentage, directly related to both bed-material load and grain size, and inversely related to discharge (gradient classes are useful in grouping streams with a similar response to flow and sediment input);
  • channel confinement, defined as the ratio of the active channel (i.e., the bankfull width) to the river valley bottom or floodplain width2
  • bankfull width, defined as the width of the stream across the top of the defined banks at which point bank overflow begins, provides a measure of stream power, and often is used as a surrogate for bankfull discharge (Q, or the dominant channel-forming flow rate, which volume of flow transports the largest portion of the annual sediment load, including bedload, over the period of record).

The stream reach scale integrates (or smoothes out) the variability inherent at the finer scale, and provides a grouping level for the stream characteristics that can be used for the identifying changes within the stream reach over time or identifying similarities and differences between stream reaches. As a consequence, these local scale factors provide the organisational framework necessary to analyse and address the spatial variability inherent in aquatic habitats (Figure 9.3).


2 U-, M-, C-constrained

 

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