123
mandated good chemical (water quality) status and good ecological status for all
surface waters by 2015. Metrics of water quality have existed for decades, but good
ecological status is more nebulous. The directive defined ecological status in terms
of the quality of the biological community and the hydrological and chemical characteristics of the river. Quality of the biological community was tied to the level of
departure from the biological community that would be expected in conditions of
minimal anthropogenic impact. Because rivers throughout Europe have been intensively manipulated—morphologically, hydrologically, chemically, and biologically—for centuries, true reference conditions are nearly impossible to find for
many types of river ecosystems in Europe. Consequently, the Water Framework
Directive initiated a surge of river research aimed at developing physical, biological,
and integrative metrics of river health that could be used to assess departure from
natural or reference conditions in diverse river ecosystems.
As noted in the first chapter, river health is an intuitively appealing concept that
is easy to communicate at a general level to non-scientists (Karr 1999). Matters
become more complicated when scientists attempt to quantify river health (Boulton
1999; Fairweather 1999; Harris and Silveira 1999). Much of the associated debate
predates the EU Water Framework Directive and is in the biological literature
because of the derivation of ideas of river health from ideas of ecosystem health
(Norris and Thoms 1999).
Biologists commonly include physical and chemical characteristics of rivers
when considering river health, as illustrated in the definition of river health as the
degree to which a river’s energy source, water quality, and flow regime, as well as
the river’s biota and habitats, match the natural conditions at all scales (Karr 1991;
Harris and Silveira 1999). Many of the metrics of river health nonetheless focus on
biological characteristics, partly because the metrics were developed by biologists
and partly because the assumption is that the biota reflect or integrate physical and
chemical characteristics of the river ecosystem. Examples include multimetric indices such as the Index of Biotic Integrity, which employs fish-community attributes
of species richness, abundance, community structure, and the health of individual
fish (Harris and Silveira 1999), and multivariate statistical methods used to discern
pattern in taxonomic composition of macroinvertebrates or other organisms (Karr
1999). Biological metrics commonly focus on some aspect of biodiversity, which is
typically defined in terms of number of species within a given ecosystem, but can be
quantified in diverse ways (Gaston and Spicer 2004). Most of the metrics of river
health described above focus on structural ecosystem components such as community composition (Estevez et al. 2017). Some scientists argue that river health is
more effectively measured by integrating measures of structure with functional
indicators such as nutrient retention or river metabolism (Bunn et al. 1999).
Cumulative metrics of river health can include measures of water quality (Bunn
et al. 1999), habitat (Maddock 1999; Norris and Thoms 1999), or flow regime
(Richter et al. 1996). Physical scientists, however, have been slower to develop metrics of physical river condition that can be used to assess departure from reference
conditions, although the Water Framework Directive spurred research on this topic
(Newson and Large 2006). Geomorphic conceptualizations of river health typically
4.5 Metrics of River Health
mandated good chemical (water quality) status and good ecological status for all
surface waters by 2015. Metrics of water quality have existed for decades, but good
ecological status is more nebulous. The directive defined ecological status in terms
of the quality of the biological community and the hydrological and chemical characteristics of the river. Quality of the biological community was tied to the level of
departure from the biological community that would be expected in conditions of
minimal anthropogenic impact. Because rivers throughout Europe have been intensively manipulated—morphologically, hydrologically, chemically, and biologically—for centuries, true reference conditions are nearly impossible to find for
many types of river ecosystems in Europe. Consequently, the Water Framework
Directive initiated a surge of river research aimed at developing physical, biological,
and integrative metrics of river health that could be used to assess departure from
natural or reference conditions in diverse river ecosystems.
As noted in the first chapter, river health is an intuitively appealing concept that
is easy to communicate at a general level to non-scientists (Karr 1999). Matters
become more complicated when scientists attempt to quantify river health (Boulton
1999; Fairweather 1999; Harris and Silveira 1999). Much of the associated debate
predates the EU Water Framework Directive and is in the biological literature
because of the derivation of ideas of river health from ideas of ecosystem health
(Norris and Thoms 1999).
Biologists commonly include physical and chemical characteristics of rivers
when considering river health, as illustrated in the definition of river health as the
degree to which a river’s energy source, water quality, and flow regime, as well as
the river’s biota and habitats, match the natural conditions at all scales (Karr 1991;
Harris and Silveira 1999). Many of the metrics of river health nonetheless focus on
biological characteristics, partly because the metrics were developed by biologists
and partly because the assumption is that the biota reflect or integrate physical and
chemical characteristics of the river ecosystem. Examples include multimetric indices such as the Index of Biotic Integrity, which employs fish-community attributes
of species richness, abundance, community structure, and the health of individual
fish (Harris and Silveira 1999), and multivariate statistical methods used to discern
pattern in taxonomic composition of macroinvertebrates or other organisms (Karr
1999). Biological metrics commonly focus on some aspect of biodiversity, which is
typically defined in terms of number of species within a given ecosystem, but can be
quantified in diverse ways (Gaston and Spicer 2004). Most of the metrics of river
health described above focus on structural ecosystem components such as community composition (Estevez et al. 2017). Some scientists argue that river health is
more effectively measured by integrating measures of structure with functional
indicators such as nutrient retention or river metabolism (Bunn et al. 1999).
Cumulative metrics of river health can include measures of water quality (Bunn
et al. 1999), habitat (Maddock 1999; Norris and Thoms 1999), or flow regime
(Richter et al. 1996). Physical scientists, however, have been slower to develop metrics of physical river condition that can be used to assess departure from reference
conditions, although the Water Framework Directive spurred research on this topic
(Newson and Large 2006). Geomorphic conceptualizations of river health typically
4.5 Metrics of River Health