358
Ecological Classification and Mapping of Aquatic Systems
that can be taken from aerial photographs include
sinuosity, channel-width variation, and channel
pattern; those readily obtained in the field include
maximum, mean, and residual depth of large pools;
linear frequency of pools greater than some threshold depth, and density and physical orientation of
coarse woody debris of a specified size range.
In recent years, the complex and diverse geomorphic and hydrologic fabric that shapes stream
ecosystems across the landscape has been increasingly appreciated by ecologists. Hynes (1975)
clearly anticipated and hailed a new focus on the
valley or catchment as a direct and indirect determinant of stream biota. The groundbreaking work
of Platts (1979) established that fish assemblages
sometimes appeared to be more closely tied to the
geomorphic class of the surrounding drainage
catchment than to measured channel or in-stream
habitat features, although in-stream physical conditions themselves are clearly at least partly correlated with landscapes (Nelson et aI., 1992).
Stemming in part from this recognition of the potential importance of broader-level ecosystem features in shaping habitats and biotic communities in
streams, hierarchical models of stream habitat organization and classification have emerged (Warren, 1979; Frissell et aI., 1986; Gregory et aI., 1991)
(Figure 24.1). These constructs place stream habitats explicitly in the context of their linkages with
the surrounding drainage catchment and terrestrial
landscape. Although this thinking has clearly
spawned important conceptual advances in stream
ecology and more recently in environmental assessment (Imhoff et aI., 1996; Jensen et aI., 1996,
1997), it remains unclear whether the approach will
eventually give rise to generic classifications in the
traditional sense of the word. Rather, hierarchical
approaches offer conceptual tools that allow more
systematic analysis, comparison, and ordination of
streams and stream habitats; they also provide an
explicit way of viewing stream conditions in relationship to the landscape in which they are embedded (Frissell et aI., 1986; Naiman et aI., 1992;
Imhoff et al., 1996; Jensen et aI., 1996, 1997).
The aquatic ECOMAP approach of the USDA
Forest Service (Maxwell et aI., 1995; see also
Jensen et aI., 1996, 1997, and previous discussion
under Section 24.2.2) exemplifies formal application of these hierarchical concepts, listing systematic criteria for defining aquatic mapping units for
inventory and ecological assessment. This approach identifies a nested set of land regions of
varying scale defining the geoclimatic and zoogeographic setting of watersheds (Figure 24.1) and
then decomposes stream networks within watersheds into valley segments, stream reaches, and
channel units, successively (similar to Frissell et
aI., 1986; see Figure 24.2). ECOMAP strives to develop the regional geographical context for local
aquatic habitat features, but brings relatively little
new clarity to the question of how specific habitat
features in streams should be described and assessed in a generic or categorical sense.
In a similar manner, ecoregions have been developed for the United States by the Environmental Protection Agency (Omernik, 1987) for broadlevel environmental monitoring and assessment
efforts. These mapping units have proved useful in
describing fish assemblage distributions across
much of the United States; however, their utility is
limited to broad-level assessments of large geographic areas (Lyons, 1989). Accordingly, prediction of fish species distribution at finer scales will
require more refined classification systems that account for local features not addressed at the regionallevel (Lyons, 1989).
The Nature Conservancy (1997) has recently offered a prototype classification of aquatic habitats
and biotic communities in the form of a dramatically truncated geographical hierarchy. Based on
the Conservancy's previous efforts to establish a
global plant community classification, this approach begins with extensive ecoregions defined at
the province and section scales, stepping down to
macrohabitat types corresponding with biotic alliances (akin to the valley segment level of Frissell
et aI., 1986, or sections of Gregory et al., 1991),
and then to habitat unit types corresponding with
biotic associations (equivalent to the riffle and pool
or channel unit scale). The intent is that after comprehensive surveys eventually a discrete set of
habitat and biotic community categories will be
identified at each of these levels. This system skips
the level of watersheds or drainage networks altogether; consequently, much information about local biogeographic controls, diversity, and natural
and impact history is not preserved. This no doubt
severely limits its potential utility for the vast majority of ecological assessments, which require
characterization and understanding of dynamic
linkages between drainage catchments and aquatic
habitats. Whether this scheme will be judged adequate as a tool for the general cataloguing and protection of biological diversity in streams remains
to be seen.
24.3.2 Lake Classification
In lakes, as for streams, the confounding influences
of geographical and biophysical covariates (hydro-
Ecological Classification and Mapping of Aquatic Systems
that can be taken from aerial photographs include
sinuosity, channel-width variation, and channel
pattern; those readily obtained in the field include
maximum, mean, and residual depth of large pools;
linear frequency of pools greater than some threshold depth, and density and physical orientation of
coarse woody debris of a specified size range.
In recent years, the complex and diverse geomorphic and hydrologic fabric that shapes stream
ecosystems across the landscape has been increasingly appreciated by ecologists. Hynes (1975)
clearly anticipated and hailed a new focus on the
valley or catchment as a direct and indirect determinant of stream biota. The groundbreaking work
of Platts (1979) established that fish assemblages
sometimes appeared to be more closely tied to the
geomorphic class of the surrounding drainage
catchment than to measured channel or in-stream
habitat features, although in-stream physical conditions themselves are clearly at least partly correlated with landscapes (Nelson et aI., 1992).
Stemming in part from this recognition of the potential importance of broader-level ecosystem features in shaping habitats and biotic communities in
streams, hierarchical models of stream habitat organization and classification have emerged (Warren, 1979; Frissell et aI., 1986; Gregory et aI., 1991)
(Figure 24.1). These constructs place stream habitats explicitly in the context of their linkages with
the surrounding drainage catchment and terrestrial
landscape. Although this thinking has clearly
spawned important conceptual advances in stream
ecology and more recently in environmental assessment (Imhoff et aI., 1996; Jensen et aI., 1996,
1997), it remains unclear whether the approach will
eventually give rise to generic classifications in the
traditional sense of the word. Rather, hierarchical
approaches offer conceptual tools that allow more
systematic analysis, comparison, and ordination of
streams and stream habitats; they also provide an
explicit way of viewing stream conditions in relationship to the landscape in which they are embedded (Frissell et aI., 1986; Naiman et aI., 1992;
Imhoff et al., 1996; Jensen et aI., 1996, 1997).
The aquatic ECOMAP approach of the USDA
Forest Service (Maxwell et aI., 1995; see also
Jensen et aI., 1996, 1997, and previous discussion
under Section 24.2.2) exemplifies formal application of these hierarchical concepts, listing systematic criteria for defining aquatic mapping units for
inventory and ecological assessment. This approach identifies a nested set of land regions of
varying scale defining the geoclimatic and zoogeographic setting of watersheds (Figure 24.1) and
then decomposes stream networks within watersheds into valley segments, stream reaches, and
channel units, successively (similar to Frissell et
aI., 1986; see Figure 24.2). ECOMAP strives to develop the regional geographical context for local
aquatic habitat features, but brings relatively little
new clarity to the question of how specific habitat
features in streams should be described and assessed in a generic or categorical sense.
In a similar manner, ecoregions have been developed for the United States by the Environmental Protection Agency (Omernik, 1987) for broadlevel environmental monitoring and assessment
efforts. These mapping units have proved useful in
describing fish assemblage distributions across
much of the United States; however, their utility is
limited to broad-level assessments of large geographic areas (Lyons, 1989). Accordingly, prediction of fish species distribution at finer scales will
require more refined classification systems that account for local features not addressed at the regionallevel (Lyons, 1989).
The Nature Conservancy (1997) has recently offered a prototype classification of aquatic habitats
and biotic communities in the form of a dramatically truncated geographical hierarchy. Based on
the Conservancy's previous efforts to establish a
global plant community classification, this approach begins with extensive ecoregions defined at
the province and section scales, stepping down to
macrohabitat types corresponding with biotic alliances (akin to the valley segment level of Frissell
et aI., 1986, or sections of Gregory et al., 1991),
and then to habitat unit types corresponding with
biotic associations (equivalent to the riffle and pool
or channel unit scale). The intent is that after comprehensive surveys eventually a discrete set of
habitat and biotic community categories will be
identified at each of these levels. This system skips
the level of watersheds or drainage networks altogether; consequently, much information about local biogeographic controls, diversity, and natural
and impact history is not preserved. This no doubt
severely limits its potential utility for the vast majority of ecological assessments, which require
characterization and understanding of dynamic
linkages between drainage catchments and aquatic
habitats. Whether this scheme will be judged adequate as a tool for the general cataloguing and protection of biological diversity in streams remains
to be seen.
24.3.2 Lake Classification
In lakes, as for streams, the confounding influences
of geographical and biophysical covariates (hydro-