25
Geomorphic Patterns,
Processes, and Perspectives
in Aquatic Assessment
Michael Hurley and Mark E. Jensen
25.1 Introduction
The variability that characterizes fluvial-aquatic
systems has confounded both interpretation and development of conceptual approaches for these dynamic systems. Because aquatic systems are fluid,
they are much more responsive to change and thus
more dynamic than terrestrial systems. This inherent dynamism is compounded by the spatial variability of geoclimatic settings and stage of recovery from natural and cultural disturbances (Graf,
1982). However, it is this same dynamic quality or
variability, often in the context of disturbance, that
is becoming the key to understanding aquatic systems and the processes operating on them (Poff and
Ward, 1989; Reeves et aI., 1995). We can best order this variability in hierarchically structured classification frameworks that link aquatic patterns and
processes with the controlling and driving variables
of different geoclimatic settings (Frissell et aI.,
1986; Maxwell et aI., 1995).
The increasing focus on more holistic watershed
and ecosystem assessments illustrates the need to
integrate disciplines and synthesize concepts. For
example, we need to think of physical processes in
the context of how they function in the ecosystem.
In this chapter we present physical processes that
create fluvial landscape heterogeneity and influence aquatic ecosystem composition and dynamics, with a primary focus on stream systems. To
begin, we offer some key historical and contemporary conceptual views of processes and processbased investigations. The process categories that
we address include geologic, climatic, hydrologic,
erosional, and sediment transport. For each of
these categories, we discuss the importance of
processes in ecosystem function, various analytical procedures, and biophysical factors that influence aquatic processes and pattern. Finally, we consider the need for incorporating spatial and temporal variability in ecosystem assessments.
25.2 Geomorphic Perspectives
How we view processes determines the usefulness
as well as limitations of a process-based approach
to landscape analyses and ecosystem assessment.
In this respect, a brief history of key geomorphic
perspectives provides context for our application of
current approaches. The current emphasis in geomorphology and landscape analysis on surface
processes, disturbance, and life-history characteristics parallels the individualistic view of biotic communities. However, during the early 20th century,
geomorphic and ecological investigations focused
on historical interpretation, description, and classification. During this period, the geographic cycle
(Davis, 1899) and the Clementsian organismic concept of community development (Clements, 1936)
were closed-system models of progressive landscape and ecosystem change. In these views, a landscape progresses through continuous erosional
stages to a climax, erosional steady state of the
peneplain; likewise, species assemblages progress
through successional states to a self-perpetuating,
steady-state climax community. Despite deficiencies, these views were popular and persist in some
form today, because they simplify ecosystem-landscape change as linear progression.
Later in the century, geologists stressed a more
quantitative, dynamic approach to counter the qualitative cyclic perspective (Strahler, 1957). In an attempt to finally displace the cyclic theory, two
landmark papers emphasize a process-based opensystems approach (Chorley, 1962) and resurrection
367
Geomorphic Patterns,
Processes, and Perspectives
in Aquatic Assessment
Michael Hurley and Mark E. Jensen
25.1 Introduction
The variability that characterizes fluvial-aquatic
systems has confounded both interpretation and development of conceptual approaches for these dynamic systems. Because aquatic systems are fluid,
they are much more responsive to change and thus
more dynamic than terrestrial systems. This inherent dynamism is compounded by the spatial variability of geoclimatic settings and stage of recovery from natural and cultural disturbances (Graf,
1982). However, it is this same dynamic quality or
variability, often in the context of disturbance, that
is becoming the key to understanding aquatic systems and the processes operating on them (Poff and
Ward, 1989; Reeves et aI., 1995). We can best order this variability in hierarchically structured classification frameworks that link aquatic patterns and
processes with the controlling and driving variables
of different geoclimatic settings (Frissell et aI.,
1986; Maxwell et aI., 1995).
The increasing focus on more holistic watershed
and ecosystem assessments illustrates the need to
integrate disciplines and synthesize concepts. For
example, we need to think of physical processes in
the context of how they function in the ecosystem.
In this chapter we present physical processes that
create fluvial landscape heterogeneity and influence aquatic ecosystem composition and dynamics, with a primary focus on stream systems. To
begin, we offer some key historical and contemporary conceptual views of processes and processbased investigations. The process categories that
we address include geologic, climatic, hydrologic,
erosional, and sediment transport. For each of
these categories, we discuss the importance of
processes in ecosystem function, various analytical procedures, and biophysical factors that influence aquatic processes and pattern. Finally, we consider the need for incorporating spatial and temporal variability in ecosystem assessments.
25.2 Geomorphic Perspectives
How we view processes determines the usefulness
as well as limitations of a process-based approach
to landscape analyses and ecosystem assessment.
In this respect, a brief history of key geomorphic
perspectives provides context for our application of
current approaches. The current emphasis in geomorphology and landscape analysis on surface
processes, disturbance, and life-history characteristics parallels the individualistic view of biotic communities. However, during the early 20th century,
geomorphic and ecological investigations focused
on historical interpretation, description, and classification. During this period, the geographic cycle
(Davis, 1899) and the Clementsian organismic concept of community development (Clements, 1936)
were closed-system models of progressive landscape and ecosystem change. In these views, a landscape progresses through continuous erosional
stages to a climax, erosional steady state of the
peneplain; likewise, species assemblages progress
through successional states to a self-perpetuating,
steady-state climax community. Despite deficiencies, these views were popular and persist in some
form today, because they simplify ecosystem-landscape change as linear progression.
Later in the century, geologists stressed a more
quantitative, dynamic approach to counter the qualitative cyclic perspective (Strahler, 1957). In an attempt to finally displace the cyclic theory, two
landmark papers emphasize a process-based opensystems approach (Chorley, 1962) and resurrection
367