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<br />as "those physical , . . variables whICh define the precise
<br />location occupied by a fish. and which would or could
<br />change with small changes in fish's location," Microhabitat
<br />variables "refer to those physical variables which appear to
<br />.".' be'ilScd by the fish to select their location," Microhabitat
<br />has both structural and hydraulic characteristics. Several
<br />variables are common to most microhabitat models: water
<br />velocity; fish (nose) velocity; water depth; fish depth; sub-
<br />strate particle size, degree of embeddedness, and percent
<br />fines; overhead cover (e.g., undercut banks, root wads,
<br />overhanging vegetation); and instream cover (e.g.. velocity
<br />shelter downstream of submerged objects, depth, surface
<br />turbulence).
<br />Bovee (1982), in discussing the change in species compo-
<br />sition in a stream from headwaters to the mouth, noted that
<br />"numerous authors have reported the addition or replace-
<br />ment of species as a function of stream order, stream size,
<br />gradient, or other descriptions of longitudinal gradations of
<br />environmental conditions. .. the 'longitudinal succession' of
<br />s~ies as a function of variables such as mean depth, mean
<br />velocity, temperature, water quality, or other characteris-
<br />tics exhibiting gradational change. This perspective might
<br />:ogicatly be defined as a macrohabitat approach to riverine
<br />ecology. "
<br />
<br />Hydraulics
<br />
<br />The purpose of hydraulic simulation is to describe the
<br />velocity distribution and water surface elevation for speci-
<br />fied discharges in a river. The assumption is made that fish-
<br />ery and water resources can be better managed if the two
<br />are linked by common physical characteristics that are a
<br />function of streamflow alterations. The important physical
<br />habitat variables that result from hydraulic simulations are
<br />the velocities, depth, wetted perimeter, channel width and
<br />surface area. The interaction between these hydraulic varia-
<br />bles and the structural features of the channel determine the
<br />dynamics of the microhabitat over time and space.
<br />
<br />Hydrology
<br />
<br />Hydrology is defined as "the applied science concerned
<br />with the waters of the earth in all its states - their occur-
<br />rences, distribution, and circulation..." (American Society
<br />of Civil Engineers 1962). In the present work, the term is
<br />restricted to concerns related to the processes in rivers. The
<br />two processes of most concern are the variations in stream-
<br />flows and sedimentation. For convenience, we discuss
<br />sedimentation as a separate process. When we use the term
<br />hydrology, we refer to the time pattern of streamflows.
<br />These streamflows may be instantaneous, daily, monthly,
<br />annual average or annual "peak flows. Although 'we do not
<br />discuss them in detail, hydrologic changes can have a sig-
<br />nificant impact on the fluvial process within the stream.
<br />
<br />.,. Sedimwltation
<br />
<br />In North American terminology, sedimentation tends ro
<br />include the process of erosion, transport, and deposition of
<br />sediment. The use of the term, however, is not clear and
<br />
<br />unambiguous. In this paper. we refer to the movement and
<br />charact;ristics of the sediment within the stream channel.
<br />and the processes that may change the channel charactens-
<br />tics. including cross-sectional morphology.
<br />The characteristics of the aquatic community within a
<br />stream are strongly related to the yield of both sediment and
<br />water from a watershed (Fig. l) (Cairns 1968; Reiser et aI.
<br />1985). The effects is a product of both the geology and cli-
<br />mate of the area. For example, the aquatic community of
<br />a stream in an arid region of granitic materials is far differ-
<br />ent from that of a stream in a humid region with bedrock
<br />ledges. The stream channel can change as a result of natu-
<br />rally occurring flows and sediment yield and as a result of
<br />changes in the amount and pattern of flows induced by man.
<br />
<br />Concepts of Riverine Habitat Analysis
<br />
<br />Habitat Models
<br />
<br />All flow I habitat models have two common elements; a
<br />procedure for describing changes in model variables as a
<br />function of discharge; and a transformation of raw values
<br />for these variables (e.g., a dissolved oxygen rDO] concen-
<br />tration of 4 mg-L -I) into biological terms (e.g., trout die
<br />if the DO concentration falls below 4 mg-L -I).
<br />Nearly all riverine habitat models that do not distinguish
<br />between micro and macro features contain variables that
<br />describe the average physical environment. These include
<br />variables such as wetted surface area, wetted perimeter,
<br />mean depth, mean velocity, average condition or percent
<br />area with specified substrate and cover, and mean, maxi-
<br />mum or minimum temperature. A subset of these models
<br />includes terms that describe the average chemical environ-
<br />ment, incorporating variables such as 00, alkalinity, nitro-
<br />gen compounds, phosphate, pH, or other chemical
<br />
<br />
<br />Geology I Clima te
<br />
<br />FIG, 1. The aquatic community as an end product of water and
<br />sediment yield from the watershed.
<br />
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