372
Geomorphic Patterns, Processes, and Perspectives in Aquatic Assessment
transpiration, percolation, and runoff. In assessments we need to understand how various biophysical factors and land use affect the processes
and functions in the hydrologic cycle, all within a
geographic spatial context.
Groundwater, an often ignored factor in aquatic
assessments, deserves special attention because it
is linked to surface-water conditions and has important geomorphic and biological functions. The
increasing recognition of groundwater influence on
the stream corridor, and particularly on aquatic
ecology through the hyporheic zone, emphasizes
the need to include the hydrological connection of
the valley and floodplain in aquatic and riparian assessment and management (Stanford and Ward,
1993). Groundwater influences fish distribution in
that fish may key in on geomorphic areas with beneficial groundwater influence (Baxter et al., 2001),
such as local temperature refugia or areas with intragravel flow for egg incubation (Keller and Kondolf, 1990). The linkage between groundwater and
watershed is also shown in patterns of upland vegetation types closely tied to subsurface hydrology
(Winkler and Rothwell, 1983), as well as in differences in riparian composition among valley
types (Harris, 1988). In both management and assessments, we should recognize that groundwater
availability is a key element for riparian vigor and
channel erosion (Kondolf et al., 1987; Beschta,
1991). Fluvial surfaces such as floodplains function as a temporary source of water storage from
high flows (bank storage). Differences in floodplain sediment composition affect bank storage
through water infiltration, storage, and release, with
variable effects on channel flow (Whiting and
Pomerants, 1997). The classifications and procedures that we use in assessments should explicitly
incorporate these various geomorphic parameters
that influence groundwater conditions (Coates,
1990a; Maxwell et aI., 1995).
The complex interaction of biophysical factors
controls the variable expression of both erosional
and hydrologic processes; such factors include climate, bedrock or surficial lithology and soil characteristics, and basin, hillslope, and channel morphology. We can order and analyze the complex
interplay of these factors in a geographic context
to characterize and predict hydrologic relations at
different scales. Base flows are influenced by the
annual precipitation regime, temporary storage in
soils and floodplains, and groundwater. Larger
flood flows are controlled by intensity and duration of precipitation, whereas basin geology and
morphology have greater influence on withinregion differences in smaller peak flows and low
flows. Hillslope delivery processes are important
in flow response and peak flows in small watersheds; in larger watersheds, routing by the stream
network controls peak flow expression. Water is
delivered to the stream as either surface flow (overland flow or saturated overland flow), subsurface
flow (throughflow), or groundwater, with losses
from evapotranspiration and to groundwater, and is
temporarily stored in soils and vegetation. Figure
25.2 depicts environmental variation in these flow
delivery processes, which are regionally controlled
by precipitation regime and vegetation density. In
general, the relative amount of surface flow is low
in forested watersheds and greater in semiarid and
arid areas (Figure 25.2). Degree of overland flow
depends on physical factors such as soil infiltration
capacity, antecedent soil moisture, rainfall intensity, and slope shape, gradient and length; it is moderated by interception by vegetation.
Vegetation is a critical factor in management and
assessments in that it influences erosion and the hydrological cycle through interception of precipitation and evapotranspiration, fog drip, interception
of surface flow, and stabilization of the soil surface. Loss of vegetation through fire or management actions can increase water yield, especially in
the growing season (Burton, 1997). Vegetation has
a strong effect on snow accumulation, as well as
timing and duration of snow melt; removal of vegetation can significantly alter timing and magnitude
of peak flows.
Antecedent conditions such as frozen soils and
soil moisture (variable source area) affect flow
variability by increasing surface flow and hence the
effect a precipitation event has on peak flow
(Dunne et aI., 1975). For example, rain on snow
events can generate a disproportionately large flow
response for a given amount of precipitation, which
may trigger slope movements and floods (Harr,
1981). Transitional rain-on-snow zones occur at
certain ranges of latitude and elevation and are an
important management and assessment consideration. Summaries are available of processes and
models of hydrology and hillslope erosion for the
Pacific Northwest (Swanston, 1991) and for regional evaluation and modeling of forest practices
on watershed hydrology and erosion processes in
humid temperate regions (EPA, 1980).
Drainage density can be used to illustrate the
complex interactions between biophysical setting
and water and erosional delivery processes (Figure
25.3). High drainage density is correlated with
faster delivery of water, higher peak flows and flow
variability, and higher sediment delivery to the
stream. Climatic parameters such as average pre-
Geomorphic Patterns, Processes, and Perspectives in Aquatic Assessment
transpiration, percolation, and runoff. In assessments we need to understand how various biophysical factors and land use affect the processes
and functions in the hydrologic cycle, all within a
geographic spatial context.
Groundwater, an often ignored factor in aquatic
assessments, deserves special attention because it
is linked to surface-water conditions and has important geomorphic and biological functions. The
increasing recognition of groundwater influence on
the stream corridor, and particularly on aquatic
ecology through the hyporheic zone, emphasizes
the need to include the hydrological connection of
the valley and floodplain in aquatic and riparian assessment and management (Stanford and Ward,
1993). Groundwater influences fish distribution in
that fish may key in on geomorphic areas with beneficial groundwater influence (Baxter et al., 2001),
such as local temperature refugia or areas with intragravel flow for egg incubation (Keller and Kondolf, 1990). The linkage between groundwater and
watershed is also shown in patterns of upland vegetation types closely tied to subsurface hydrology
(Winkler and Rothwell, 1983), as well as in differences in riparian composition among valley
types (Harris, 1988). In both management and assessments, we should recognize that groundwater
availability is a key element for riparian vigor and
channel erosion (Kondolf et al., 1987; Beschta,
1991). Fluvial surfaces such as floodplains function as a temporary source of water storage from
high flows (bank storage). Differences in floodplain sediment composition affect bank storage
through water infiltration, storage, and release, with
variable effects on channel flow (Whiting and
Pomerants, 1997). The classifications and procedures that we use in assessments should explicitly
incorporate these various geomorphic parameters
that influence groundwater conditions (Coates,
1990a; Maxwell et aI., 1995).
The complex interaction of biophysical factors
controls the variable expression of both erosional
and hydrologic processes; such factors include climate, bedrock or surficial lithology and soil characteristics, and basin, hillslope, and channel morphology. We can order and analyze the complex
interplay of these factors in a geographic context
to characterize and predict hydrologic relations at
different scales. Base flows are influenced by the
annual precipitation regime, temporary storage in
soils and floodplains, and groundwater. Larger
flood flows are controlled by intensity and duration of precipitation, whereas basin geology and
morphology have greater influence on withinregion differences in smaller peak flows and low
flows. Hillslope delivery processes are important
in flow response and peak flows in small watersheds; in larger watersheds, routing by the stream
network controls peak flow expression. Water is
delivered to the stream as either surface flow (overland flow or saturated overland flow), subsurface
flow (throughflow), or groundwater, with losses
from evapotranspiration and to groundwater, and is
temporarily stored in soils and vegetation. Figure
25.2 depicts environmental variation in these flow
delivery processes, which are regionally controlled
by precipitation regime and vegetation density. In
general, the relative amount of surface flow is low
in forested watersheds and greater in semiarid and
arid areas (Figure 25.2). Degree of overland flow
depends on physical factors such as soil infiltration
capacity, antecedent soil moisture, rainfall intensity, and slope shape, gradient and length; it is moderated by interception by vegetation.
Vegetation is a critical factor in management and
assessments in that it influences erosion and the hydrological cycle through interception of precipitation and evapotranspiration, fog drip, interception
of surface flow, and stabilization of the soil surface. Loss of vegetation through fire or management actions can increase water yield, especially in
the growing season (Burton, 1997). Vegetation has
a strong effect on snow accumulation, as well as
timing and duration of snow melt; removal of vegetation can significantly alter timing and magnitude
of peak flows.
Antecedent conditions such as frozen soils and
soil moisture (variable source area) affect flow
variability by increasing surface flow and hence the
effect a precipitation event has on peak flow
(Dunne et aI., 1975). For example, rain on snow
events can generate a disproportionately large flow
response for a given amount of precipitation, which
may trigger slope movements and floods (Harr,
1981). Transitional rain-on-snow zones occur at
certain ranges of latitude and elevation and are an
important management and assessment consideration. Summaries are available of processes and
models of hydrology and hillslope erosion for the
Pacific Northwest (Swanston, 1991) and for regional evaluation and modeling of forest practices
on watershed hydrology and erosion processes in
humid temperate regions (EPA, 1980).
Drainage density can be used to illustrate the
complex interactions between biophysical setting
and water and erosional delivery processes (Figure
25.3). High drainage density is correlated with
faster delivery of water, higher peak flows and flow
variability, and higher sediment delivery to the
stream. Climatic parameters such as average pre-