376
Geomorphic Patterns, Processes, and Perspectives in Aquatic Assessment
dIe, 1992). Fire strongly influences historic and
contemporary sediment and channel conditions
(Meyer et aI., 1995) and is an essential consideration in assessments. Fire intensity and extent determine the amount of vegetation removed, exposure of soil, and soil hydrophobisity (water
repellency), which in turn influence surface runoff
and erosion (Beschta, 1990; DeBano et aI., 1996).
We must also recognize how regional constraints
of precipitation regimes and physiographic setting
affect geographic variation in fire cycles and resultant erosional regimes (Swanson, 1981). Another consideration in forested watersheds is roadrelated sediment production and transfer, which
vary in different bedrock lithologies (Duncan and
Ward, 1985).
Both physiographic and compositional properties of watersheds influence spatial variability in
slope stability, erosional processes, and efficiency
and rates of sediment production and delivery
(Nolan and Marron, 1985). Of particular concern
in watershed management is the efficiency of sediment delivery from the watershed to the channel.
Watershed physiographic factors related to sediment delivery efficiency include watershed morphology and drainage density; local factors are
hills lope shape and morphology (such as slopelength), valley confinement, and hillslope and riparian vegetation characteristics. Higher-relief watersheds have greater sediment yields in part due
to greater mass wasting erosion, reduced storage of
sediment in floodplains, and greater sediment transport potential (Swanson et aI., 1987). Valley confinement is a good channel morphometric because
it reflects the relative impingement of the hillslope
on the channel and effective delivery of sediment
from hills lope processes.
Physical factors alone do not adequately account
for slope stability, as shown by variation in feedback initiation mechanisms among different bedrock types (Onda, 1992). Bedrock lithology is a primary determinant in slope stability and sediment
production in that it influences geotechtile properties, basin and hillslope morphology, and soil characteristics, but unfortunately these factors are not
incorporated in assessments in a consistent manner. The role of geology is exemplified in the Pacific Northwest, where spatial patterns, types, and
frequency and magnitude of mass movement
processes are related to geologic factors such as
hillslope relief, lithology, geologic contact areas,
and structural dip slopes (Swanston, 1978; Graham,
1985; Swanson and Leinkaemper, 1985; Poulin,
1991). Both watershed management and assessment need to consider spatial distribution of physiographic features that influence mass wasting,
such as bedrock hollows, because they significantly
affect frequency-magnitude relations of erosion
and sediment dynamics (Dunne, 1998). Also, geologic processes such as earthquakes can generate
episodic mass wasting and excessive sediment
loading to channels (Pearce and Watson, 1986),
which can be a significant regional consideration
in sediment dynamics.
Because of the dynamic nature and variability of
erosional processes and initiation mechanisms in
different geoclimatic settings, modeling and measuring erosion is problematic, and this complexity
is compounded in mountainous areas managed for
multiple resources. Furthermore, sediment modeling has not adequately addressed the current desire
to continue land-use activities in watersheds while
maintaining the natural frequency and magnitude of
erosional processes, especially in the context of ecological function. An assessment strategy that needs
further refinement is modeling and analyzing sediment at multiple scales (Arnett, 1979; Ahnert,
1987). Benda and Dunne (1997) address the dynamic nature of sediment supply with stochastic
models at different basin scales. These models emphasize probability occurrence of combinations of
fire, precipitation events, and mass wasting and
show promise for regional characterization of sediment dynamics. Another approach is hierarchical
physiographic land classification, which provides a
useful spatial context for the controlling factors that
influence geologic processes and land management
(Godfrey, 1978). This type of framework has particular promise for assessments, especially if coupled with GIS-based models for spatial analysis of
erosion. Other more traditional models used to assess land management impacts, slope stability, and
mass wasting erosional processes are outlined by
Megahan (1996) and in the WREN document (EPA,
1980). The universal soil loss equation, originally
developed for agricultural applications and surface
erosion, has been applied to forested areas (e.g.,
EPA, 1980), but is not effective for mass wasting
erosional processes. Sediment rating curves are useful for determining realized and expected sediment
delivery for individual basins and lithologies or for
regional comparisons (EPA, 1980).
25.5.2 Sediment Transport, Storage,
and Stream Habitat
Sediment transport is a key but weak link in ecosystem assessments. Although the mechanics of sediment transport are well studied, the application to
Geomorphic Patterns, Processes, and Perspectives in Aquatic Assessment
dIe, 1992). Fire strongly influences historic and
contemporary sediment and channel conditions
(Meyer et aI., 1995) and is an essential consideration in assessments. Fire intensity and extent determine the amount of vegetation removed, exposure of soil, and soil hydrophobisity (water
repellency), which in turn influence surface runoff
and erosion (Beschta, 1990; DeBano et aI., 1996).
We must also recognize how regional constraints
of precipitation regimes and physiographic setting
affect geographic variation in fire cycles and resultant erosional regimes (Swanson, 1981). Another consideration in forested watersheds is roadrelated sediment production and transfer, which
vary in different bedrock lithologies (Duncan and
Ward, 1985).
Both physiographic and compositional properties of watersheds influence spatial variability in
slope stability, erosional processes, and efficiency
and rates of sediment production and delivery
(Nolan and Marron, 1985). Of particular concern
in watershed management is the efficiency of sediment delivery from the watershed to the channel.
Watershed physiographic factors related to sediment delivery efficiency include watershed morphology and drainage density; local factors are
hills lope shape and morphology (such as slopelength), valley confinement, and hillslope and riparian vegetation characteristics. Higher-relief watersheds have greater sediment yields in part due
to greater mass wasting erosion, reduced storage of
sediment in floodplains, and greater sediment transport potential (Swanson et aI., 1987). Valley confinement is a good channel morphometric because
it reflects the relative impingement of the hillslope
on the channel and effective delivery of sediment
from hills lope processes.
Physical factors alone do not adequately account
for slope stability, as shown by variation in feedback initiation mechanisms among different bedrock types (Onda, 1992). Bedrock lithology is a primary determinant in slope stability and sediment
production in that it influences geotechtile properties, basin and hillslope morphology, and soil characteristics, but unfortunately these factors are not
incorporated in assessments in a consistent manner. The role of geology is exemplified in the Pacific Northwest, where spatial patterns, types, and
frequency and magnitude of mass movement
processes are related to geologic factors such as
hillslope relief, lithology, geologic contact areas,
and structural dip slopes (Swanston, 1978; Graham,
1985; Swanson and Leinkaemper, 1985; Poulin,
1991). Both watershed management and assessment need to consider spatial distribution of physiographic features that influence mass wasting,
such as bedrock hollows, because they significantly
affect frequency-magnitude relations of erosion
and sediment dynamics (Dunne, 1998). Also, geologic processes such as earthquakes can generate
episodic mass wasting and excessive sediment
loading to channels (Pearce and Watson, 1986),
which can be a significant regional consideration
in sediment dynamics.
Because of the dynamic nature and variability of
erosional processes and initiation mechanisms in
different geoclimatic settings, modeling and measuring erosion is problematic, and this complexity
is compounded in mountainous areas managed for
multiple resources. Furthermore, sediment modeling has not adequately addressed the current desire
to continue land-use activities in watersheds while
maintaining the natural frequency and magnitude of
erosional processes, especially in the context of ecological function. An assessment strategy that needs
further refinement is modeling and analyzing sediment at multiple scales (Arnett, 1979; Ahnert,
1987). Benda and Dunne (1997) address the dynamic nature of sediment supply with stochastic
models at different basin scales. These models emphasize probability occurrence of combinations of
fire, precipitation events, and mass wasting and
show promise for regional characterization of sediment dynamics. Another approach is hierarchical
physiographic land classification, which provides a
useful spatial context for the controlling factors that
influence geologic processes and land management
(Godfrey, 1978). This type of framework has particular promise for assessments, especially if coupled with GIS-based models for spatial analysis of
erosion. Other more traditional models used to assess land management impacts, slope stability, and
mass wasting erosional processes are outlined by
Megahan (1996) and in the WREN document (EPA,
1980). The universal soil loss equation, originally
developed for agricultural applications and surface
erosion, has been applied to forested areas (e.g.,
EPA, 1980), but is not effective for mass wasting
erosional processes. Sediment rating curves are useful for determining realized and expected sediment
delivery for individual basins and lithologies or for
regional comparisons (EPA, 1980).
25.5.2 Sediment Transport, Storage,
and Stream Habitat
Sediment transport is a key but weak link in ecosystem assessments. Although the mechanics of sediment transport are well studied, the application to