25.5 Erosional and Sediment Processes
25.5 Erosional and
Sediment Processes
25.5.1 Erosional Processes
The challenge before us in ecosystem assessment
of erosion and sediment is twofold. First, we need
to connect erosional and sediment processes to
ecosystem functions and biota in some meaningful
way. Second, we need to decipher habitat conditions and management impacts in the context of the
complex natural historical-temporal and spatial
variability inherent in these processes. It is essential that we recognize that the long-term function
of the watershed is to transport sediment as the
landscape is eroded. Erosional processes and sediment production are a natural part of this function,
which can be altered or accelerated by changes in
climate, fire, or land use. Sediment delivery to the
channel has a duality in aquatic ecosystems in that
it is necessary to create and maintain habitat, yet
can negatively effect biota. Stream habitat is simply part of the sediment, water, and organic material delivered to and stored in streams (Lisle, 1983).
In this view, pools are temporarily stored water in
channel depressions that are scoured at high flows,
and riffles flow over sediment deposits. The concept of the sediment budget is important in watershed assessments, because it links stream habitat
composition and dynamics to watershed processes
of sediment delivery, transport, and storage (Swanson et aI., 1987; Swanston, 1991; Dunne, 1998).
Hillslope erosional processes transfer materials
such as boulders and large wood to the stream,
adding structure to the stream, and in some instances greatly improve fish habitat in structurally
deficient channels (Benda et aI., 1998). However,
in areas of intensive management, increased frequency or episodic mass wasting over large areas
and consequent channel adjustment can be detrimental to fish habitat and rearing (Poulin, 1991).
For effective assessments, we need to recognize
the types of processes involved in sediment production and transport and the triggering and
transport mechanisms for these processes. There
are essentially three sediment sources: hillslopes,
floodplain features, and in-stream storage. The watershed functions of erosion-sediment are sediment
production, delivery, storage, and transport. Sediment sources from hillslope processes include surface erosion and slope movement processes; the
relative types and contribution of these processes
vary with geoclimatic setting, location within a watershed, and watershed history or condition. Swanson et al. (1982) provide a good summary of sediment transfer processes applicable for steepland
375
humid temperate climates. Sources of in-stream
sediment include storage in bedforms and erosion
of fluvial surfaces. The floodplains of wide alluvial valleys can be a significant in-channel source
of sediment from accelerated lateral or vertical
channel migration (e.g., as bank erosion; Beschta,
1991). Most erosional processes involve forces of
water and gravity, which are countered by resistance by vegetated soil, soil cohesion, and regolith
geotechtile properties. Surface erosion is the dominant process in regions with overland flow, and
slope movement processes are more prevalent in
areas predominated by subsurface flow (Coates,
1990b). Surface erosion requires detachment, transport, and deposition of soil particles and is primarily expressed as rill, gully or sheet erosion and
secondarily as rainsplash and dry ravel. Soil mass
movement types are classified by the rate of delivery, size and type of materials, and type of delivery (Varnes, 1979). Triggering mechanisms for
mass movement include precipitation and soil properties that affect soil saturation, cohesion, and increased pore pressures (Coates, 1990b). We should
also consider that groundwater and subsurface flow
processes could be important agents of erosion and
landform development in a variety of settings
(Dunne, 1990).
Erosional processes, sediment delivery, and sensitivity to disturbance are controlled by the same
multiscaled biophysical factors that affect hydrology. Climate controls erosion processes through
degree of vegetation development and the precipitation and storm regime. Peltier (1975) has formalized coarse-scale controls on the effectiveness
of climate and vegetation on erosional processes
for regions of the United States. These regional climatic controls are reflected in sediment load and
drainage density, both of which are highest in semiarid climates (Langbein and Schumm, 1958). Geomorphic sensitivity to change in vegetation depends on the effectiveness of vegetation on
controlling geomorphic processes, which changes
with geoclimatic setting and factors such as slope
and soil cohesion (Swanson, 1981). Vegetation influences erosion by moderating precipitation impact, soil moisture, and water delivery processes
and by stabilizing the soil surface. Changes in landscape vegetation from fire, grazing, tree removal,
or climate change may lower threshold response
and cause a precipitation event to have a much
larger impact on the system than it would have had
under previous vegetation conditions. In forested
regions, for example, rooting strength is an important factor in slope stability on steep hillslopes, and
the change in timing and degree of slope instability depends on vegetation and site conditions (Si-
25.5 Erosional and
Sediment Processes
25.5.1 Erosional Processes
The challenge before us in ecosystem assessment
of erosion and sediment is twofold. First, we need
to connect erosional and sediment processes to
ecosystem functions and biota in some meaningful
way. Second, we need to decipher habitat conditions and management impacts in the context of the
complex natural historical-temporal and spatial
variability inherent in these processes. It is essential that we recognize that the long-term function
of the watershed is to transport sediment as the
landscape is eroded. Erosional processes and sediment production are a natural part of this function,
which can be altered or accelerated by changes in
climate, fire, or land use. Sediment delivery to the
channel has a duality in aquatic ecosystems in that
it is necessary to create and maintain habitat, yet
can negatively effect biota. Stream habitat is simply part of the sediment, water, and organic material delivered to and stored in streams (Lisle, 1983).
In this view, pools are temporarily stored water in
channel depressions that are scoured at high flows,
and riffles flow over sediment deposits. The concept of the sediment budget is important in watershed assessments, because it links stream habitat
composition and dynamics to watershed processes
of sediment delivery, transport, and storage (Swanson et aI., 1987; Swanston, 1991; Dunne, 1998).
Hillslope erosional processes transfer materials
such as boulders and large wood to the stream,
adding structure to the stream, and in some instances greatly improve fish habitat in structurally
deficient channels (Benda et aI., 1998). However,
in areas of intensive management, increased frequency or episodic mass wasting over large areas
and consequent channel adjustment can be detrimental to fish habitat and rearing (Poulin, 1991).
For effective assessments, we need to recognize
the types of processes involved in sediment production and transport and the triggering and
transport mechanisms for these processes. There
are essentially three sediment sources: hillslopes,
floodplain features, and in-stream storage. The watershed functions of erosion-sediment are sediment
production, delivery, storage, and transport. Sediment sources from hillslope processes include surface erosion and slope movement processes; the
relative types and contribution of these processes
vary with geoclimatic setting, location within a watershed, and watershed history or condition. Swanson et al. (1982) provide a good summary of sediment transfer processes applicable for steepland
375
humid temperate climates. Sources of in-stream
sediment include storage in bedforms and erosion
of fluvial surfaces. The floodplains of wide alluvial valleys can be a significant in-channel source
of sediment from accelerated lateral or vertical
channel migration (e.g., as bank erosion; Beschta,
1991). Most erosional processes involve forces of
water and gravity, which are countered by resistance by vegetated soil, soil cohesion, and regolith
geotechtile properties. Surface erosion is the dominant process in regions with overland flow, and
slope movement processes are more prevalent in
areas predominated by subsurface flow (Coates,
1990b). Surface erosion requires detachment, transport, and deposition of soil particles and is primarily expressed as rill, gully or sheet erosion and
secondarily as rainsplash and dry ravel. Soil mass
movement types are classified by the rate of delivery, size and type of materials, and type of delivery (Varnes, 1979). Triggering mechanisms for
mass movement include precipitation and soil properties that affect soil saturation, cohesion, and increased pore pressures (Coates, 1990b). We should
also consider that groundwater and subsurface flow
processes could be important agents of erosion and
landform development in a variety of settings
(Dunne, 1990).
Erosional processes, sediment delivery, and sensitivity to disturbance are controlled by the same
multiscaled biophysical factors that affect hydrology. Climate controls erosion processes through
degree of vegetation development and the precipitation and storm regime. Peltier (1975) has formalized coarse-scale controls on the effectiveness
of climate and vegetation on erosional processes
for regions of the United States. These regional climatic controls are reflected in sediment load and
drainage density, both of which are highest in semiarid climates (Langbein and Schumm, 1958). Geomorphic sensitivity to change in vegetation depends on the effectiveness of vegetation on
controlling geomorphic processes, which changes
with geoclimatic setting and factors such as slope
and soil cohesion (Swanson, 1981). Vegetation influences erosion by moderating precipitation impact, soil moisture, and water delivery processes
and by stabilizing the soil surface. Changes in landscape vegetation from fire, grazing, tree removal,
or climate change may lower threshold response
and cause a precipitation event to have a much
larger impact on the system than it would have had
under previous vegetation conditions. In forested
regions, for example, rooting strength is an important factor in slope stability on steep hillslopes, and
the change in timing and degree of slope instability depends on vegetation and site conditions (Si-