25.5 Erosional and Sediment Processes
stream habitat is not adequately understood, especially in the context of variability at different spatial and temporal scales. Fluvial processes of sediment transport and storage are directly linked with
ecosystem concepts of habitat formation, distribution, and dynamics. To better connect stream habitat with properties and processes of the watershed,
recognized geomorphic features of channels, such
as channel units, bedforms, and structural elements,
can be perceived as habitat features (Frissell et aI.,
1986; Lisle, 1986; Grant et aI., 1990). Sediment
transport and bed composition are highly variable
in space and time (VanSickle and Beschta, 1983;
Benda et aI., 1998), confounding the development
of methods and regulatory standards for impacts on
fish (Chapman and McLeod, 1987). Excess sediment from extreme natural events and land use
causes adjustments in channel morphology, and the
impacts on aquatic habitat can be quite variable
spatially and temporally. Sediment input from
large-scale events tends to travel through the basins
as sediment slugs or waves. Valley confinement has
a strong influence on rates and patterns of transport of sediment waves (Meade et aI., 1990). Conversely, channel morphology and bedform size
change over time as sediment is eroded and transported from landslide-derived in-channel sediment
deposits (Perkins, 1989). Pool volume (Hilton and
Lisle, 1993) is a sensitive index for monitoring the
effect that transport of high sediment volumes has
on habitat parameters of pool density and volume
(Lisle, 1982).
Fish spawning success depends in part on the
timing and extent of bed scour. Thus the distribution and interannual variability of salmonid populations are related to the geographic range and timing of geomorphic, land-use, and hydrologic factors
that influence bed stability (Kondolf et aI., 1991;
Montgomery et al., 1996). Channel or bed stability indices can potentially assess both geographic
variation in stability and the sensitivity of channels
to disturbance (Kappesser, 1993; Olsen et aI.,
1997). Elevated fine-sediment content is considered detrimental to fish egg development and affects overwintering habitat for juvenile fish in icecovered streams. Furthermore, increases in fine
sediment can differentially increase sediment transport, bed stability, and channel adjustment (Buffington, 1995). Indices based on surface bed material size have been used to infer changes in
sediment transport and monitor fine sediment, but
have limitations in application and interpretation
(Potyondy and Hardy, 1994; Kondolf, 1995a).
Other methods for measurement and analysis of
channel and sediment characteristics related to fish377
eries are outlined in various manuals (Platts et aI.,
1983; Chapman and McLeod, 1987; SchuettHames et aI., 1993).
It is essential that assessments identify the dynamics of long-term sediment supply from fire
regimes, climate fluctuations, and historical land
use. It is only in this context of historical process
dynamics that we can interpret contemporary conditions in fluvial surfaces, sediment storage, habitat characteristics, and fish distribution (Costa,
1975; Swanson, 1981; Meyer et aI., 1995; Benda
et aI., 1998). For instance, streams may have inherited a sediment regime from the Pleistocene, reflected in variations of floodplain and terrace morphology (McDowell, 1983), that strongly influence
contemporary processes (Heed, 1981).
We also need to consider that sediment transport
processes and resultant fluvial landforms vary
within a watershed, and their distribution is controlled by watershed and channel size, slope, and
morphology (Ohmori and Shimazu, 1994). The
travel behavior of hyperconcentrated flows (e.g.,
debris torrents) can demonstrate the influence of
basin characteristics on sediment transport. Geoclimatic factors affect the initiation processes and
composition (water and sediment composition) of
hyperconcentrated flows, which in tum influence
travel behavior, channel impact, and depositional
characteristics (fluvial landforms) of these flows
(Costa, 1988). For instance, flows with higher water content and finer sediment composition tend to
flow farther and stop at lower gradients than those
with lower water content. Travel and deposition of
debris torrents are also controlled by basin morphometry, such as stream network configuration,
tributary junction angles, and valley slope and confinement (Benda, 1990). Thus assessments must incorporate variation in geoclimatic factors and basin
characteristics that affect both regional and withinbasin behavior of sediment transport mechanisms,
such as debris torrents (Swanson et aI., 1987;
Benda, 1990).
Large wood is an important geomorphic agent
controlling sediment transport and channel morphology (Keller and Swanson, 1979). Wood steps
in steeper streams function in dissipation of energy
(Heed, 1981), which results in localized areas of
lower gradient and scour for pool formation. Wood
steps also function in sediment storage (Mosley,
1980; Bojonell et aI., 1993) and act as in-channel
storage buffers to sediment transferred from hillslopes. Watershed assessments and riparian management schemes need to consider input of large
wood from both local sources and transport
processes, such as debris torrents and landslides,
stream habitat is not adequately understood, especially in the context of variability at different spatial and temporal scales. Fluvial processes of sediment transport and storage are directly linked with
ecosystem concepts of habitat formation, distribution, and dynamics. To better connect stream habitat with properties and processes of the watershed,
recognized geomorphic features of channels, such
as channel units, bedforms, and structural elements,
can be perceived as habitat features (Frissell et aI.,
1986; Lisle, 1986; Grant et aI., 1990). Sediment
transport and bed composition are highly variable
in space and time (VanSickle and Beschta, 1983;
Benda et aI., 1998), confounding the development
of methods and regulatory standards for impacts on
fish (Chapman and McLeod, 1987). Excess sediment from extreme natural events and land use
causes adjustments in channel morphology, and the
impacts on aquatic habitat can be quite variable
spatially and temporally. Sediment input from
large-scale events tends to travel through the basins
as sediment slugs or waves. Valley confinement has
a strong influence on rates and patterns of transport of sediment waves (Meade et aI., 1990). Conversely, channel morphology and bedform size
change over time as sediment is eroded and transported from landslide-derived in-channel sediment
deposits (Perkins, 1989). Pool volume (Hilton and
Lisle, 1993) is a sensitive index for monitoring the
effect that transport of high sediment volumes has
on habitat parameters of pool density and volume
(Lisle, 1982).
Fish spawning success depends in part on the
timing and extent of bed scour. Thus the distribution and interannual variability of salmonid populations are related to the geographic range and timing of geomorphic, land-use, and hydrologic factors
that influence bed stability (Kondolf et aI., 1991;
Montgomery et al., 1996). Channel or bed stability indices can potentially assess both geographic
variation in stability and the sensitivity of channels
to disturbance (Kappesser, 1993; Olsen et aI.,
1997). Elevated fine-sediment content is considered detrimental to fish egg development and affects overwintering habitat for juvenile fish in icecovered streams. Furthermore, increases in fine
sediment can differentially increase sediment transport, bed stability, and channel adjustment (Buffington, 1995). Indices based on surface bed material size have been used to infer changes in
sediment transport and monitor fine sediment, but
have limitations in application and interpretation
(Potyondy and Hardy, 1994; Kondolf, 1995a).
Other methods for measurement and analysis of
channel and sediment characteristics related to fish377
eries are outlined in various manuals (Platts et aI.,
1983; Chapman and McLeod, 1987; SchuettHames et aI., 1993).
It is essential that assessments identify the dynamics of long-term sediment supply from fire
regimes, climate fluctuations, and historical land
use. It is only in this context of historical process
dynamics that we can interpret contemporary conditions in fluvial surfaces, sediment storage, habitat characteristics, and fish distribution (Costa,
1975; Swanson, 1981; Meyer et aI., 1995; Benda
et aI., 1998). For instance, streams may have inherited a sediment regime from the Pleistocene, reflected in variations of floodplain and terrace morphology (McDowell, 1983), that strongly influence
contemporary processes (Heed, 1981).
We also need to consider that sediment transport
processes and resultant fluvial landforms vary
within a watershed, and their distribution is controlled by watershed and channel size, slope, and
morphology (Ohmori and Shimazu, 1994). The
travel behavior of hyperconcentrated flows (e.g.,
debris torrents) can demonstrate the influence of
basin characteristics on sediment transport. Geoclimatic factors affect the initiation processes and
composition (water and sediment composition) of
hyperconcentrated flows, which in tum influence
travel behavior, channel impact, and depositional
characteristics (fluvial landforms) of these flows
(Costa, 1988). For instance, flows with higher water content and finer sediment composition tend to
flow farther and stop at lower gradients than those
with lower water content. Travel and deposition of
debris torrents are also controlled by basin morphometry, such as stream network configuration,
tributary junction angles, and valley slope and confinement (Benda, 1990). Thus assessments must incorporate variation in geoclimatic factors and basin
characteristics that affect both regional and withinbasin behavior of sediment transport mechanisms,
such as debris torrents (Swanson et aI., 1987;
Benda, 1990).
Large wood is an important geomorphic agent
controlling sediment transport and channel morphology (Keller and Swanson, 1979). Wood steps
in steeper streams function in dissipation of energy
(Heed, 1981), which results in localized areas of
lower gradient and scour for pool formation. Wood
steps also function in sediment storage (Mosley,
1980; Bojonell et aI., 1993) and act as in-channel
storage buffers to sediment transferred from hillslopes. Watershed assessments and riparian management schemes need to consider input of large
wood from both local sources and transport
processes, such as debris torrents and landslides,