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an important source of sediment if these channels are not
<br />protected by buffer zones.
<br />While soil erosion is important from the standpoint of
<br />site productivity, it is the delivery of sediment to the
<br />stream channel that is of greatest concern and the focus
<br />of most regulatory efforts. Total suspended sediment
<br />(TSS) is the concentration of solid particles in the water
<br />column, and this is usually expressed as milligrams per
<br />liter (mg L -'). Turbidity is an optical measurement of the
<br />water's ability to diffract light, and is measured in Neph-
<br />elometric Turbidity Units (NTU) (Stednick, 1991). Both
<br />turbidity and TSS vary tremendously over time with
<br />streamflow. Water quality standards in some states limit
<br />either the absolute amount or relative increase in these
<br />parameters. In Colorado there are no state standards for
<br />turbidity or TSS, but the state has an embeddedness stan-
<br />dard (embeddedness refers to the degree to which a large
<br />particle is buried by finer particles). The embeddedness
<br />standard is related to macroinvertebrate productivity in a
<br />reference reach, but specific criteria and definitions have
<br />not been defined or adopted in Colorado.
<br />Suspended sediment also can be a concern because fine
<br />particles have large surface areas per unit mass. These
<br />surface areas are reactive and may adsorb and absorb
<br />various water quality constituents, including phosphorus,
<br />introduced chemicals, and petroleum products. Because
<br />phosphorus has a low solubility in water, phosphorus ex-
<br />ports are often correlated with the amount of suspended
<br />sediment transport. Hence the delivery and deposition of
<br />suspended sediment can affect aquatic resources both
<br />physically and chemically.
<br />An increase in the amount of coarser particles in the
<br />stream channel is another potential concern. Bedload
<br />is defined as the transport of large particles by rolling
<br />or bouncing along the streambed. Bedload movement
<br />is difficult to measure and is not used as a water qual-
<br />ity standard (MacDonald et al., 1991). An increase in
<br />bedload may be a major concern from the standpoint of
<br />aquatic habitat, reservoir sedimentation, channel mor-
<br />phology, and channel stability. The difficulty of directly
<br />measuring sediment loads means that changes in sedi-
<br />ment inputs may be more readily detected by monitoring
<br />the physical features of the channel — such as pool vol-
<br />umes, amount of bank erosion, and bed material particle
<br />size — rather than direct measurements (State of Idaho,
<br />1987; MacDonald et al., 1991; MacDonald, 1993).
<br />The amount of suspended sediment and bedload in
<br />streams is largely governed by the characteristics of the
<br />drainage basin, and these include the geology, vegetation,
<br />23
<br />precipitation, and topography, and land use. In- channel
<br />sediment sources due to scour or bank erosion can be
<br />important and should be distinguished from hillslope
<br />sources because all of the eroded material is delivered or
<br />accessible to the stream.
<br />To achieve stream stability, a longer -term equilibrium
<br />must be sustained between the amount of sediment
<br />entering the stream and the amount of sediment being
<br />transported through the channel. Landuse activities that
<br />significantly change the amount of runoff or sediment
<br />can upset this balance and result in unwanted physical
<br />and biological changes (State of Idaho, 1987). Increases
<br />in the frequency of high flows usually increase sediment
<br />transport rates, and an increase in the size or duration
<br />of peakflows can decrease channel stability, increase
<br />turbidity and increase sediment concentrations (Brown
<br />et al., 1974; Troendle and Olsen, 1994).
<br />The storage and routing of sediment is critical to the
<br />generation and persistence of downstream effects. Large
<br />amounts of sediment can be stored in the channel or on
<br />floodplains, terraces, and alluvial fans, and most studies
<br />show a large decrease in unit area sediment yields with
<br />increasing basin size (Walling, 1983). Larger sediment
<br />particles do not travel as rapidly and are typically more
<br />persistent in the channel network than finer particles
<br />(NCASI, 1999a). However, the storage and routing of
<br />sediment is highly variable in time and space, and this
<br />makes it difficult to predict or quantify the changes in
<br />sediment loads as a result of land use activities (NCASI,
<br />1999a). Most studies suggest that suspended sediment
<br />concentrations and sediment yields decrease as a nega-
<br />tive exponential after site disturbance (Leaf, 1974; Bes-
<br />chta, 1978; Ketcheson and Megahan, 1996). The tempo-
<br />ral patterns in sediment production and delivery should
<br />be considered in land use planning (Stednick, 1987).
<br />The forest practices with the greatest potential for caus-
<br />ing erosion and stream sedimentation are road construc-
<br />tion and intensive site preparation. Careful planning and
<br />implementation of forest practices can minimize adverse
<br />effects on water quality.
<br />3.2.1. Roads
<br />Numerous studies have identified unpaved roads as a
<br />major source of sediment in forested watersheds (Elliott,
<br />2000). Specific sources include the road tread, cutslope,
<br />inside ditch, sidecast or fill material, and areas subjected
<br />to concentrated road drainage (Elliot, 2000). The highest
<br />erosion rates typically occur during road construction,
<br />and road erosion rates generally increase with road
<br />maintenance and the amount and type of traffic. Recre-
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