Increases in suspended sediment concentrations can also reduce the value of water for domestic and agricultural use. High suspended sediment levels can reduce the effectiveness of treatment processes and increase maintenance costs by clogging or reducing the capacity of filtration systems. Visible turbidity is aesthetically undesirable for domestic use and can be associated with higher bacterial concentrations. Suspended sediment can also be deposited in, and reduce, the capacity of irrigation ditches, storage ponds, and water tanks.
Most of the time, streams are capable of carrying more suspended sediment than they actually contain. In such cases sediment concentration is "supply limited" and an increase in erosion by running water anywhere in a watershed will usually cause an increase in suspended sediment load downstream. In studies where researchers have considered the effects of both increased peak flows and increased sediment supply, the increases in sediment supply have been consistently judged to be more important in causing an increase in suspended sediment concentrations.
Roads are one of the most significant causes of increased erosion. Road construction exposes large areas of mineral soil to removal by rainwater and snowmelt. Sediment is easily delivered to water courses during wet periods because roads and their drainage ditches frequently intersect stream channels. Generally, the greater the number of stream crossings, the greater the number of sites where sediment can readily be delivered to channels. Fine-grained soils are particularly sensitive to such surface erosion.
The erosion and transport of sediment from roads is exacerbated by the greatly reduced infiltration capacity of mineral soils on cut banks, running surfaces, and fill slopes, caused by compaction and the loss of organic horizons. Roads and skid trails also intercept and concentrate surface runoff so that it has more energy to erode even stable soils. Roads in areas with higher rainfall and snowmelt rates tend to exhibit higher levels of erosion than roads in drier areas. Roads can also cause rapid mass movements and result in very large increases in sediment loads.
The negative effects of roads and skid trails can be moderated by laying them out to avoid the more sensitive sites, using appropriate road and drainage structure construction techniques, installing waterbars on non-permanent roads, and adopting a variety of other erosion control techniques.
Road density for total sub-basin (Indicator #3 and #8): Measure the total length of all roads in the watershed and divide by the total sub-basin area.
Density of roads on erodible soils (Indicator #4): Measure the total length of all roads located on erodible soils (as defined in Appendix 6 and mapped out on the base map) and divide by the total sub-basin area.
Road density within 100 m of a stream (Indicator #5): Measure the total length of all roads located within 100 m of any stream identified on the TRIM or forest cover maps and divide by the total sub-basin area.
Road density within 100 m of a stream and on erodible soils (Indicator # 6): This is probably the most important indicator in the surface erosion section. The two most important factors in determining how much fine sediment will be delivered to streams from road running surfaces are the proximity of the road to the stream and the parent material on which the road was built. This indicator attempts to quantify this hazard.
To obtain this value, measure the total length of roads that are located within 100 m of any stream and on erodible soils (as defined in Appendix 6) and divide by the total watershed area.
Density of active stream crossings (Indicator # 7): It has been frequently documented that stream crossing are often a chronic source of fine-textured material to streams. This can be either directly from the building of the stream crossing or indirectly from delivery of fine sediments along road ditches that empty directly into a stream. To obtain this value, count the total number of stream crossings in the watershed (all streams visible on TRIM or forest cover maps) and divide by the total watershed area. Active stream crossings are defined as those crossings that are still presently being used or will be maintained in a coordinated access management plan.