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.