How does sediment get into rivers?
Soil is eroded from fields during rainstorms during periods of the year when the soil is bare. Autumn sowing exacerbates this problem because the soil is exposed for a longer period and during the wettest part of the year. On sloping fields water flows into little rills and picks up silt and clay particles which eventually get washed out of the field into a watercourse. Where livestock cause a problem, sediment is introduced where banks are poached by their hooves or where overstocking occurs, by rainwater off bare soil. What harm does fine sediment do? Fine sediment can have serious effects on river ecology. Silt, clay and fine sand can clog the pores and crevices within a salmon redd. This prevents the through flow of water and the eggs or alevins can suffocate. They can also bind up gravel making redds harder to dig in the first place. Sediment, especially sand, can fill up crevices and cracks among the stones on the riverbed where juvenile salmon would otherwise take refuge. Fine s
