The role of organic matter in sediment budgets in forested terrain

The production and transport of suspended particulate matter usually are major components of a sediment budget, but the organic and inorganic fractions of this material are not commonly differentiated. In four forested streams in the northwestern United States, the organic content of suspended sediment samples ranged from 10 to 80 weight percent for individual flood events. For a given stream, as a percentage of suspended sediment, organic content was highest during base flows and the early rising and late falling limbs of hydrographs, but on an annual cumulative basis, most organic flux occurred during a few days of high flow. By weight, the inorganic component of suspended sediment dominated the annual sediment flux in three of the catchments, but organics represented more than half the suspended sediment load in an old growth redwood stream. Although commonly minor by weight, organic suspended sediment can have important effects on aquatic biological communities, turbidity measurements, and eutrophication in estuaries.