ABSTRACT
Relatively infertile modern sediment may be deposited on floodplains or on colluvial soils. Modern sediment results from culturally accelerated erosion. It may take various forms, such as over-bank splays, fans, or vertical accretion deposits. Infertile sand or gravel deposits commonly cause this type of damage, but silts and clays derived from subsoil erosion, if low in nutrient elements, are also harmful in some areas. General principles of stream and valley sedimentation, criteria for recognition of modern valley deposits, and the relationship of stream and valley sedimentation of flood-control problems were outlined by Happ et al. (1940). The degree of damage by deposition of infertile materials depends on the type of material, the depth of deposit, and the rate of deposition, as well as on the productivity of the land in its original state. As an example, consider two areas of silt loam floodplain soils that have been damaged by sand. One has been damaged by a deposit of 16 inches of sand added gradually, at the rate of an inch each year for 16 years. It has been possible to mix the sand with the surface 8 inches (plow depth) of soil each year. Therefore, although the productive capacity of the soil has diminished to 50% of the original capacity, it is still in production. In contrast, the other area has received a deposit of only 8 inches of sand, all of which was deposited in a year. Under normal practices, there would not be much mixing of this sand with the old soil below. Unless special treatment is used, it might be taken out of production and for all practical purposes lost to cultivation.
