Deposition of sediments during a flood event on seasonally flooded forestsof the lower Orinoco River and two of its black-water tributaries, Venezuela

Deposition of suspended sediments and their associated nutrients were estim ated during the flood event of 1995 in the seasonally flooded forests of th e Mapire and Caura Rivers, two black-water tributaries of lower Orinoco, an d on two islands of this white-water river. The deposition spanned a wide r ange from 0.07 kg m(-2) in the depositional bar forests of the Mapire River to 73.60 kg m(-2) on the flooded forests of the Orinoco Island site called Jarizo. This variation is associated with the dynamic nature of sediment m obilization, transport and deposition, as well as with the different geomor phic environments and erosion processes upstream from the study sites. The deposited sediment in all the study areas was highly quartzitic with a rela tively high content of kaolinite and goethite. Only in the sediment of the Orinoco Islands was mica (illite) identified in a relatively high proportio n. These mineralogical results reflect the intense weathering processes in the catchment areas of the study rivers. The chemical composition of the de posited sediments showed a great variability among the different study area s, which is in part related to the mineral composition of the sediments and their particle size distribution. The highest concentrations of K, Ca and Mg were found on the clay sediments of the Orinoco agricultural island. The total amount of deposited nutrients varied over a wide range, which is inf luenced by the amount of deposited sediments. In the Jarizo Island site of lower Orinoco were deposited the largest amount of nutrients. In the floode d forests of the Mapire River, the nutrient contribution by the deposited s ediments to the nutrient cycling is relatively low in the depositional-bar forests and practically nonexistent in the forests sites on terraces.