Rivers are the major natural transport mechanism responsible for moving detritus and dissolved solids on the land surface of the Earth. The dissolved load of rivers is dependent on the nature of the sediments and rocks underlying the drainage basin (see Meybeck, Chapter 4, this volume). The organic carbon and fixed nitrogen carried by rivers are related to the vegetation and climate. The solid load of the rivers is a function of relief, climate, and geology of the drainage basin. Variability is a characteristic of rivers; the global ratio of solid to dissolved load is about 4 to 1, but among major rivers it ranges from 80 to 0.1. The fluvial transport system has potential for great variations in response to climate change.

In most instances, measurement of fluxes associated with rivers did not begin until after human activities had modified the landscape, so that the primeval natural fluxes are not known. Even large rivers can be perturbed by large-scale agriculture, impoundment, and associated urbanization. Where these have not occurred, rivers may be in their natural state. Thus, the Amazon, Orinoco, Zaire, Yukon, and MacKenzie rivers along with many Siberian rivers may still be representative of the natural state. Most subtropical and temperate rivers have been significantly perturbed by anthropogenic activity. Indeed, human activities may have altered so many natural conditions that their effect pervades all aspects of material flux.

The alteration of sediment on the ocean floor through reaction with ocean bottom water and with pore waters results in fluxes both into and out of the sediment. The global scale of these fluxes and the time scales on which they may change are only beginning to be understood. Martin and Sayles (Chapter 10, this volume) present a review of the current state of knowledge of these important processes and associated fluxes.