Rivers material transport

The material transported by rivers consists of dissolved ions (dissolved load), sediment suspended in the flow (suspended load), and sediment transported along the bed of the river (bedload). The total load and the proportion of the load represented by these phases varies widely among rivers in different environments. In particular, climate, topography, and erosion influence the amount and composition of riverine sediment loads. 

Stallard, R. F., Koehnken, L., and Johnsson, M. J. (1991). Weathering processes and the composition of inorganic material transported through the Orinoco River system, Venezuela and Colombia. Geoderma 51,133-165. 

The freshwater cycle is an important link in the carbon cycle as an agent of erosion and as a necessary condition for terrestrial life. Although the amount of carbon stored in freshwater systems is insignificant as a carbon reservoir (De Vooys, 1979 Kempe, 1979a), about 90% of the material transported from land to oceans is carried by streams and rivers. 

In 1986, J.I. Hedges and colleagues (see reference) reported on how dissolved and particulate organic material transported by rivers can provide... 

The rivers play a major role in the transfert of carbon and mineral nutrients from land to the sea and influence significantly the biogeochemical processes operating in coastal waters. Quantification of the material transport, both in the dissolved and particulate forms, has been attempted by several authors in the past (Clarke, 1924 Holeman, 1968 Garrels McKenzie, 1971 Martin et al., 1980 Meybeck, 1982 Milliman Meade, 1983). Depending on the type of sampling techniques and methods of calculations employed there are differences in the reported fluxes. A major problem in such calculations is the paucity of reliable data from some of the major rivers of the world especially of Asia (see e.g. Milliman Meade, 1983). Additionally the difficulty of obtaining representative samples from the rivers will adversely affect flux calculations. Most of the inferences drawn on the nature and transport of riverine materials rest on data collected randomly - at different points in time and space. Seasonal variations in the transport of materials are very common in some of the major world rivers, and in some cases more than 60 % of the material transport occurs within a very short period of time. Furthermore, available data are not always comparable since the analytical techniques used differ from river to river. 

The sources of particulate phosphorus to the seabed include detrital inorganic and organic material transported by rivers to the ocean, biogenic material produced in the marine water column that sinks to the seabed, and atmospheric... 

The geological record shows that this material-transport mechanism has operated for at least 3.8 billion years. New sediments are derived either from older sedimentary rocks or from newly generated or ancient igneous and metamorphic rock. The average chemical composition of suspended sediment in rivers, sedimentary mudrock and the upper continental crust is quite similar (Table 4.1). 

This suggests that rivers represent an important pathway of material transport (Table 4.2) and that sedimentary mudrocks record crustal composition during material cycling. 

Sediments — A general term used to describe material on the bottom and in suspension in water and the suspended material transported by a stream or river, the unconsolidated sand and gravel deposits of river valleys and coastlines, and materials deposited on the floor of lakes and oceans. 

Weathering, atmospheric deposition, and the fixation of atmospheric gases are the ultimate sources of the material transported by rivers. These... 

The composition of dissolved and solid material transported by rivers in mountain belts of the humid... 

The competence of a river to transport its load is demonstrated by the largest boulder it is capable of moving it varies according to the velocity of a river and its volume, being at a maximum during flood. It has been calculated that the competence of a river varies as the sixth power of its velocity. The capacity of a river refers to the total amount of sediment that it carries. It varies according to the size of the soil and rock material that form the load, and the velocity of the river. When the load consists of fine particles, the capacity is greater than when it is comprised of coarse material. Usually, the capacity of a river varies as the third power of its velocity. 

Rivers transport suspended sediments derived from the disintegration of basin surface layers. With reduced velocity, sediment is deposited in the river channel. The finest material is carried to the sea. It has been estimated that the average mechanical denudation rate for continents is 0.056 mm year (35). This is based on a total suspended load of 13.5 x 10 metric tons year (S). Presently, about two-thirds of the world s total suspended sediment load derives from Southern Asia and large Pacific Islands. Berner has estimated the increase in sediment loss in the U.S. and world since prehuman times to be approximately 200% (35). Current estimated erosion rate from the major land forms is provided in Table I. The relatively recent construction of large sediment trapping dams that normally caused sediment to be deposited in river valleys or transported to the ocean has drastically reduced sediment yields in great rivers. 

The physical transport of particles in a river occurs by two primary modes bedload and suspended load. Bedload consists of material moved along the bed of the river by the tractive force exerted by flowing water. Bedload may roll or hop along the bottom, and individual particles may remain stationary for long periods of time between episodes of movement. Suspended load consists of material suspended within the flow and that is consequently advected by flowing water. Rivers and streams are naturally turbulent, and if the upward component of turbulence is sufficient to overcome the settling velocity of a particle, then it will tend to remain in suspension because the particles become resuspended before they can settle to the bottom of the flow. Suspended load consists of the finest particles transported by a river, and in general is composed of clay- and silt-sized... 

A fundamental distinction among landscapes is whether the net sediment flux (the total load carried by the river) is limited by the ability of erosional processes to carry sediment (transport-limited environments) or the availability of erodible material (weathering-limited environments). In general, soil-mantled landscapes can... 

---