Carl F. Nordin

Sediment loads in the Amazon River

ESTIMATES published in the past 20 years of the sediment load delivered to the sea by the Amazon River range from 4 to 10 × 108 tonnes yr−1 Estimates published around 1960, when few data were available on either the sediment concentration or the river discharge, were in the range 9–10 × 108 tonnes yr−1 (refs 1, 2). Estimates published in 1967 and 1968, after significant new data had been collected, were in the lower range of 4–5 × 108 tonnes yr−1 (refs 3, 4). Our more comprehensive data collected mostly since 1970 (and especially in 1977) suggest that the earlier higher estimates may have been more nearly correct. We show here that the mean annual load of suspended sediment at Óbidos, Brazil is between 8 and 9 × 108 tonnes yr−1. Most of this sediment is discharged onto the continental shelf.

Non-uniform vertical distribution of fine sediment in the Amazon River

IN studies of sediment transport in rivers, it is frequently assumed that suspended particles finer than a certain size (usually of the order of 60 µm) are uniformly concentrated from river bed to water surface1,2. The assumption seems to be mostly a matter of convenience; it is well established that the vertical concentration gradient for sediment particles suspended in rivers is directly proportional to fall velocity of the particles and inversely proportional to shear velocity of the flow3. For very fine particles suspended in highly turbulent flows, concentration gradient may be small; the assumption has been used, therefore, to justify collection of near-surface samples in streams where velocities are high and turbulence is intense2. Our recent studies of sediment in suspension in the Amazon River show that this assumption is not valid for deep rivers and that appreciable errors result from using concentrations of surface samples to represent concentration of suspended sediment and its associated adsorbed constituents through the entire stream depth.

TIME SERIES ANALYSES OF WATER AND SEDIMENT DISCHARGES

Abstract The techniques of time series analysis applied to studies of long-term water and sediment discharges for four stations of the Rio Grande show that cyclic models can account for less than 25 percent of the variance in the water series and 15 percent in the sediment series. The annual cycles of water discharge and sediment discharge are significantly correlated, with systematic variations from station to station resulting from erratic tributary inflow and variable irrigation demands.

Transporte de sedimentos no rio Amazonas

From new data on the sediment in the Amazon River, we estimate the mean annual load of suspended sediment at Obidos, Brazil, to be 0.8-0.9x109 tonnes per year. Most of this sediment is discharged onto the continental shelf.

Distribuição vertical não uniforme de sedimento fino no rio Amazonas ()

Particulas suspensas de sedimentos menores que 60μm nao estao uniformemente distribuidas nas aguas dos rios Amazonas e Solimoes. Concentracoes destas particulas finas na superficie da agua sao somente a metade da concentracao do que em amostras integradas atraves de profundidade. Ate particulas de 10μm estao menos concentradas perto da superficie do que a maiores profundidades. Estimativas de producao de sedimento e fluxo de constituintes quimicos cm suspensoes que sao baseadas somente em amostra da superficie, estao passiveis de conter erros significativamente grandes.