Factors controlling variations in river sediment loads

Sediment transfer from continents to oceans via rivers is one of the important processes regulating river-bank stabilization, soil formation, biogeochemical cycling of elements, crust evolution and many other earth-related processes. Due to changes in continental positions during the geologic past, water flow and sediment loads in rivers have also shown variations during different time periods. Recent estimations budget sediment flux from rivers to oceans of about 18 × 109 tons annually. However, it is estimated that the present-day sediment load in rivers has been greatly altered due to large-scale human perturbations. Factors such as relief, channel slope, basin size, seasonality of rains and tectonic activities control sediment loads in rivers. Human interventions in the form of reservoirs for water storage have impounded and trapped huge sediment loads on the continental parts. Similarly, land use patterns also had their effects on sediment flux to the oceans. Rivers flowing over the Pacific islands have large sediment yield as also the Himalayan rivers, Ganga and Brahmaputra, due to high relief and tectonic disturbances, whereas rivers in North America, China and Africa show decrease in sediment flux due to trapping of sediments in the reservoirs.[1]

World-Wide Delivery of River Sediment to the Oceans

New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about  tons) contribute about  tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about  tons annually; bedload and flood discharges may account for an additional  tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.[2]

River-Sediment Inputs to Major Deltas

River sediments are distributed unevenly in space and time, and they are markedly susceptible to human influences. Half the world’s river sediment is derived from the Himalayan region and its environs. Most of the remainder is derived from other tectonically active regions such as the western Pacific islands, the Andes, and southern Alaska. River-sediment loads are variable at many time scales: seasonal, annual, decadal, and longer. The storage of sediment in river systems confounds our ability to predict the delivery of sediment to coastal zones. Natural river-sediment loads are increased by deforestation and crop farming, and decreased by dams and reservoirs.[3]

Distribution of river sediment in channels

This article reviews the methods for calculating the distribution of river sediments in channels. As a result of the analysis, initial methods for the distribution of sediments on the channels was developed for straight and unstable rivers. A mathematical model that describes the distribution of river sediments during uneven movement in the structures has been developed. The distinctive feature of this model is that the distribution of sediments in the stream depends on the fractions. In the hydraulic structures, a mathematical model of turbulent flow was proposed during the uneven movement and based on this model; a method of calculating sediment distribution was developed in a non-linear motion. Comparative analysis of model of calculation of river sediments in constructions was verified with analysis of natural field conditions. The analysis of the results shows that the difference between the measured values and calculated values is 4-8%.[4]

The behavioural characteristics of sediment properties and their implications for sediment fingerprinting as an approach for identifying sediment sources in river basins

Sediment fingerprinting is a technique that is increasingly being used to improve the understanding of sediment dynamics within river basins. At present, one of the main limitations of the technique is the ability to link sediment back to their sources due to the non-conservative nature of many of the sediment properties. The processes that occur between the sediment source locations and the point of collection downstream are not well understood or quantified and currently represent a black-box in the sediment fingerprinting approach. The literature on sediment fingerprinting tends to assume that there is a direct connection between sources and sinks, while much of the broader environmental sedimentology literature identifies that numerous chemical, biological and physical transformations and alterations can occur as sediment moves through the landscape. The focus of this paper is on the processes that drive particle size and organic matter selectivity and biological, geochemical and physical transformations and how understanding these processes can be used to guide sampling protocols, fingerprint selection and data interpretation. The application of statistical approaches without consideration of how unique sediment fingerprints have developed and how robust they are within the environment is a major limitation of many recent studies. This review summarises the current information, identifies areas that need further investigation and provides recommendations for sediment fingerprinting that should be considered for adoption in future studies if the full potential and utility of the approach are to be realised.[5]

Reference

[1] Chakrapani, G.J., 2005. Factors controlling variations in river sediment loads. Current science, pp.569-575.

[2] Milliman, J.D. and Meade, R.H., 1983. World-wide delivery of river sediment to the oceans. The Journal of Geology, 91(1), pp.1-21.

[3] Meade, R.H., 1996. River-sediment inputs to major deltas. In Sea-level rise and coastal subsidence (pp. 63-85). Springer, Dordrecht.

[4] Arifjanov, A., Samiev, L., Apakhodjaeva, T. and Akmalov, S., 2019, December. Distribution of river sediment in channels. In IOP Conference Series: Earth and Environmental Science (Vol. 403, No. 1, p. 012153). IOP Publishing.

[5] Koiter, A.J., Owens, P.N., Petticrew, E.L. and Lobb, D.A., 2013. The behavioural characteristics of sediment properties and their implications for sediment fingerprinting as an approach for identifying sediment sources in river basins. Earth-Science Reviews, 125, pp.24-42.