Rivers, streams and floodwater carry sediment which can deposit in aquatic ecosystems which can alter the flow of water in these systems, reduce water depth, impact on water quality and smother habitats. Particle sizes of sediments and their distributions are fundamental properties that have a major influence on many other processes, including susceptibility to settling out, transport and deposition, porosity, permeability, chemical reactivity and agricultural productivity.
While water processes are often easy to see and measure, the movement of sediment can be more difficult to quantify. Despite this, an understanding of the erosion, transport, and deposition of sediment within wetlands is vitally important in their management. As well as being a building block for many wetland processes and wetland morphology, sediment can also be a source of nutrients or other contaminants into a system.
A sediment budget may be constructed for a wetland. The budget considers the rate and volume of the sediment that is entering the wetland from different sources, what component of this resides in the wetland, and how much exits the system.
Understanding the sources and transport of sediment in a catchment involves consideration of the types of sediment present, how they have been formed and derivation, and how they are liberated by a combination of weathering and erosion. The processes of transport, erosion and deposition may vary spatially both between and within catchments.
Sediments can be described based on the material they have been derived from. These include:
Weathering and erosion processes
Weathering is the in-situ disintegration of a rock or sediment into smaller parts:, erosion is the transport of sediment away from the site. Weathering processes include physical, chemical and biological processes.
Physical/Mechanical: this weathering process involves the effect of a change in temperature on rocks. For example, frost shattering where water freezes in cracks and forces the rock apart. Another example is insolation weathering where the rock is heated and cooled causing expansion and contraction that breaks the rock surface apart.
Chemical: this weathering process involves the minerals in the rock being altered into different minerals or salts, usually by the addition of water. The changes in the mineral properties of the rock can make it more prone to erosion. For example, iron ‘rusting’ into iron oxide is a chemical weathering process resulting from the addition of oxygen to iron minerals. Another example is hydrolysis, where minerals such as feldspars react with carbonic acid in rainwater to produce clays. This hydrolysis process can cause the disintegration of rocks such as granites. The harder quartz constituents in granite remain as sands while the surrounding feldspar changes to clay and can be more easily eroded leaving the sand behind.
The change from a solid rock into a liquid solution and gas can occur in limestones. The addition of acids, such as carbonic acid or acid rain, changes the calcite into a soluble ion. The result is that in limestone areas weathering and erosion creates specific forms known as karst. Subterranean caves can cause surface rivers to suddenly disappear underground, or for rivers to only flow when the water table is high.
Biological: this weathering process involves to the effect of living organisms on rocks. For example, tree roots growing through cracks in rocks to find water can prize the rock apart. Another example is when bacteria, algae and lichens produce chemicals to break down the rock they live on in order to acquire nutrients they need for survival. There are also animals which use rocks for protection, making holes in the rock by scraping away the grains or secreting acid to dissolve the rock.
Sediment transport processes
Sediment can be transported by wind (aeolian) or water (fluvial and marine) processes. Aeolian processes tend to have limited transport capacity usually transporting sand grains or finer grained sediments. In riverine wetland systems fluvial processes dominate. The coarser sediments are carried along the riverbed by rolling making up the bedload. Intermediately sized sediments may be carried in suspension where they are moved in the water column but may also interact with the riverbed. The wash load is the fine sediment that is constantly suspended in the water column.
Transport of sediment is a balance between the shear stress ‘pulling’ and the shear strength ‘resisting’. The velocity, density and depth of water control the shear stress put on the particle, where high velocity deep flows have the highest pulling forces. The shear strength is lowest in sands and increases with particle sizes larger than sand. The sediment shear strength also increases as the particle size decreases into silts and clays as they have cohesive forces that bind the particles together.
Whilst the largest floods may transport very high sediment loads the combination of magnitude and frequency should be considered when determining the dominant sediment transporting flows. More frequent medium flows may occur often enough that they transport more sediment than the infrequent large flows. The size of the river channel may naturally adjust so that the dominant sediment transporting flows are kept within the channel.
Catchment scale variability in erosion and deposition
To build a sediment budget the sources and sinks of sediment need to be understood. In a riverine system the catchment can be conceptually divided into an upland supply area, a mid-catchment transfer zone and a downstream depositional zone.
Upstream environments are often dominated by sediment delivered from hillslopes by processes such as overland flow, landslides and gullying. Overland flow can deliver sediment into the channel by sheet flow, however, the more concentrated the flow is into small channels the greater its transport capacity. This is one of the reasons that gullies can relatively contribute much greater volumes of sediment.
Sediment can be deposited into short term stores such as in-stream bars, benches, or islands. When floods occur sediment may move into storage on the floodplain with a longer residency time. The transfer zone in a river system includes sediment going in and out of storage as well as being transported from upstream.
Erosion of riverbanks also occurs alongside deposition. The processes of erosion can be divided into three main categories.
The downstream reaches of a river are frequently a lower energy environment. Both the processes of erosion and deposition occur but often at slower rates compared to upstream. The lower energy available to transport and erode sediment means that depositional processes tend to dominate.
Goudie, A. 1988. The encyclopaedic dictionary of physical geography. Basil Blackwell Ltd. Oxford, UK.
Last updated: 23 December 2021
This page should be cited as:
Department of Environment and Science, Queensland (2021) Sediment processes, WetlandInfo website, accessed 1 July 2022. Available at: https://wetlandinfo.des.qld.gov.au/wetlands/ecology/processes-systems/sediment/