Sediments and suspended solids
Sediments consist of fine particles of mostly inorganic material such as mud and silt washed into a stream as a result of land cultivation and construction. They may also arise from demolition and mining operations where these activities take place. The presence of solid particulate material suspended in the flowing water is the reason why many rivers look brown in colour, especially in the rainy season. The particles are called suspended solids while they are carried (suspended) in flowing water. When they settle to the bottom, they are called sediments.
Organic matter means anything that is derived from living organisms, i.e. all plants and animals. Inorganic matter has a mineral, rather than biological, origin meaning it comes from rocks and other non-living sources.
Large quantities of inorganic matter, in the form of suspended solids, may reduce light penetration into the water which can affect the growth of plants. Sediments may even suffocate organisms on the river bed. River water may also contain organic matter, such as human and animal wastes, which can deplete (reduce) the oxygen in the water if the river is slow-flowing (see Box 17.1). This can lead to anaerobic conditions which may create unsightly conditions and cause unpleasant odours.
Box 17.1 Oxygen in water
Many aquatic (water living) organisms depend on oxygen dissolved in the water to survive. Aquatic animals include fish, amphibians and many invertebrate species such as insect larvae, snails and worms. Their supply of oxygen in the water is maintained from atmospheric oxygen in the air above the water and from oxygen produced by green aquatic plants by the process of photosynthesis. Fast-flowing, turbulent water will be aerated (gain oxygen) more than still water because the boundary between air and water is more active.
If organic pollutants such as human and animal wastes are released into a water body, bacteria will use the waste as food and break it down into simpler, less harmful substances. As they do this, the bacteria will use up the dissolved oxygen from the water. This is called deoxygenation. If the quantity of organic pollution is high, then all the oxygen from the water may be used up leading to anaerobic (without oxygen) conditions. This is unlikely in a river where the water is moving but can happen in lakes or slow-flowing channels.
Inorganic solids, such as mud and silt, do not have this effect because they are inert (stable and inactive) and cannot be used as food by bacteria.