The Types of Deposits Found at the Mouth of Streams

Sedimentary deposits at stream mouths can help determine the health of a stream and its ecosystem. Too much sediment can block sunlight, in turn killing plants that are an essential supply of food and shelter for aquatic life. It can also clog fish gills and feeding filters of invertebrates. Nests and eggs in the water can be negatively affected. Increased flooding risks and a decrease in the availability of safe drinking water are concerns for populations living near a stream with excessive sediment. Controlling erosion and pollutants entering upstream can benefit the stream and neighboring communities.

Erosion

• Water erodes rock surfaces as it flows over them. The size of rock particles washed into a stream or eroded from the stream bed is dependent on the speed of the water and the composition of the rock. The smaller the particles, the longer they will be suspended in the water and more likely they will find their way to the mouth of the stream and be deposited. Clay particles, minerals and soil are also washed into streams from their watersheds. Poor land management can increase erosion.

Organic Matter

• Organic matter can make up to 15% of river silt. This includes skeletons and shells that have been pulverized by the action of the stream as well as decomposed fish and plant life. Organic compounds such as DDT, sewage and emissions from fossil fuels are a direct result of human activities. Although organic, some of these pollutants are harmful to animals and humans. DDT, for instance, is a pesticide that was banned from use in the 1970s when it was identified as a possible carcinogen. Its persistent presence in river sediment today is testament to how resistant this compound is to breakdown in the environment.

Chemicals

• Fertilizers and pesticides from farms and gardens can be washed into streams. Improperly maintained landfills have been known to discharge chemicals from medical, household and industrial waste, including trace elements of arsenic, copper, mercury and lead. These chemicals negatively affect fish, plants, animals and insect life in and around the streams. A stream slows where it enters a lake or merges with another stream. Without the forward momentum, its ability to suspend chemicals and other particles in the water decreases. Sediment is formed as these chemicals and particles drop to the riverbed.

Solid Pollutants

• A wide range of solid waste is dumped into the world's streams and rivers. Not all these objects make it to the mouth and get dumped as sediment. Those that stay upstream break down slowly under the relentless rush of water, and the components can be deposited at the stream's mouth. The Living Lands and Waters Organization has documented a dizzying array of items their volunteers have cleaned from rivers, including bicycles, washing machines, boats, chains, tires, toilets, toys, shopping carts, lawn mowers and life jackets.