Erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another location where it is deposited. Erosion is distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by dissolution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.
A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in British English usage, aits or eyots.
In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.
The term bed load or bedload describes particles in a flowing fluid that are transported along the stream bed. Bed load is complementary to suspended load and wash load.
In physical geography, a channel is a type of landform consisting of the outline of a path of relatively shallow and narrow body of water or of other fluids, most commonly the confine of a river, river delta or strait. The word is generally used to refer to a natural formation of a narrow and is cognate to canal, which is more commonly used to denote an artificial formation.
The Exner equation is a statement of conservation of mass that applies to sediment in a fluvial system such as a river. It was developed by the Austrian meteorologist and sedimentologist Felix Maria Exner, from whom it derives its name.
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks, mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.
The suspended load of a flow of fluid, such as a river, is the portion of its sediment uplifted by the fluid's flow in the process of sediment transportation. It is kept suspended by the fluid's turbulence. The suspended load generally consists of smaller particles, like clay, silt, and fine sands.
Sediments deposited into lakes that have come from glaciers are called glaciolacustrine deposits. In some European geological traditions, the term limnoglacial is used. These lakes include ice margin lakes or other types formed from glacial erosion or deposition. Sediments in the bedload and suspended load are carried into lakes and deposited. The bedload is deposited at the lake margin while the suspended load is deposited all over the lake bed. Glaciolacustrine deposits commonly form varves, which are annually deposited layers of silt and clay, where silt is deposited during the summer, and clay during the winter.
Stream load is a geologic term referring to the solid matter carried by a stream. Erosion and bed shear stress continually remove mineral material from the bed and banks of the stream channel, adding this material to the regular flow of water. The amount of solid load that a stream can carry, or stream capacity, is measured in metric tons per day, passing a given location. Stream capacity is dependent upon the stream's velocity, the amount of water flow, and the gradation.
Wash load is similar to a suspended load, but wash load sediment never interacts with the bed load. All of the sediment in the wash load stays suspended in the water throughout the channel. Wash load refers to a river's ability to move sediment through a channel.
Dissolved load is the portion of a stream's total sediment load that is carried in solution, especially ions from chemical weathering. It is a major contributor to the total amount of material removed from a river's drainage basin, along with suspended load and bed load. The amount of material carried as dissolved load is typically much smaller than the suspended load, though this is not always the case, particularly when the available river flow is mostly harnessed for purposes such as irrigation or industrial uses. Dissolved load comprises a significant portion of the total material flux out of a landscape, and its composition is important in regulating the chemistry and biology of the stream water.
A bar in a river is an elevated region of sediment that has been deposited by the flow. Types of bars include mid-channel bars, point bars, and mouth bars. The locations of bars are determined by the geometry of the river and the flow through it. Bars reflect sediment supply conditions, and can show where sediment supply rate is greater than the transport capacity.
Stream power, originally derived by R. A. Bagnold in the 1960s, is the amount of energy the water in a river or stream is exerting on the sides and bottom of the river. Stream power is the result of multiplying the density of the water, the acceleration of the water due to gravity, the volume of water flowing through the river, and the slope of that water. There are many forms of the stream power formula with varying utilities, such as comparing rivers of various widths or quantifying the energy required to move sediment of a certain size. Stream power is closely related to other criteria such as stream competency and shear stress. Stream power is a valuable measurement for hydrologists and geomorphologists tackling sediment transport issues as well as for civil engineers, who use it in the planning and construction of roads, bridges, dams, and culverts.
Three components that are included in the load of a river system are the following: dissolved load, wash load and bed material load. The bed material load is the portion of the sediment that is transported by a stream that contains material derived from the bed. Bed material load typically consists of all of the bed load, and the proportion of the suspended load that is represented in the bed sediments. It generally consists of grains coarser than 0.062 mm with the principal source being the channel bed. Its importance lies in that its composition is that of the bed, and the material in transport can therefore be actively interchanged with the bed. For this reason, bed material load exerts a control on river channel morphology. Bed load and wash load together constitute the total load of sediment in a stream. The order in which the three components of load have been considered – dissolved, wash, bed material – can be thought of as progression: of increasingly slower transport velocities, so that the load peak lags further and further behind the flow peak during any event.
A bedrock river is a river that has little to no alluvium mantling the bedrock over which it flows. However, most bedrock rivers are not pure forms; they are a combination of a bedrock channel and an alluvial channel. The way one can distinguish between bedrock rivers and alluvial rivers is through the extent of sediment cover.
In hydrology stream competency, also known as stream competence, is a measure of the maximum size of particles a stream can transport. The particles are made up of grain sizes ranging from large to small and include boulders, rocks, pebbles, sand, silt, and clay. These particles make up the bed load of the stream. Stream competence was originally simplified by the “sixth-power-law,” which states the mass of a particle that can be moved is proportional to the velocity of the river raised to the sixth power. This refers to the stream bed velocity which is difficult to measure or estimate due to the many factors that cause slight variances in stream velocities.
Hydraulic roughness is the measure of the amount of frictional resistance water experiences when passing over land and channel features. One roughness coefficient is Manning's n-value. Manning's n is used extensively around the world to predict the degree of roughness in channels. Flow velocity is strongly dependent on the resistance to flow. An increase in this n value will cause a decrease in the velocity of water flowing across a surface.
Subglacial streams are conduits of glacial meltwater that flow at the base of glaciers and ice caps. Meltwater from the glacial surface travels downward throughout the glacier, forming an englacial drainage system consisting of a network of passages that eventually reach the bedrock below, where they form subglacial streams. Subglacial streams form a system of tunnels and interlinked cavities and conduits, with water flowing under extreme pressures from the ice above; as a result, flow direction is determined by the pressure gradient from the ice and the topography of the bed rather than gravity. Subglacial streams form a dynamic system that is responsive to changing conditions, and the system can change significantly in response to seasonal variation in meltwater and temperature. Water from subglacial streams is routed towards the glacial terminus, where it exits the glacier. Discharge from subglacial streams can have a significant impact on local, and in some cases global, environmental and geological conditions. Sediments, nutrients, and organic matter contained in the meltwater can all influence downstream and marine conditions. Climate change may have a significant impact on subglacial stream systems, increasing the volume of meltwater entering subglacial drainage systems and influencing their hydrology.
