Streams, Rivers, and Deltas
by Owen Borville
January 30, 2021
Learning, Geology, Science
The water cycle is a continuous cycle that occurs on Earth’s surface and includes precipitation as rain or snow falling on the land surface. Evaporation or transpiration from plants returns water back to the atmosphere. Water percolates down into the Earth to become groundwater. Runoff over the land surface flows into streams, rivers, and eventually lakes and oceans. Finally, evaporation from lakes and oceans sends water back into the atmosphere. Runoff is the flow of water over land surfaces (including paved surfaces) commonly from precipitation when the ground is saturated with water. Infiltration capacity is the maximum rate the soil or landmass can absorb water from rainfall events.
Laminar flow describes the flow of a stream that is low velocity, orderly and streamlined, and flows in a straight line. Turbulent flow is stream flow which increases as stream velocity increases and can lift and carry sediment grains downstream. The swirling turbulent flow of a very rapid stream can break chunks of fractured bedrock off the channel bottom.
Sheet flow is a thin layer of water flowing over land. Channel flow is a type of surface water flow which occurs in a channel shaped depression and can be the size of a small stream or a large river. Base flow is groundwater that flows into a stream channel.
The gradient or slope of a stream is the vertical drop over a fixed distance. Stream velocity is dependent on the shape of the channel and the roughness of the channel. Friction near the banks of the stream and the bottom reduces the velocity of the flow. A shallow channel would have a relatively lower velocity as well as a narrow channel. The stream velocity would be the highest at the center surface of the stream. The discharge of a stream is the amount of water in a stream that passes a specific point in a given period of time. Stage is the height of the water level at a particular point in a stream.
Dissolved load is the dissolved material moving downstream in the river that cannot be seen and will not settle out of the water except through evaporation of the water and precipitation of the dissolved material. The suspended load of a stream carries sediment or sedimentary particles in suspension and includes silts and clays which are light enough to be swept along in the current without touching the bottom of the stream bed. Bed load is the sediment which is moved across the bed of a river or stream channel and contains the largest particles in the stream bed. The competence of a stream refers to the heaviest particles a stream can erode and thus transport. Stream capacity is the maximum amount of solid load (including bed load and suspended load) a stream can carry and depends on the discharge and the velocity of the stream. Saltation consists of particles transported as bed load such as sand grains, which stay close to the bed of the stream but are not in contact continuously. These particles move along in a series of steps. These steps include a particle jumping up into the flow. Then the particle is pulled forward by the discharge. The particle then hits the bottom and bounces up again or ejects another particle from the bed of the stream up into the flow.
A braided stream or river is characterized by a complex network of branches that continuously separate and reunite. Streams braid when they have a much greater sediment load than they can carry. Point bars are islands of sediment deposition which form in braided streams. Braided streams are commonly found in areas with little vegetation. Braided streams are also called anastomosing streams.
A meandering stream is a stream channel which forms sinuous alternating curve shapes. The outer side of the channel is eroded (called a cut bank) and the eroded sediment is deposited on the inner side of the following meander channel (called a point bar). The curved channels are called meanders.
Oxbow lakes are long, curved shaped lakes formed when a meandering stream breaks one of its meandering channels to form a straighter channel.
Point bars are islands of sediment common in braided streams which form where sediment saturated rivers flow across broad and easily eroded slopes. Point bars also commonly occur in meandering rivers, which typically flow across wide valleys and lowlands in a series of S-shaped curves where erosion and deposition occur continuously along the banks of the river. On the outside of a curve where velocity is high and erosion normally takes place, cut banks supply the sediment to the next point bar deposited on the low velocity inside meander curve. Cut banks are erosional features that occur in meandering streams on the outer part of the meander due to the high velocity of the stream.
A floodplain is a relatively flat surface adjacent or along both sides of a stream or river. During floods, when the stream overflows its banks, water flows over the flood plain. Streams construct flood plains that accommodate their maximum flood capacity.
A levee is a natural or manmade earthen barrier along the edge of a stream, lake, or river which can protect the adjacent land from flooding. Natural levees contain sediment deposited on each edge or margin of the stream in which the levee rises vertically as sediment builds up during flooding. This can cause the stream level to rise higher than the floodplain level. Avulsion occurs when a river channel breaks away at a particular point and creates a new channel while the old channel is partially or completely drained of water.
Types of Deltas
A delta is the accumulation of sediment at the mouth of a river or where a river enters a larger body of water such as an ocean, sea, or lake. A prograding delta occurs as river velocity decreases near the delta and sediment is deposited, which enlarges the delta. This process is called progradation and causes sea level to be lowered. As sediment is deposited at the delta, the finest grained sediment is deposited at the bottom and at the farthest distance from the mouth of the river. Progressively coarser and larger grained sediment is deposited on top of and behind the finer grained sediment. Three main types of deltas describe their formation: river dominated deltas, tide dominated deltas, and wave dominated deltas.
River dominated deltas form when river distributary channels prograde or extend sediment into the sea. The distributary channels of river dominated deltas extend farther seaward than in other types of deltas. The distributary channel also changes direction over time so that there is one active distributary channel and several former distributary channels. When sediment builds up excessively, the river slows down and eventually shifts to a faster pathway to the sea. The older channel is abandoned and the river shifts to a new channel travelling in a different direction toward the sea. The deltas of the older channels are submerged underwater. A classic example of a river dominated delta is the Mississippi River delta.
Tide dominated deltas are strongly influenced by erosion from tidal processes caused by large tides. Tide dominated deltas produce long sand bars and ridges which are perpendicular to the shoreline. Tides cause the sea level to rise and fall and are caused by the gravitational attraction of the Earth, the Sun, and the Moon. Tide dominated deltas have a dendritic structure and have many distributary channels. New distributaries are formed during floods or storm surges. Tide dominated deltas can be partially submerged into the sea. An example of a tide dominated delta is the Ganges Delta in India and Bangladesh.
Wave dominated deltas are strongly influenced by wave erosion but the amount of sediment deposition is more than the erosion removes. Wave dominated deltas have a shape that resembles the Greek letter Delta. The Nile River delta is an example of a wave dominated delta.
Alluvial Fans: Alluvium is sediment transported and deposited by streams. Alluvial fans are fan-shaped piles of sediment that form where a rapidly flowing mountain stream enters a relatively flat valley. As water slows down, sediment or alluvium is deposited gradually into a fan shape.
A drainage basin is a sloping land area drained by a stream in which all of the precipitation entering the basin flows into the stream system and in most cases exits at a single point. Several drainage patterns exist:
Dendritic drainage is the drainage pattern of a river resembling tree branches in which smaller or secondary branches merge into larger branches and eventually merge into the main channel that exits the drainage basin at the bottom of the slope. A rectangular drainage pattern is controlled by joints and fractures in the bedrock. In rectangular drainage, streams tend to form right angles as the streams follow the depressions formed by joints and fractures. A trellis drainage pattern occurs where alternating layers of strong erosion resistant rocks and weaker, softer rocks cause the streams to commonly branch out as a pattern reflecting the weaker valley rocks and the stronger hill forming rocks. An annular drainage pattern in rivers and streams forms a circular or ring pattern commonly caused by dome or basin features.
Parallel drainage is a drainage pattern in streams consisting of main channels and branching channels that are parallel to each other in the down slope direction. Radial drainage is a drainage pattern that carries water away from a central high point such as a volcano or mountain. Anastomosing or anabranching describes a river pattern which has divided into several interconnected channels flowing adjacent to each other and separated by small islands.
Base level is the maximum depth that a stream can erode its valley. The ultimate base level that a stream can theoretically erode is sea level. A graded stream is a stream having the correct slope and channel characteristics to maintain the velocity required to transport the material supplied to the steam. A graded stream is in an equilibrium state where it is neither eroding nor depositing material but is simply transporting the material. Changes in the base level, discharge, channel shape or size, or sediment load will result in erosion or deposition until a new equilibrium state is found.
A stream terrace is a former floodplain of a stream which is exposed because the stream is flowing at a lower level. An incised meander is a meander of a river that is cut or eroded well below the level of the former floodplain of the river and into the bedrock. Incised meanders result from uplift of the land by plate tectonics and down-cutting erosion along the thalweg, or the deepest part of a river channel. The down-cutting is so rapid that the river maintains a meandering pattern while deepening its valley. Incised meanders can produce large canyons, as seen in Utah. Superposed streams are streams that have cut through a ridge or mountain lying in the path of the stream. Superposed streams occur because the stream has been imposed on a landscape in which the previous landscape structure has been eroded and removed.
by Owen Borville
January 30, 2021
Learning, Geology, Science
The water cycle is a continuous cycle that occurs on Earth’s surface and includes precipitation as rain or snow falling on the land surface. Evaporation or transpiration from plants returns water back to the atmosphere. Water percolates down into the Earth to become groundwater. Runoff over the land surface flows into streams, rivers, and eventually lakes and oceans. Finally, evaporation from lakes and oceans sends water back into the atmosphere. Runoff is the flow of water over land surfaces (including paved surfaces) commonly from precipitation when the ground is saturated with water. Infiltration capacity is the maximum rate the soil or landmass can absorb water from rainfall events.
Laminar flow describes the flow of a stream that is low velocity, orderly and streamlined, and flows in a straight line. Turbulent flow is stream flow which increases as stream velocity increases and can lift and carry sediment grains downstream. The swirling turbulent flow of a very rapid stream can break chunks of fractured bedrock off the channel bottom.
Sheet flow is a thin layer of water flowing over land. Channel flow is a type of surface water flow which occurs in a channel shaped depression and can be the size of a small stream or a large river. Base flow is groundwater that flows into a stream channel.
The gradient or slope of a stream is the vertical drop over a fixed distance. Stream velocity is dependent on the shape of the channel and the roughness of the channel. Friction near the banks of the stream and the bottom reduces the velocity of the flow. A shallow channel would have a relatively lower velocity as well as a narrow channel. The stream velocity would be the highest at the center surface of the stream. The discharge of a stream is the amount of water in a stream that passes a specific point in a given period of time. Stage is the height of the water level at a particular point in a stream.
Dissolved load is the dissolved material moving downstream in the river that cannot be seen and will not settle out of the water except through evaporation of the water and precipitation of the dissolved material. The suspended load of a stream carries sediment or sedimentary particles in suspension and includes silts and clays which are light enough to be swept along in the current without touching the bottom of the stream bed. Bed load is the sediment which is moved across the bed of a river or stream channel and contains the largest particles in the stream bed. The competence of a stream refers to the heaviest particles a stream can erode and thus transport. Stream capacity is the maximum amount of solid load (including bed load and suspended load) a stream can carry and depends on the discharge and the velocity of the stream. Saltation consists of particles transported as bed load such as sand grains, which stay close to the bed of the stream but are not in contact continuously. These particles move along in a series of steps. These steps include a particle jumping up into the flow. Then the particle is pulled forward by the discharge. The particle then hits the bottom and bounces up again or ejects another particle from the bed of the stream up into the flow.
A braided stream or river is characterized by a complex network of branches that continuously separate and reunite. Streams braid when they have a much greater sediment load than they can carry. Point bars are islands of sediment deposition which form in braided streams. Braided streams are commonly found in areas with little vegetation. Braided streams are also called anastomosing streams.
A meandering stream is a stream channel which forms sinuous alternating curve shapes. The outer side of the channel is eroded (called a cut bank) and the eroded sediment is deposited on the inner side of the following meander channel (called a point bar). The curved channels are called meanders.
Oxbow lakes are long, curved shaped lakes formed when a meandering stream breaks one of its meandering channels to form a straighter channel.
Point bars are islands of sediment common in braided streams which form where sediment saturated rivers flow across broad and easily eroded slopes. Point bars also commonly occur in meandering rivers, which typically flow across wide valleys and lowlands in a series of S-shaped curves where erosion and deposition occur continuously along the banks of the river. On the outside of a curve where velocity is high and erosion normally takes place, cut banks supply the sediment to the next point bar deposited on the low velocity inside meander curve. Cut banks are erosional features that occur in meandering streams on the outer part of the meander due to the high velocity of the stream.
A floodplain is a relatively flat surface adjacent or along both sides of a stream or river. During floods, when the stream overflows its banks, water flows over the flood plain. Streams construct flood plains that accommodate their maximum flood capacity.
A levee is a natural or manmade earthen barrier along the edge of a stream, lake, or river which can protect the adjacent land from flooding. Natural levees contain sediment deposited on each edge or margin of the stream in which the levee rises vertically as sediment builds up during flooding. This can cause the stream level to rise higher than the floodplain level. Avulsion occurs when a river channel breaks away at a particular point and creates a new channel while the old channel is partially or completely drained of water.
Types of Deltas
A delta is the accumulation of sediment at the mouth of a river or where a river enters a larger body of water such as an ocean, sea, or lake. A prograding delta occurs as river velocity decreases near the delta and sediment is deposited, which enlarges the delta. This process is called progradation and causes sea level to be lowered. As sediment is deposited at the delta, the finest grained sediment is deposited at the bottom and at the farthest distance from the mouth of the river. Progressively coarser and larger grained sediment is deposited on top of and behind the finer grained sediment. Three main types of deltas describe their formation: river dominated deltas, tide dominated deltas, and wave dominated deltas.
River dominated deltas form when river distributary channels prograde or extend sediment into the sea. The distributary channels of river dominated deltas extend farther seaward than in other types of deltas. The distributary channel also changes direction over time so that there is one active distributary channel and several former distributary channels. When sediment builds up excessively, the river slows down and eventually shifts to a faster pathway to the sea. The older channel is abandoned and the river shifts to a new channel travelling in a different direction toward the sea. The deltas of the older channels are submerged underwater. A classic example of a river dominated delta is the Mississippi River delta.
Tide dominated deltas are strongly influenced by erosion from tidal processes caused by large tides. Tide dominated deltas produce long sand bars and ridges which are perpendicular to the shoreline. Tides cause the sea level to rise and fall and are caused by the gravitational attraction of the Earth, the Sun, and the Moon. Tide dominated deltas have a dendritic structure and have many distributary channels. New distributaries are formed during floods or storm surges. Tide dominated deltas can be partially submerged into the sea. An example of a tide dominated delta is the Ganges Delta in India and Bangladesh.
Wave dominated deltas are strongly influenced by wave erosion but the amount of sediment deposition is more than the erosion removes. Wave dominated deltas have a shape that resembles the Greek letter Delta. The Nile River delta is an example of a wave dominated delta.
Alluvial Fans: Alluvium is sediment transported and deposited by streams. Alluvial fans are fan-shaped piles of sediment that form where a rapidly flowing mountain stream enters a relatively flat valley. As water slows down, sediment or alluvium is deposited gradually into a fan shape.
A drainage basin is a sloping land area drained by a stream in which all of the precipitation entering the basin flows into the stream system and in most cases exits at a single point. Several drainage patterns exist:
Dendritic drainage is the drainage pattern of a river resembling tree branches in which smaller or secondary branches merge into larger branches and eventually merge into the main channel that exits the drainage basin at the bottom of the slope. A rectangular drainage pattern is controlled by joints and fractures in the bedrock. In rectangular drainage, streams tend to form right angles as the streams follow the depressions formed by joints and fractures. A trellis drainage pattern occurs where alternating layers of strong erosion resistant rocks and weaker, softer rocks cause the streams to commonly branch out as a pattern reflecting the weaker valley rocks and the stronger hill forming rocks. An annular drainage pattern in rivers and streams forms a circular or ring pattern commonly caused by dome or basin features.
Parallel drainage is a drainage pattern in streams consisting of main channels and branching channels that are parallel to each other in the down slope direction. Radial drainage is a drainage pattern that carries water away from a central high point such as a volcano or mountain. Anastomosing or anabranching describes a river pattern which has divided into several interconnected channels flowing adjacent to each other and separated by small islands.
Base level is the maximum depth that a stream can erode its valley. The ultimate base level that a stream can theoretically erode is sea level. A graded stream is a stream having the correct slope and channel characteristics to maintain the velocity required to transport the material supplied to the steam. A graded stream is in an equilibrium state where it is neither eroding nor depositing material but is simply transporting the material. Changes in the base level, discharge, channel shape or size, or sediment load will result in erosion or deposition until a new equilibrium state is found.
A stream terrace is a former floodplain of a stream which is exposed because the stream is flowing at a lower level. An incised meander is a meander of a river that is cut or eroded well below the level of the former floodplain of the river and into the bedrock. Incised meanders result from uplift of the land by plate tectonics and down-cutting erosion along the thalweg, or the deepest part of a river channel. The down-cutting is so rapid that the river maintains a meandering pattern while deepening its valley. Incised meanders can produce large canyons, as seen in Utah. Superposed streams are streams that have cut through a ridge or mountain lying in the path of the stream. Superposed streams occur because the stream has been imposed on a landscape in which the previous landscape structure has been eroded and removed.
