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Geological action of river

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The document discusses the geological action of rivers and their role in shaping landscapes. It describes how river systems erode, transport, and deposit sediment through various processes like meandering, delta formation, and flooding. Key landforms created by rivers include valleys, floodplains, terraces, and alluvial fans. The document also explains how urbanization has impacted river flow and sediment transport.

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Geological Action of River By: Mr. M.R. Siddique
MAJOR CONCEPTS 1. Running water is part of Earth’s hydrologic system and is the most important agent of erosion. Stream valleys are the most abundant and widespread landforms on the continents. 2. A river system consists of a main channel and all of the tributaries that flow into it. It can be divided into three subsystems: (a) a collecting system, (b) a transporting system, and (c) a dispersing system. 3. The most important variables in stream flow are (a) discharge, (b) gradient, (c) velocity, (d) sediment load, and (e) base level. 4. The variables in a stream constantly adjust toward a state of equilibrium. 5. Rivers erode by (a) removal of regolith, (b) downcutting of the stream channel by abrasion, and (c) headward erosion. 6. As a river develops a low gradient, it deposits part of its load on point bars, on natural levees, and across the surface of its floodplain. 7. Most of a river’s sediment is deposited where the river empties into a lake or ocean. This deposition commonly builds a delta at the river’s mouth. In arid regions, many streams deposit their loads as alluvial fans at the base of steep slopes. 8. The origin and evolution of the world’s major rivers are controlled by the tectonic and hydrologic systems.
EROSION BY RUNNING WATER Missouri River (A) A map of an area in Missouri shows the regional patterns of the valleys formed by Missouri river system .The area is about 200 km across. (B) A detailed view of the area reveals an intricate network of streams and valleys within the tributary regions of the large streams. (C) At even higher resolution, many smaller streams and valleys in the main drainage system are revealed
FLOW OF WATER AROUND A MEANDER BEND in a river follows a corkscrew pattern. Water on the outside of the bend is forced to flow faster than that on the inside of the curve. This difference in velocity, together with normal frictional drag on the channel walls, produces a corkscrew pattern. As a result, erosion occurs on the outside bank, where velocity is greatest, and deposition occurs on the inside of the bend, where velocity is at a minimum. Erosion on the outside of the meander bend and deposition on the inside cause the stream channel to migrate laterally.
MOVEMENT OF THE SEDIMENT LOAD Saltation Rolling Suspension Dissolved ions in a stream is accomplished in a variety of ways. Mud is carried in suspension. Particles that are too large to remain in suspension are moved by sliding, rolling, and saltation. Some ions are dissolved and carried in solution. Increases in discharge, due to heavy rainfall or spring snowmelt, can flush out all of the loose sand and gravel, so the bedrock is eroded by abrasion.
URBANIZATION HAS AFFECTED THE AMOUNT AND RATE OF RUNOFF. (B) Surface runoff vs. infiltration in urban stage areas. From 0% to 10% filters into the subsurface, and from 90% to 100% moves as surface runoff. Peak flow is very high and occurs in a short period of time. (A) Surface runoff vs. infiltration under natural conditions. From 80% to 100% of the surface water filters into the subsurface, and from 0% to 20% flows through the drainage system. Runoff is distributed over a long period with a small peak flow.
THE TOOLS OF EROSION Erosion are sand and gravel. Transported by a river, they act as powerful abrasives, cutting through the bedrock as they are moved by flowing water. The abrasive action of sand and gravel cut this vertical gorge through resistant limestone in the Grand Canyon, Arizona.
MEANDERS Entrenched meanders of the Colorado River resulted from down cutting of the river channel more than 300 m.
POTHOLES Potholes are eroded in a streambed by sand, pebbles, and cobbles whirled around by eddies. These potholes on the floor of the Blyde River Canyon in South Africa are about 3 m across.
FALLS The Niagara River originated as the last glacier receded from the area and water flowed from Lake Erie to Lake Ontario over the Niagara Escarpment. Erosion causes the waterfalls to migrate upstream at an average rate of about 1 m/yr.
Niagara Falls presents a spectacular scene as large volumes of water fall vertically over the cliffs of limestone. The falls are 70 m high and have migrated head ward more than 11.5 km in the last 12,300 years.
Headword erosion is constantly extending the drainage upslope so that the network of tributaries is enlarged and consumes the flat, undissected upland. Water flows as a sheet down the undissected regional slope. As it converges toward the head of a tributary valley, its velocity and volume are greatly increased, so its ability to erode also increases. The tributary valley is thus eroded headward, up the regional slope.
Stream piracy occurs where a tributary with a high gradient rapidly erodes headward and captures a tributary of another stream.
(A) Initially, a dendritic pattern formed on horizontal sedimentary rocks, which cover the older, eroded folds. (B) Regional uplift causes erosion to remove the horizontal sediments, thereby exposing the older, folded rocks at the surface. The dendrite drainage pattern is then superposed, or placed on, the folded rocks. C) Main streams cut across both resistant and nonresistant rock by channel abrasion. (D) Rapid headward erosion along exposures of weak rocks results in stream capture and modification of the dendritic pattern to a trellis pattern.
The effects of erosion of the eastern Grand Canyon are seen in this space photograph.The river flows from Lees Ferry, in the upper left, toward the lower right. At Lees Ferry, the river is just beginning to cut through the sedimentary rock sequence and has produced a profile like the one shown in the computer model. Downstream, uplift has permitted the river to cut deeper, and it has produced a sequence of profiles of alternating cliffs and slopes formed on resistant and nonresistant rock bodies. (Photograph courtesy of NASA)
Bluff Yazoo stream Backswamp Floodplain Cutoff Oxbow lake Point bar Meander bend Natural levees Alluvium Meander neck Bedrock Oxbow lake Yazoo stream Bluff
The major features of a floodplain include meanders, point bars, oxbow lakes, natural levees, back swamps, and stream channels. A stream flowing around a meander bend erodes the outside curve and deposits sediment on the inside curve to form a point bar. The meander bend migrates laterally and is ultimately cut off, to form an oxbow lake. Natural levees build up the banks of the stream, and back swamps develop on the lower surfaces of the floodplain. Yazoo streams have difficulty entering the main stream because of the high natural levees and thus flow parallel to it.
Stream meanders evolve because erosion occurs on the outside of a curved stream channel, where velocity is greatest, and deposition occurs on the inside of the curve, where velocity is low. An irregularity deflects stream flow (A) to the opposite bank and erosion begins. This start the development of a meander loop (B–D). At the same time, sediment is deposited on the inside of the bend, forming point bars. The meander enlarges and migrates laterally (C–D). Continued growth of the meander bends ultimately cuts off the channel and forms an oxbow lake (D).
Geological action of river
(A) A stream cuts a valley by normal downcutting and headward erosion processes. (B) Changes in climate base level, or other factors that reduce flow energy cause the stream to partially fill its valley with sediments, forming a broad, flat floor.
(C) An increase in flow energy causes the stream to erode through the previously deposited alluvium. A pair of terraces is left as a remnant of the former floodplain. (D) The stream shifts laterally and forms lower terraces as subsequent changes cause it to erode through the older valley fill. The evolution of stream terraces involves the deposition of sediment in a stream valley, subsequent change in the stream’s gradient, and renewed downcutting.These changes can be initiated by various factors that affect a stream’s capacity to transport sediment, such as changes in climate, changes in base level, or regional uplift.
DELTAS As a river enters a lake or the ocean, its velocity suddenly diminishes, and most of its sediment load is deposited to form a delta.
Geological action of river
The history of the Mississippi Delta involves repeated avulsion of the main channel, which has formed seven sub deltas. Most of the sediment is deposited in a small sector of the delta front. A major break in the natural levee upstream eventually diverts the entire flow to some other sector, and the process is repeated. Wave action then erodes the inactive bird-foot deltas. Previous sub deltas are indicated by numbers (1–6) according to age; (7) is the present sub delta. The active distributaries system (6 and 7) has built a major bird-foot delta in the last 500 years, the details of which are shown in this satellite photograph. (Courtesy of NASA/GSFC/METI/ERSDAC/JAROS.)
TYPES OF DELTAS. The shape of a delta depends on the balance between fluvial and marine processes. The most important processes include the ability of waves and tides to rework the sediment in the delta. The Mississippi, Nile, Mekong, and Niger deltas are dominated by different processes.
ALLUVIAL FANS Alluvial fans form in arid regions where streams enter dry basins and deposit their sediment load as the stream gradient becomes smaller.This fan is in Death Valley, California.
FLOODS Flooding is the overflow of water from the stream channel onto adjacent land that is usually dry. It is a natural process in all river systems and has occurred throughout all of geologic time. Floods of the Ganges River of Bangladesh are caused by the monsoons, which occur between June and September each year. Floods inundate the low-lying delta of the river. (Brian Blake/John Hillelson Agency)
Geological action of river

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Geological action of river

  • 1. Geological Action of River By: Mr. M.R. Siddique
  • 2. MAJOR CONCEPTS 1. Running water is part of Earth’s hydrologic system and is the most important agent of erosion. Stream valleys are the most abundant and widespread landforms on the continents. 2. A river system consists of a main channel and all of the tributaries that flow into it. It can be divided into three subsystems: (a) a collecting system, (b) a transporting system, and (c) a dispersing system. 3. The most important variables in stream flow are (a) discharge, (b) gradient, (c) velocity, (d) sediment load, and (e) base level. 4. The variables in a stream constantly adjust toward a state of equilibrium. 5. Rivers erode by (a) removal of regolith, (b) downcutting of the stream channel by abrasion, and (c) headward erosion. 6. As a river develops a low gradient, it deposits part of its load on point bars, on natural levees, and across the surface of its floodplain. 7. Most of a river’s sediment is deposited where the river empties into a lake or ocean. This deposition commonly builds a delta at the river’s mouth. In arid regions, many streams deposit their loads as alluvial fans at the base of steep slopes. 8. The origin and evolution of the world’s major rivers are controlled by the tectonic and hydrologic systems.
  • 3. EROSION BY RUNNING WATER Missouri River (A) A map of an area in Missouri shows the regional patterns of the valleys formed by Missouri river system .The area is about 200 km across. (B) A detailed view of the area reveals an intricate network of streams and valleys within the tributary regions of the large streams. (C) At even higher resolution, many smaller streams and valleys in the main drainage system are revealed
  • 4. FLOW OF WATER AROUND A MEANDER BEND in a river follows a corkscrew pattern. Water on the outside of the bend is forced to flow faster than that on the inside of the curve. This difference in velocity, together with normal frictional drag on the channel walls, produces a corkscrew pattern. As a result, erosion occurs on the outside bank, where velocity is greatest, and deposition occurs on the inside of the bend, where velocity is at a minimum. Erosion on the outside of the meander bend and deposition on the inside cause the stream channel to migrate laterally.
  • 5. MOVEMENT OF THE SEDIMENT LOAD Saltation Rolling Suspension Dissolved ions in a stream is accomplished in a variety of ways. Mud is carried in suspension. Particles that are too large to remain in suspension are moved by sliding, rolling, and saltation. Some ions are dissolved and carried in solution. Increases in discharge, due to heavy rainfall or spring snowmelt, can flush out all of the loose sand and gravel, so the bedrock is eroded by abrasion.
  • 6. URBANIZATION HAS AFFECTED THE AMOUNT AND RATE OF RUNOFF. (B) Surface runoff vs. infiltration in urban stage areas. From 0% to 10% filters into the subsurface, and from 90% to 100% moves as surface runoff. Peak flow is very high and occurs in a short period of time. (A) Surface runoff vs. infiltration under natural conditions. From 80% to 100% of the surface water filters into the subsurface, and from 0% to 20% flows through the drainage system. Runoff is distributed over a long period with a small peak flow.
  • 7. THE TOOLS OF EROSION Erosion are sand and gravel. Transported by a river, they act as powerful abrasives, cutting through the bedrock as they are moved by flowing water. The abrasive action of sand and gravel cut this vertical gorge through resistant limestone in the Grand Canyon, Arizona.
  • 8. MEANDERS Entrenched meanders of the Colorado River resulted from down cutting of the river channel more than 300 m.
  • 9. POTHOLES Potholes are eroded in a streambed by sand, pebbles, and cobbles whirled around by eddies. These potholes on the floor of the Blyde River Canyon in South Africa are about 3 m across.
  • 10. FALLS The Niagara River originated as the last glacier receded from the area and water flowed from Lake Erie to Lake Ontario over the Niagara Escarpment. Erosion causes the waterfalls to migrate upstream at an average rate of about 1 m/yr.
  • 11. Niagara Falls presents a spectacular scene as large volumes of water fall vertically over the cliffs of limestone. The falls are 70 m high and have migrated head ward more than 11.5 km in the last 12,300 years.
  • 12. Headword erosion is constantly extending the drainage upslope so that the network of tributaries is enlarged and consumes the flat, undissected upland. Water flows as a sheet down the undissected regional slope. As it converges toward the head of a tributary valley, its velocity and volume are greatly increased, so its ability to erode also increases. The tributary valley is thus eroded headward, up the regional slope.
  • 13. Stream piracy occurs where a tributary with a high gradient rapidly erodes headward and captures a tributary of another stream.
  • 14. (A) Initially, a dendritic pattern formed on horizontal sedimentary rocks, which cover the older, eroded folds. (B) Regional uplift causes erosion to remove the horizontal sediments, thereby exposing the older, folded rocks at the surface. The dendrite drainage pattern is then superposed, or placed on, the folded rocks. C) Main streams cut across both resistant and nonresistant rock by channel abrasion. (D) Rapid headward erosion along exposures of weak rocks results in stream capture and modification of the dendritic pattern to a trellis pattern.
  • 15. The effects of erosion of the eastern Grand Canyon are seen in this space photograph.The river flows from Lees Ferry, in the upper left, toward the lower right. At Lees Ferry, the river is just beginning to cut through the sedimentary rock sequence and has produced a profile like the one shown in the computer model. Downstream, uplift has permitted the river to cut deeper, and it has produced a sequence of profiles of alternating cliffs and slopes formed on resistant and nonresistant rock bodies. (Photograph courtesy of NASA)
  • 16. Bluff Yazoo stream Backswamp Floodplain Cutoff Oxbow lake Point bar Meander bend Natural levees Alluvium Meander neck Bedrock Oxbow lake Yazoo stream Bluff
  • 17. The major features of a floodplain include meanders, point bars, oxbow lakes, natural levees, back swamps, and stream channels. A stream flowing around a meander bend erodes the outside curve and deposits sediment on the inside curve to form a point bar. The meander bend migrates laterally and is ultimately cut off, to form an oxbow lake. Natural levees build up the banks of the stream, and back swamps develop on the lower surfaces of the floodplain. Yazoo streams have difficulty entering the main stream because of the high natural levees and thus flow parallel to it.
  • 18. Stream meanders evolve because erosion occurs on the outside of a curved stream channel, where velocity is greatest, and deposition occurs on the inside of the curve, where velocity is low. An irregularity deflects stream flow (A) to the opposite bank and erosion begins. This start the development of a meander loop (B–D). At the same time, sediment is deposited on the inside of the bend, forming point bars. The meander enlarges and migrates laterally (C–D). Continued growth of the meander bends ultimately cuts off the channel and forms an oxbow lake (D).
  • 20. (A) A stream cuts a valley by normal downcutting and headward erosion processes. (B) Changes in climate base level, or other factors that reduce flow energy cause the stream to partially fill its valley with sediments, forming a broad, flat floor.
  • 21. (C) An increase in flow energy causes the stream to erode through the previously deposited alluvium. A pair of terraces is left as a remnant of the former floodplain. (D) The stream shifts laterally and forms lower terraces as subsequent changes cause it to erode through the older valley fill. The evolution of stream terraces involves the deposition of sediment in a stream valley, subsequent change in the stream’s gradient, and renewed downcutting.These changes can be initiated by various factors that affect a stream’s capacity to transport sediment, such as changes in climate, changes in base level, or regional uplift.
  • 22. DELTAS As a river enters a lake or the ocean, its velocity suddenly diminishes, and most of its sediment load is deposited to form a delta.
  • 24. The history of the Mississippi Delta involves repeated avulsion of the main channel, which has formed seven sub deltas. Most of the sediment is deposited in a small sector of the delta front. A major break in the natural levee upstream eventually diverts the entire flow to some other sector, and the process is repeated. Wave action then erodes the inactive bird-foot deltas. Previous sub deltas are indicated by numbers (1–6) according to age; (7) is the present sub delta. The active distributaries system (6 and 7) has built a major bird-foot delta in the last 500 years, the details of which are shown in this satellite photograph. (Courtesy of NASA/GSFC/METI/ERSDAC/JAROS.)
  • 25. TYPES OF DELTAS. The shape of a delta depends on the balance between fluvial and marine processes. The most important processes include the ability of waves and tides to rework the sediment in the delta. The Mississippi, Nile, Mekong, and Niger deltas are dominated by different processes.
  • 26. ALLUVIAL FANS Alluvial fans form in arid regions where streams enter dry basins and deposit their sediment load as the stream gradient becomes smaller.This fan is in Death Valley, California.
  • 27. FLOODS Flooding is the overflow of water from the stream channel onto adjacent land that is usually dry. It is a natural process in all river systems and has occurred throughout all of geologic time. Floods of the Ganges River of Bangladesh are caused by the monsoons, which occur between June and September each year. Floods inundate the low-lying delta of the river. (Brian Blake/John Hillelson Agency)

Editor's Notes

  1. Me.siddique@rediff.com