Mountain Rivers

• Mountain Rivers - Basic

  • Rivers in mountain systems begin where there is sufficient amount of flow in a landscape, the form of channel is initiated by erosion
  • The channel head marks the boundary between diffusive and advective sediment transport processes
  • DRAW – diffusive = hillslope processes, advective = fluvial processes
• Stream Power- Mountain Rivers

  Stream power = the amount of energy that the water in a river is exerting on the sides and bottom of the river

  • Stream power = density of water * g * discharge * slope
  • Specific stream power = stream power / channel width
  • Rapid changes in stream power can occur. E.g., if water was stuck behind a chunk of glacial ice, when the system fails it would be released in a big burst (increases stream power and sediment transport)
  • Humans can also influence stream power
• Mechanisms of Bedrock Erosion

  • Solution -the removal of rocks that are carbonate rich (e.g. limestone, marble) through carbonic acid → slowly dissolved
  • Hydrolysis – water breaks down into clays and silts through the chemical reaction
  • Abrasion- sculpting, polishing, flute casts formed by sediments hitting rocks
  • Cavitation - water bubbles stuck to the surface of the rock, split so that high velocity of water hits the rock and impacts DRAW
• Sediments- Mountain Rivers

  •  Sediments are both the producer and a product of erosion
  • They are a control on the rate of bedrock erosion as they can act as both a tool for erosion, or cover the bedrock to protect it from erosion
  • Sediments are responsible for the erosion of bed rock in river systems
  • There exists a critical shear stress threshold for the erosion of bedrock. When the stream power is high enough, such as in flood events, large shear stresses erode the rocks.
  • The changes in rock competency also effect the erosion rate and stream power required to eroded rocks
• Bedrock Incision 


  • The rate of bedrock incision is dependent on the sediment supply rate to the channel
  • The bedrock erosion rate is highest when there is a balance between sediment supply to the channel and sufficient bedrock to erode
• Bedrock River Dynamics

  • The change in environmental conditions overtime leads to adjustment of the river system
  • As sediment supply is decreased, but slope and discharge increase, there is a transformation to a transport-limited river channel, with exposed bedrocks (DRAW)
  • Or, as sediment supply is increased while discharge and slope decrease, there is a transformation to an aggradation river -> where sediment deposits is greater than sediment erosion/transportation
  • Bedrock rivers occur where bedrock load is greater then suspended load
• Fill terraces

  • Identifiable by their stair-step appearance
  • Show previously deposited sediments by a river
  • Show the change in water level through time
• Straths

  • Eroded platforms into valleys
• Climatic and environmental changes- Moutain Rivers

  • A feature of climate and environmental change are fill terraces and straths.
  • Tectonics – If a river experienced uplift, there would be a change in slope, this change in slope would influence stream power and effect the pattern of deposition or erosion
  • Climatic changes- If there was increased rain during a certain period, the stream power of the river would increase, impacting transport of depositional sediments as well as erosion. And if there was less rain this would decrease the stream power and have the same effect.
• Mountain River Profiles

  • Longitudinal profile- rivers adjust towards a smooth longitudinal profile where the slope begins to gradually decrease further downstream -> towards a state of minimum energy expenditure
  • Rivers experience a dynamic equilibrium. They continuously adjust to changes in flow, sediment supply, base levels to satisfy the balance between erosive and resistance forces.
  • Long profiles try to balance changes in base levels by creating knickpoints or depositing deltas
• Knickpoints


  • When the base level regresses, a knickpoint propagates upstream. As the base level continues to fall, knickpoints are created further up to remain graded to the original base level
  • The retreat of knickpoints is a key mechanism for conveying base level fall
  • 2 main types of knickpoints: break in gradient knickpoints and break in elevation knickpoints
  • Knickpoints can be either stationary or migratory depending on the lithology, base level and tectonic settings of a river
• Break in Gradient knickpoints

  • Abrupt changes in gradient of slope in a river
• Break in Elevation knickpoints

  • Sudden change in the elevation of the river
  • These are the knickpoints commonly associated with base level changes