Rivers Key Terms Flashcards
Attrition
The impacting of particles upon one another, causing their gradual reduction in size.
Base level
The theoretical level down to which (but not below) the river can erode its valley. The ultimate base level is sea level.
Base flow/groundwater flow
Process by which water moves downslope below the water table under the influence of gravity
Bed load
The coarser material carried along the bed of a river by the force of streamflow
Capacity
The total load actually transported by the river
Catchment area
The area of land which water drains into a river
Cavitation
A form of hydraulic action where bubbled of air collapse and resulting shockwaves hit and slowly weaken poorly consolidated banks. Slowest and least effective erosion process.
Channel efficiency
The ability of a river channel to transport its water and its load
Competence
The diameter of the largest particle that a river can carry at a given velocity
Discharge
The volume of water passing a fixed position in a given time. Cross sectional area x average velocity. Measured in cumecs.
Drainage basin
The area of land drained by a river and its tributaries
Entrainment
The process by which sediment/rocks are drawn into a moving body of water
Ephemeral channel
A channel which only contains water after intermittent downpours of rain, separated by longer periods when channel is dry.
Evaporation
The process by which water is transferred from the earth’s surface to the atmosphere. It represents an output from the drainage basin.
Evapotranspiration
Process accounting for the total output of water vapour from the drainage basin system to the atmosphere through a combination of evaporation and transpiration
Floodplain
The area on either side of a river channel consisting of alluvium across which the river spills when it’s over bank full
Graded profile
The concave long profile of a stream where erosion, transportation and deposition are in a state of equilibrium. Discharge and available energy are in balance with the available load.
Groundwater
The water filling all the pore spaces in the zone of saturation
Groundwater store
Th water held below water table in aquifers
Headward erosion
Erosion occurring at the source of a stream, resulting in the lowering of the land surface, allowing stream to rise further back or further uphill. Possible for streams to erode through watershed allowing them to capture streams in other valleys.
Helicoidal flow
A type of corkscrew flow superimposed on the primary downstream flow of a river. Flow moves across river from inside bank of meander to outside. The flow eroded meanders on outside bend and deposits on inside.
Hjulstrom’s curve
A logarithmic graph showing the relationship between stream velocity and particle size (competence)
Hydraulic Action / Scour
A process of fluvial erosion whereby water washes beds and banks. As water flows towards other banks of the meander, water and air is compressed into cracks and blasts out material as pressure is released.
Hydraulic radius
Measure of channel efficiency. Divide cross sectional area by wetted perimeter. The higher the ratio = the more efficient. Efficiency increases downstream - depth and width increase so less discharge is in contact with the bed and banks = less friction slows it down
Hydrograph
Storm hydrograph records the surge in discharge due to a single rainfall event. Plots discharge against time. Stormflow shows surface and subsurface discharge due to the storm, and base flow shows continuous release of groundwater into river.
Flashy hydrograph
Rapid response to storm with short lag time and steep rising limb
Hydrological cycle
Movement of water between the land, air and sea
Incised meanders
A river meander which cuts down deeply into the bedrock due to rejuvenation
Infiltration
Process by which water moves downwards from the surface to soil pores.
Infiltration capacity
The maximum rate which water can be absorbed by a soil or rock in a given condition
Infiltration rate
The rate of water absorption expressed in millimetres per minute
Interception
Process by which incoming precipitation is trapped by vegetation and buildings. Water is then transferred to the earth’s surface or evaporated.
Interlocking Spurs
Protrusions of higher land characteristic of the upper sections of a river valley resulting from the river’s winding course and more resistant rocks.
Isovel
A lining joining pointed of equal velocity in a river channel.
Knick point
A break of slope in the long profile of river due to rejuvenation. Because of the renewed down cutting a new long profile is created and where this intersects the former long profile a Knick point is created.
Lag time
The time between peak precipitation and peak discharge
Laminar flow
Horizontal movement of water
Levee
A bank of sediment along the edge of a river channel, deposited naturally with floodwaters. Often raised and strengthened artificially to contain floodwaters.
Load
Pebbles, particles and dissolved matter carried by a river
Long profile
A river course from its source in an upland area along its channel to its mouth.
Meander
A bend in a river with a sinuosity ratio of more than 1:5
Natural levee
An embankment on the sides of a river formed by the deposition of silt during flooding
Overland flow / surface run off
Process by which water moves downslope over the earth’s surface in unconfined channels under the influence of gravity. Saturated overland flow occurs when excess water flows across the ground surface due to subsurface being saturated.
Percolation
Process by which water moves vertically downwards through soil and rock
Permeability
Ability of rocks, soil or sediment to permit water to flow through it. Permeable rocks allow water to pass through them via bedding planes and joints e.g. Limestone. Impermeable rocks like granite produce more run off and streams.
Point bar
Sediment deposited under water on the inside bed of a meander
Pools and riffles
The alternating pattern of deep pools and is reaches where a river flows across gravel bars. Distribution if regularly spaced with the distance between pools being 5-7 times greater than the channel width.
Porous rocks
Allow water to pass through them via pores e.g. Sandstone and chalk
Recessional limb
Part of the hydrograph showing the speed at which the discharge declines after a peak back to its pre storm level
Precipitation
Process by which water is transferred from the atmosphere to surface in the form of snow, sleet, hail, rain, fog. Input into drainage system.
Recharge
Water added to groundwater or soil to replace that lost by evaporation or transpiration
River regime
Variation in discharge from January to December
Rejuvenation
The state of a river when it’s energy has been revitalised leading to an increase in erosional capability. Results from a fall in base level, due to fall in sea level or uplift of land.
Rising limb
The part of a flood hydrograph which shows how quickly floodwaters rise due to storm event
River cliff / Bluff
Erosional landform on the outside bend of a meander
Saltation
Bed load is transported by bounding motion of lighter particles
Sinuosity
The curving nature of the meander. The actual channel length divided by the straight line distance.
Soil moisture budget
The balance between moisture inputs and outputs in a soil over time.
Soil moisture deficit
A shortage of soil moisture. Evapotranspiration > precipitation- in summer
Soil moisture recharge
A period after summer or drought when lost moisture has to be replaced
Soil moisture surplus
Occurs after a period of recharge when precipitation > evapotranspiration (winter). Soils saturated = surface run off dominates.
Dissolved load / Solute
Part of load that’s in solution
Solution
Process by which minerals in rocks making up the bed and bank are dissolved by carbonic or humic acids in water. Occurs continuously and unaffected by discharge or velocity.
Suspension
Lighter material held in water flow by turbulent eddies.
Stem flow
Intercepted precipitation which runs down stems of plants to ground
Storm hydrograph
Graph showing discharge against time following single precipitation event
Suspended load
Solid particles carried within the current but not touching bed
Thalweg
The longitudinal profile of a streambed along its line of max velocity
Through fall
Incoming precipitation which falls through vegetation and is NOT intercepted
Through flow
Process by which water moves through the soil under the influence of gravity
Traction
Bed load is transported by rolling of heavy particles
Transpiration
Process by which water is transferred from vegetation to atmosphere via stomata on leaves - output from drainage system
Water table
The upper boundary of the saturated rock beneath which cracks and pores are full of water. Upper surface of ground water layer. Rises in winter and falls in summer in response to variations in precipitation
Watershed
The boundary between one drainage basin and another marked by a ridge of high land beyond which precipitation will fall into adjacent basins
Wetted perimeter
Total length of water in a cross section of a river in contact with the bed and banks
Abrasion
Process of fluvial erosion whereby beds and banks are eroded by water borne sediments
