Key Terms

📖 Key Terms

Hydraulic action

Erosion by the force and pressure of moving water entering cracks in rock, compressing air, and weakening the rock structure

Abrasion (corrasion)

Erosion of the bed and banks by the river's own load — particles are dragged across surfaces, wearing them away like sandpaper

Attrition

The wearing down of load particles as they collide with each other and with the bed — particles become smaller and rounder downstream

Corrosion (solution)

Chemical erosion of soluble rock (especially limestone and chalk) by slightly acidic river water — rock is dissolved and carried away in solution

Traction

Transportation of large boulders and cobbles by rolling them along the riverbed

Saltation

Transportation of medium-sized pebbles in a bouncing or hopping motion along the riverbed

Suspension

Transportation of very fine particles (silt and clay) carried within the body of the flowing water — makes rivers appear brown or cloudy

Solution

Transportation of dissolved minerals within the water — invisible, but significant in limestone and chalk catchments

Discharge

The volume of water passing a given point in a river per second, measured in cumecs (m³/s) — increases downstream as tributaries add water

Competence

The maximum size of particle a river can carry at a given velocity — increases with velocity; when velocity drops, competence drops and deposition occurs

Capacity

The total amount (volume) of load a river can carry — increases with discharge

Meander

A sinuous (S-shaped) bend in a river formed by unequal erosion on the outside of bends and deposition on the inside

River cliff

A steep, near-vertical bank on the outside of a meander bend, formed by lateral erosion undercutting the bank

Point bar (slip-off slope)

A gentle slope of deposited sediment on the inside of a meander bend, built up by reduced velocity and helical flow

Ox-bow lake

A crescent-shaped lake formed when a highly exaggerated meander is cut off from the main channel, usually during a flood event

Floodplain

The flat, wide valley floor flanking the lower course of a river, built up from alluvium deposited by repeated flooding events over thousands of years

Levée

A natural embankment of sediment built up alongside the river channel by repeated deposition of coarse material during flood events

Alluvium

Fine silt and clay deposited by a river during flooding — builds the floodplain and creates highly fertile agricultural soils

Interlocking spur

Projecting ridges of resistant rock in the upper course that the river winds between, creating a zigzag plan view — they interlock like the teeth of a zip

Plunge pool

A deep, circular pool at the base of a waterfall formed by the abrasive action of falling water and swirling boulders

Gorge

A steep-sided, narrow valley formed in the wake of a retreating waterfall

Hydraulic radius

The ratio of cross-sectional area to wetted perimeter — a measure of channel efficiency; higher hydraulic radius = lower friction = faster flow