Cross-profile = section taken sideways across a river channel and/or valley
○ Channel cross-profile = only includes the river
○ Valley cross-profile = includes channel, valley floor and slopes
Cross-profile changes downstream:
● Upper course
○ Very narrow → only few metres wide
○ Very shallow → less than 0.25m deep
● Middle course
○ Becomes wider by several metres
○ Much deeper (over a metre deep)
● Lower course
○ Becomes wider
■ Small river, 5 – 10m wide
■ Large river (eg/ Severn) →
3.2km wide at estuary mouth
Reasons for change:
○ Upper course
■ River erodes bed by hydraulic and abrasive action
■ Tributaries increase volume of water and therefore kinetic energy
■ Allows it to cut a much deeper channel with increased distance
○ Middle course
■ Channel becomes wider as gradient becomes less steep
■ Less vertical erosion → lateral erosion is dominant and makes channel wider
Valley cross-profile changes downstream:
● Upper course → shallow, narrow channel
○ Steep v-shaped valley
○ Vertical erosion dominant (hydraulic action/abrasion/attrition)
○ Hard rock → valley sides not broadened out by weathering/erosion
○ Slopes remain steep
● Middle course → wider, deeper channel
○ Valley broadens out
○ Small flood plain
○ Less steep gradient
■ River can meander
■ Lateral erosion
○ Rate of weathering increases on softer rocks
○ More energy used in lateral erosion than vertical erosion
■ Not able to remove weathered material on the river bed
■ Material builds up valley floor
● Lower course → widest, deepest channel
○ Large expanse of floodplain
○ Deposition from floods builds up flood plain
○ Builds up and widens valley
