Running Water as a Geomorphic Agent

Running Water as a Geomorphic Agent

• In humid regions with heavy rainfall, running water is a significant agent in land surface degradation.
• Running water has two components: overland flow (as a sheet on the land surface) and linear flow (as streams and rivers in valleys).

Erosional Landforms

• Vigorous, youthful rivers on steep gradients are associated with erosional landforms.
• Stream channels become gentler over time due to erosion, leading to increased deposition.

Depositional Forms

• Streams flowing over steep slopes lead to depositional forms on a small scale.
• Over medium to gentle slopes, streams contribute to larger-scale deposition.

Effect on Landforms

• Continued erosion reduces downward cutting and increases lateral erosion, transforming hills and valleys into plains.
• Sheet erosion by overland flow occurs on the land surface.
• Overland flow concentrates into narrow to wide paths, forming rills and gullies.
• Friction of flowing water removes materials from the land surface, leading to rill formation.
• Rills and gullies further deepen, widen, lengthen, and unite to form valleys.

Stages of Valley Development

• Early stages: down-cutting, removing irregularities like waterfalls.
• Middle stages: slower stream bed cutting and severe lateral erosion of valley sides.
• Valley sides are gradually reduced to lower slopes, forming a peneplain (lowland with faint relief).

Stages of River

Youth Stage (Erosion dominates)

• Few streams, poor integration, and shallow V-shaped valleys.
• Trunk streams may have narrow floodplains or none at all.
• Broad and flat stream divides with marshes, swamps, and lakes.
• Meanders may develop on broad upland surfaces, possibly entrenching themselves.
• Waterfalls, Georges, and rapids may exist in areas with exposed hard rock bodies.

Mature Stage (Transportation dominates)

• Plenty of streams with good integration, increasing water volume with the confluence of many tributaries.
• Vertical erosion slowly starts to replace with lateral erosion or erosion from both sides of the channel.
• River channel causes the gradual disappearance of its V-shaped valley (not completely).
• Trunk streams have wider floodplains, allowing for alluvial fans, piedmont alluvial plains, meanders.
• Flat and broad inter-stream areas, swamps, and marshes of youth disappear.
• Sharp stream divides with no waterfalls or rapids.

Old Age Stage (Deposition dominates)

• Few smaller tributaries with gentle gradients.
• Streams meander freely over vast floodplains with natural levees, oxbow lakes, etc.
• Broad and flat stream divides with lakes, swamps, and marshes.
• Landscape mostly at or slightly above sea level.

Erosion Types

• Corrasion or Abrasion: rock particles break off additional rock fragments through bouncing, scraping, and dragging.
  • Vertical corrosion: acts downward.
  • Lateral corrosion: acts on both sides.
• Corrosion or Solution: chemical or solvent action of water on soluble or partly soluble rocks.
• Hydraulic Action: mechanical loosening and sweeping away of material by the sheer force of river water itself.

Transportation types

• Traction: heavier and larger rock fragments like gravel, pebbles, etc. are forced by the flow of the river to roll along its bed.
• Saltation: fragments of rocks bounce and jump along the riverbed.