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Gravity's Effect on Erosion & Deposition

Gravity is a constant force of attraction that makes all materials on the earth tend to fall toward the center of the planet. It attempts to create an equilibrium by moving materials from areas of higher elevations to lower elevations. By itself gravity moves materials downhill through various processes such as landslides, mudslides, and avalanches. Collectively these are all forms of erosion, the process of moving sediment from one location to another. Gravity is the force powering erosion, but the most important agent of erosion is water.

The Erosion Process
- Erosion can be broken down further into a sequence of three primary events: (1) Detachment or breaking the rock substrate down by physical weathering into smaller particles or by chemical weathering into compounds which can either be carried or dissolved in water; (2) Entrainment which results from water running down the mountains and valleys as the force of gravity pulls the particulate matter and dissolved material along in its flow; and (3) Transport of the particle matter can occur as long as the velocity of the transport water medium is fast enough to move material horizontally along with its flow.
Water Velocity and Stream Volume
- Eroded particles and chemicals are carried along in the water. The amount of eroded particulate matter carried by a stream or river is its sediment load. The amount of load the water can carry depends on its volume of water flow, its velocity, and how the stream curves, runs straight or meanders down its course. The water volume results from the rainfall and snow melt amounts picked up by the stream at the higher elevations. Its velocity depends on the slope as well as the volume of the stream. The size of the particles transported by the water will increase as the velocity increases.
  
  There is a direct relationship between the erosion and the velocity rate, in that the erosion increases with the increase in velocity. A fast flowing stream can carry microscopic sized material up to pebble sizes and even boulders in extreme cases. The fastest water flows naturally on the outside of a curve so the curve erodes away more quickly. The water moves at its fastest rate in the center of a straight moving stream.
The Deposition Process
- When the water reaches level ground, the flow slows down and the water drops its sediment load gradually in direct relationship to its decrease in velocity. The larger particles fall out first, and as it continues to slow down, the pebbles, sands and clays will eventually all settle out.
  
  In curved streams, the water velocity is slower on the inside of the curves, which results in deposition build up on the inner curves while the faster flowing water on the outside curves picks up sediment and erodes them. Because of this process, the courses of streams and rivers tend to move sideways.
Other Gravitational Effects on Erosion
- The most dangerous and sudden examples of gravity-caused movement are landslides, rock falls and avalanches. Large amounts of rainfall acts with gravity to cause normally dry clay or sandy soils with sparse vegetation to form mudflows or mudslides. These usually follow river channels and wash out bridges, trees, and buildings in their path of destruction.

Nature