Streams do a large of geological work. Flowing water from upstream toward downstream possesses a tremendous amount of energy. The geological work of streams is to erode the valley, transport the eroded material, and deposit the same in the lower reaches at favorable sites.
The stream causes erosion in four ways: (i) chemical action, (ii) hydraulic action, (iii) abrasion, and (iv) attrition.
(1). Chemical Action:
It includes the solvent and chemical action of water on country rocks. The chemical decay works along joints and cracks and thus helps in breaking the bedrocks.
(2). Hydraulic Action:
The swiftly flowing water hammers the uneven faces of jointed rocks exposed along its channel and removes the jointed blocks. This process of erosion is called Hydraulic-action erosion. At the bottom of water-falls, the channels are eroded at an enormously rapid ratio by the hydraulic action.
The flowing water uses rock fragments such as pebbles gravels, and sands as a tool for scratching and grinding the sides and floor of the valley. This process of erosion is called “Abrasion”.
Attrition is the breaking down of the transported material due to their mutual collision with one another. The attrition caused the larger pieces of rock to become more rounded and finer/smaller in size.
In addition to the above, the streams acquire their load by many other means. Much of the material carried by a stream is contributed by underground water, overland flow, and mass wasting.
The amount of solid material transported by a stream is called its load. The streams transport it in three ways: (i) Transportation in dissolved form, (ii) transportation in suspended condition, and (iii) Transportation along the bottom (bedload).
(1). Dissolved Load:
The dissolved load is brought to the stream by groundwater. Some amount of it is also acquired directly from soluble rocks, which occur along the stream’s course.
(2). Suspended Load:
Suspended Load forms the major portion of the load carried by streams. Usually only smaller particles such as clay and silt travel in the suspension, but during floods, much larger particles are carried this way.
(3). Bed Load:
The forward force of moving water acts more directly on the larger grains at the bottom pushing, rolling, and sliding them along. The particles moved in this way constitute the “bedload” of a stream. Locally the medium size material may travel partly by rolling as bed load and partly in suspension This process of intermittent jumping is called “Saltation”. In saltation, the heavy particles are lifted occasionally for a few seconds by a swift eddy.
The velocity of a stream is affected by a number of factors, including gradient, channel size and shape, load, and discharge. The increase of velocity increases the transporting power of a river as much as the 6th power of the velocity.
Transporting Power ∝V6
It means that during floods the transporting power of a stream suddenly rises very much and it becomes capable of moving big boulders which would otherwise remain quite immovable.
The loose rock materials transported by a stream downstream, are deposited where the velocity of flowing water is reduced. The sorting of the material takes place automatically as the large and heavier particles settle quickly while the smaller and lighter ones continue their journey further ahead. The material which a stream deposits as sediment is called “alluvium” or “alluvial deposits”.