Rainwater that lands on the ground, apart from losses such as infiltration and evaporation, flows along a certain direction and path under the neutralizing action.

This type of water flow is called surface runoff. Surface runoff erodes the ground over the long term, forming gullies, which eventually develop into streams and converge to form rivers.

The formation and development of river valleys involve not only the erosive action of rivers but also transportation and deposition processes. The carrying capacity of a river depends on the velocity of its flow. Generally, the carrying capacity is greater in the upper reaches than in the lower reaches during flood periods compared to dry seasons. Significant sediment sorting is used in the processes of transportation and deposition.

In the upper reaches, the riverbed consists mostly of large boulders, while in the middle reaches, it comprises small gravel and coarse sand, and in the lower reaches, only sand and mud are present. The landforms caused by river deposition mainly include alluvial fans, floodplains, and deltas.

The formation and evolution of rivers are also influenced by various natural factors such as tectonic movements, climate change, and changes in groundwater levels. Human activities can also affect rivers, such as excessive development and pollution, which can influence the evolution process of rivers.

The valley through which a river flow is called a river valley and the part of the valley floor occupied by water flow is called the riverbed or channel. Facing downstream, the left bank of the river is called the left bank, and the right bank is called the right bank. Rivers serve as a major pathway in the hydrological cycle. Rivers have the smallest surface area and volume among all Earth's water bodies, yet their relationship with humans is the most intimate.

Segmentation of Rivers

Rivers are generally divided into five segments from upstream to downstream based on characteristics such as the features of their valleys and riverbeds, bed gradients, erosion and deposition levels, flow velocity and discharge, and variations in water conditions.

(1) Source:

The place where a river originates is called its source, typically the area where the river begins. Rivers often originate from mountains, but some may originate from plains, lakes, marshes, or springs.

(2) Upper Reaches:

The section of the river directly connected to its source is called the upper reaches. Its characteristics include steep gradients, rapid flow, strong downcutting action, narrow valleys with high mountains on both sides, often forming V-shaped valleys, with many rapids and waterfalls, and significant fluctuations in flow during floods.

(3) Middle Reaches:

Below the upper reaches are the middle ones, characterized by gentler bed gradients, weaker downcutting action, gradual river widening due to erosion on both sides, and significant erosion and deposition, resulting in shoals or sandbars in the riverbed.

(4) Lower Reaches:

The lowest section of the river is called the lower reaches, characterized by gentle bed gradients and slow flow velocity, mostly in a state of sedimentation, with shoals and sandbars prevalent, wide valleys, high flow rates, and meandering channels.

(5) River Mouth:

When a river flows into the ocean, a lake, or another river, the point of entry is called the river mouth.

Due to the sudden expansion of the river mouth and the decrease in flow velocity, combined with the influence of tides, large amounts of sediment accumulate, forming multi-channel delta river mouths. Some rivers disappear into deserts without a river mouth.

On Earth, rivers are the source of life, shaping the landscape and carrying the development of human civilization. However, rivers' formation and evolution are manifestations of natural forces and are influenced by human activities.

Therefore, it is incumbent upon each of us to take responsibility for protecting the ecological environment of rivers, promoting the natural evolution of rivers, and living in harmony with nature. Only in this way can we protect our Earth, the home we rely on for survival, and keep it forever vibrant and beautiful.