Stream erosion has shaped the landscape of the planet. Constructional areas are limited, and include volcanoes, river plains and deltas, coral reefs, and the surfaces of glaciers and ice sheets. Mountain ranges are built by forces within the Earth, but individual mountains are only the remnants left behind by the erosion of the valleys that separate them. The continually eroding valleys are the active, evolving part of the landscape.

Rivers have been credited with up to 90 percent of the total sediment transported to the sea, although this journey can take many human lifetimes. All of the sediments that eventually end up in the sea were at some time, a part of the continental landscape.

Rivers and Landscape Shaping

Stream erosion occurs when water flowing through a channel is able to transport sediment downstream. The amount of sediment transported depends mainly on the volume of flow, which is related to the size of the drainage basin and can fluctuate immensely. Volume of streamflow, or discharge, is measured in cubic meters per second. Most riverbed erosion occurs during times of high discharge.

New landscapes can be created by constructional forces, such as a volcanic eruption, the melting of a glacier, or the uplift of the ocean floor. On a new landscape, river runoff follows an irregular path downhill towards sea level, forming lakes in closed basins and then overflowing to continue downhill. This pre-mature drainage is called a consequent system because it is a direct consequence of the preexisting landscape.  As time goes on, the drainage network becomes more streamlined. Water cuts new paths and consolidates others as it follows the path of least resistance to sea level. Drainage networks become sophisticated but simplified, as they shape the landscape and adjust to their climate region.

As rocks erode, rivers gradually change their courses to keep their path of least resistance. Where a river crosses rocks resistant to erosion, it generally cuts a steep, narrow canyon. This occurs because the river is unable to erode the underlying rock at the rate that the river erodes the rock downstream, creating a steep gradient. An example of this process is a wide, open valley in the mountains with a flat running stream, while upstream you find a steep, narrow gorge cut through rocks that were unable to be eroded at the rate the valley was.

Sediment Transportation

There are three ways sediment is transported in a river: in a solution, suspended, or as bed load.

Solution load - These are sediments that are dissolved in the water. Solution load is generally higher where water flow is derived from mainly groundwater and where local bedrock, such as limestone, is prone to chemical weathering.

Suspended load - These are sediments that are not dissolved in the water, but are present in them. They are suspended in the water. These usually include fine sediments such as clay and silt. They are suspended mainly by turbulence, and will settle out when they have been dormant for awhile.

Bed load - These are sediments that roll, skip, or slide across the stream bed. There will be periods of time that these sediments are settled at the bottom of the bed, and times when they are being transported, depending on turbulence.

This photo taken by me from a plane flying near Las Vegas, Nevada clearly shows the effect of stream erosion on this landscape. Over time, the stream has eroded the surrounding bedrock and was able to maintain its flow, making it a subsequent stream. 

For more information, visit
The Water Encyclopedia

The American Geosciences Institute