Rivers are a unique feature in the geography of the earth. They provide dry inland areas with much needed fresh water and allow climates of every type to support life. Agriculture is supported by rivers and humans benefit from their existence in many ways.
Rivers Follow the Path of Least Resistance
Rivers twist and turn, shaping the environment around them just as they are shaped by the existing physical geography of the surrounding landscape.
Rivers typically follow the path of least resistance- from their headwaters to their outlets in the sea they are constantly moving around rocks and eroding valleys, growing and changing as they flow and age.
Rivers function to get from their beginnings, or headwaters, to sea level through the most efficient path possible. Rivers change in kinetic energy, water flow rates, velocity, discharge and more as they go from start to finish.
You may notice when looking at the path of a river that it gains width and depth as it gets closer to its endpoint at sea level. The river gains energy as it flows down the slope (or gradient) from its headwaters to sea level which can vary depending on the landscape and the river.
The Erosive Process of a River’s Flow
The river’s kinetic energy (or the energy that comes from the moving of the water as it flows downhill) is what causes the majority of the erosion to the geography of the river.
Water moving past and over rocks, dirt, and other materials erodes them and often sweeps them along to be deposited further downstream.
Rivers naturally try to create a balance between the amount of kinetic energy and water flow they have to be equal from start to finish, which greatly contributes to the erosion we see around rivers. In order to achieve this balance rivers erode their banks, change their paths, and transport and deposit sediment along their way.
Sections of a River
Rivers have three sections or courses: the upper course, middle course and lower course.
The upper course of a river typically sees much erosion of bedrock in order to achieve the equilibrium mentioned above. The upper course is also at the highest altitude as this is where the headwaters of the river originate. Rivers in the upper course erode down vertically which often creates a steep channel profile in what becomes the river valley.
The middle course sees a river at a slightly lower altitude than the upper course, but is still maintains a sense of trying to acquire equilibrium in flow and shape. The discharge and velocity of the river still allows the water to erode the banks and chart a course of least resistance through lateral erosion.
The lower course is unique as this is the stage in which a river has reached sea level. Here we find features not found in the middle and upper courses like floodplains and river deltas that have been formed by the sediment deposits of rocks and dirt gathered from upstream.
Four Different Types of River Erosion
Rivers participate in four different types of erosion in their path from their headwaters to sea level: abrasion, hydraulic action, solution, and attrition.
Abrasion, hydraulic action, and solution are all the ways in which a river erodes the bank and river bed through friction between water, soil, rocks, and other natural material.
Attrition has to do with the friction and interactions of the bed load, or the chunks of rock and other sedimentary materials that are transported by the river downstream.
Rivers Slowly Change the Geography Around Them
Over time rivers slow in their erosion of the geography around them but are never quite done changing the landscape completely.
Young rivers erode the rock and soil quickly in order to achieve equilibrium between the altitude of their headwaters and their final expulsion at sea level. Rivers take the path of least resistance, skirting around tougher rocks or materials that aren’t as easily broken down by the strength of the water.
Waterfalls, rapids and the movement of a river around hills or mountains are all examples of ways in which a river will move around geography it cannot change quickly.
As the river flows it gains width and depth before depositing the sediment from upstream into river deltas and floodplains, which grow and spread; these floodplains are extremely fertile and can, with time, prove to be viable agricultural areas.
As a river ages it begins to smooth out the geography of the land around it. Canyons are created, nearby hills are flattened and the river bed has broadened.
Old Rivers Versus Young Rivers
Old rivers have lesser gradients than do young rivers, and their floodplains are wide; young rivers have incredibly steep gradients and rarely a floodplain to speak of.
Mature and old rivers erode laterally as opposed to young rivers, which are eroding in many different ways. Old rivers often appear incredibly muddy and slow-moving, as the sediment they carry is suspended through all layers of the river as it make its way downstream.
Evidence of young, mature and old rivers can be seen around the world; rivers in each stage of development can be seen twisting and turning through each unique landscape they come across on their quest to reach sea level. Rivers continue to be dynamic and ever-changing.
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Geology – Grand Canyon. (n.d.). NPS.gov. https://www.nps.gov/grca/learn/nature/grca-geology.htm
River geomorphology and Geomorphic change. (2019, August 3). USGS.gov. https://www.usgs.gov/centers/sbsc/science/river-geomorphology-and-geomorphic-change
Water Encyclopedia: Science and Issues. Stream Erosion and Landscape Development. http://www.waterencyclopedia.com/St-Ts/Stream-Erosion-and-Landscape-Development.html