Action of Running Water (Rivers)

Water as an Energy System:

Running water is a significant agent of denudation.

Denudation is the geological process in which running water, ice, wind, or waves, lower and level the Earth’s surface, causing a reduction in the elevation and relief of landforms and landscapesA landscape is the visible features of an area of land, its landforms, and how they integrate with natural or man-made features, often considered in terms of their aesthetic appeal. A... More.

As a river flows, it creates channels, and it also transports and deposits materials as it flows through. This occurs throughout hundreds or even thousands of kilometres through the river channel. The running water of rivers plays an essential role in the erosion, transportation and deposition of materials.

A river channel is characterised by dimensions. These dimensions consist of the length, width, depth, gradient and perimeter of the river channel.

Terms Associated with Rivers:

The following are important terms associated with Rivers:

• Gradient / Slope is the angle between the river surface and the horizon.

• Depth, usually measured in meters, is the vertical distance from the surface to the bed of the river.

• Velocity is the rate of movement or speed at which the water flows in a channel. The velocity is less at the sides and bed, but higher at the centre. Frictional drag is the cause of this difference in velocity.

• Width, usually measured in meters, is the distance across the surface of the river from bank to bank.

• Source of a River: This refers to where a river starts or begins, usually in highlands or mountainous areas. This is where water from precipitationPrecipitation is any liquid or frozen water that forms in the atmosphere and falls back to the Earth. It comes in many forms, like rain, sleet, and snow. More or melting snow and ice starts to flow downhill.

• Watershed: A watershed is an area of land where all the rainwater and surface water flow into the same river, lake, or ocean. The source of a river, or its headwaters, is located within a watershed.

• River Basin / Drainage Basin / Catchment Area: A river basin collects water from many watersheds and sends it all to a main river. Every stream and tributary has its own watershed, which drains to a larger stream or wetland. These streams, ponds, wetlands, and lakes are part of a river basin.

In many areas, a watershed is also referred to as a river basin.

• Watershed Divide / Drainage Divide: These are the highlands that separate these drainage areas or the highland areas that separate two or more rivers, river basins, or watersheds. Water flows in different directions on either side.

• Tributaries: These are smaller rivers or streams that join together to form a larger one.

• The Confluence of a River: This is the meeting point of two different rivers.

• River EnergyEnergy is the ability to do work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, and electrical energy. Units of Energy: The SI unit... More: This can also be known as the velocity or speed of a river. The efficiency of a river to erode and transport the eroded materials depends very much on its velocity.

• Distributaries: This refers to the channels formed by the division of a river as it flows into the sea. They are usually found in the delta region of a river.

• River Regime: This refers to the seasonal changes in the volume of water in a river in a year. When there is one period of high volume and one period of low volume within a year, this is a single regime. However, when there are two distinct periods of high volume in a year, this is a double regime.

• The Course of a River: This is the path or channel by which the river flows.

• The Mouth of a River: This is where the river ends or where it enters the sea, ocean or lake.

Riverbed vs. Riverbank:

The riverbed is the ground at the bottom of a river, while the riverbank is the land alongside the river, bordering its edges. Essentially, the riverbed is the channel through which the water flows, while the riverbank is the land that contains the river within its channel.

Factors Affecting the Velocity of a River:

The level of erosion, deposition and transportation achieved by a river depends on several factors, such as; the flow of water through its channel, the amount of energy possessed by the river, the gradient/slope of its bed, the slope of the channel, the volume of the river as well as the nature of its flow.

When the volume of water and the gradient increase, the energy increases. The volume increases from the source to the mouth.

The following factors affect the velocity of a river;

a. The Volume of Water Released: The larger the volume of water released by a river, the more its velocity. However, as the volume decreases during the dry season, its velocity also reduces.

b. The Slope of the River Valley: Gradient/slope is the angle between the river surface and the horizon. The steeper the slope, the higher the velocity of the water in the river.

c. The Shape of the River Valley: It takes more energy to flow through a flat-wide valley than through a narrow-deep valley because the former has a larger surface area.

d. Amount and Size of Materials: The greater the speed of a river, the greater the materials or loads it can carry or move.

A river uses its energy to:

• overcome friction along its channels.
• transport its load.
• erode the river valley. It is this energy that is trapped by man to generate electricity (Hydroelectric power).

Laminar vs. Turbulent Flow:

A River can flow in two ways: laminar flow and turbulent flow. Laminar flow is described as the flow of the river parallel to its bed, while turbulent flow is the cyclical movement of the river. Turbulent flow can also be described as a surging flow of water on uneven slope surfaces. The laminar flow and turbulent flow are illustrated in the diagrams below;

It is worthy of note that at the upper part of a river’s course, the channel is steeper, that is, the gradient is steeper with less volume of water. In this part of the river’s course, it is often thought that the river flows fastest, but this is usually not the case due to frictional drag and turbulent flow with lots of eddies.

The middle course of a river has more energy and a high volume of water. The gradient here is gentle, and lateral (sideways) erosion widens the river channel. The river channel is also deepened here. A larger river channel means there is less friction, so the water flows faster.

The ability of flowing water to carry load or materials depends on the slope, the volume of water and the river energy it possesses. Where the velocity of a river’s flow is very high and the volume of water is adequate to detach materials and break them away, the load will be larger. As the energy and gradient decrease, the river drops the heavier load and flows along with the lighter load until the river’s energy reaches a near-zero level, where essentially all of its sediment load is deposited.