Bed load

Rocks move in a river.

They stay on the bottom. This is called bed load.

These rocks move in many ways. They can roll along. They can slide. They can even hop.

Rocks hit each other as they move. They also hit the river bed. This makes them smaller.

All this hitting makes them smooth. They lose their rough edges.

Rocks far downstream are small and round. It is cool to see how rivers change rocks.

Rivers carry many things. Some parts stay on the bottom. This is called bed load.

Bed load is made of small pieces. These pieces move in a few ways. They can roll along the bottom. They can also slide or hop. This hopping is called saltation.

As rocks move, they hit each other. They also hit the river bed. This is called abrasion. This hitting makes the rocks smaller. It also makes them smooth and round. This is called downstream fining.

Smaller rocks move more easily. Large rocks stay put more often. But some big rocks help small ones. This is called the hiding effect. Small rocks hide behind large ones. This helps them move at the same time. The water must push hard to move them. This push is called shear stress. It must beat the pull of gravity.

Rivers do more than just flow with water. They also carry many pieces of earth along their paths. This material is called sediment. One specific type is known as bed load. Bed load is the sediment that moves along the bottom of a stream. It is different from suspended load or wash load. These different types of sediment work together in the water.

Bed load moves in a few distinct ways. Some pieces roll along the river bed. Other pieces slide across the bottom. Some particles move by hopping along the floor. This hopping movement is called saltation. As these rocks move, they hit one another. They also strike the walls of the river channel. This hitting is known as abrasion.

These movements change the rocks over time. This change is called downstream fining. Rocks near the top of a stream are often large and rough. As they travel downstream, they become smaller and more rounded. This happens because of attrition and abrasion. The rocks collide and lose their rough edges. Smaller pieces are also easier to move than large ones.

Scientists study how these particles move. Parker and Klingeman described the hiding effect in 1982. This effect happens when larger particles stick out from the bed. Small particles stay shielded and hidden by the big ones. This means many sizes move at nearly the same shear stress. Shear stress is the force that pushes the sediment. It must overcome the pull of gravity.

There are special ways to measure this movement. One way uses the Shields stress. This looks at the ratio of forces acting on a particle. It uses friction velocity and particle density. It also uses the diameter of the particle and gravity. The Meyer-Peter-Müller formula also helps scientists. It describes the bed load flux under uniform flow. This helps us understand how much sediment moves.

Bed load is a specific type of sediment found in flowing fluids. Most often, this occurs in rivers and streams. Bed load refers to the particles that move along the bottom of a stream bed. It is one of three ways sediment travels through water. The other two types are known as suspended load and wash load. Understanding bed load helps scientists study how rivers shape the landscape.

Particles in the bed load move through three distinct motions. Some pieces move by rolling along the bottom surface. Other particles move by sliding across the bed. A third method is called saltation, which means the particles hop along the floor. These movements are driven by the force of the water. As the water flows, it exerts pressure on the sediment. This pressure determines how the particles will travel.

As bed load travels downstream, it undergoes a process called downstream fining. This means the sediment becomes smaller and more rounded. This change happens because of two main processes: attrition and abrasion. Abrasion occurs when stones strike the walls of the river channel. Attrition happens when the stones collide with each other. These collisions remove the rough textures from the particles. Consequently, the rocks lose their sharp edges and shrink in size.

Selective transport also plays a major role in how sediment changes. Smaller particles are usually easier to move than larger ones. This is because the shear stress required to move a grain is linearly proportional to its diameter. However, scientists Parker and Klingeman discovered a complication in 1982. They described what is known as the hiding effect. In this process, larger particles protrude from the river bed. These large particles shield and hide the smaller particles. Because of this shielding, nearly all grain sizes can be moved by the same amount of shear stress.

To understand when movement begins, scientists look at the Shields stress. This is a ratio between two different types of forces. One force is hydrodynamic, which is the destabilizing force of the water. The other is gravitational, which is the stabilizing force holding the particle down. The Shields stress calculation uses friction velocity and particle density. It also includes the diameter of the particle and the pull of gravity. If the water's force exceeds this critical value, the sediment will begin to move.

There are mathematical ways to predict how much sediment will move. One method is the Meyer-Peter-Müller formula. This formula calculates the bed load flux, which is the amount of sediment moving per unit width. It works under conditions of uniform flow and equilibrium. The formula states that the flux is proportional to the excess of shear stress. This excess is the difference between the actual stress and a critical value. Specifically, the relationship is expressed as a monotonically increasing nonlinear function. This is often written as a power law.

Bed load is a fundamental part of geomorphology and sedimentology. Geomorphology is the study of how landforms are shaped. Sedimentology is the study of how sediment is deposited and moved. By studying bed load, researchers can understand how rivers transport material across the Earth. They can see how the energy of water changes the physical shape of the ground. This knowledge connects the movement of tiny grains to the massive changes in river valleys.