Spark
Loading...

Fluvial Sediment Properties and Transport

Fluvial sediment properties and transport involve the movement of rock and soil particles by rivers and streams. This process shapes landscapes through erosion, transportation, and deposition, creating features like ripples, dunes, floodplains, and deltas. The size and type of sediment a river can carry depend on the water's velocity and the particle's weight and shape. Larger particles, such as sand and gravel, are typically moved during high-energy events like floods, while finer particles like silt and clay remain suspended in the water. Understanding fluvial sediment processes is crucial because rivers transport massive amounts of material annually, influencing ecosystems, shaping landforms, and affecting human activities. These processes also provide insights into Earth's geological history, as preserved sediments reveal past environmental conditions and events.

Figures (4)

Middle Triassic marginal marine sequence of siltstones and sandstones, southwestern Utah.
Mi Vida uranium mine in redox mudstones near Moab, Utah
Heavy minerals (dark) deposited in a quartz beach sand (Chennai, India).
Centripetal desiccation cracks (with a dinosaur footprint in the center) in the Lower Jurassic Moenave Formation at the St. George Dinosaur Discovery Site at Johnson Farm, southwestern Utah.

Sediment in rivers is transported as either bedload (the coarser fragments which move close to the bed) or suspended load (finer fragments carried in the water). There is also a component carried as dissolved material. For each grain size there is a specific flow velocity at which the grains start to move, called entrainment velocity.

However the grains will continue to be transported even if the velocity falls below the entrainment velocity due to the reduced (or removed) friction between the grains and the river bed. Eventually the velocity will fall low enough for the grains to be deposited. This is shown by the Hjulström curve.

A river is continually picking up and dropping solid particles of rock and soil from its bed throughout its length. Where the river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up.

Areas where more particles are dropped are called alluvial or flood plains, and the dropped particles are called alluvium. Even small streams make alluvial deposits, but it is in floodplains and deltas of large rivers that large, geologically-significant alluvial deposits are found. The amount of matter carried by a large river is enormous.

It has been estimated that the Mississippi River annually carries 406 million tons of sediment to the sea, the Yellow River 796 million tons, and the Po River in Italy 67 million tons. The names of many rivers derive from the color that the transported matter gives the water. For example, the Yellow River (Huang He) in China is named after the hue of the sediment it carries, and the White Nile is named for the clay it carries.

Key Points

  • Fluvial sediment refers to the solid particles of rock and soil that are picked up, transported, and deposited by rivers and streams.
  • Bed load is the coarser fragment of sediment in a river that moves close to the river bed by rolling, sliding, or saltating.

Terms

Tap a term for a plain-language explanation.

Sources & licensing(4)