Rivers

A River's Journey

Nearly all rivers begin their journey in mountainous regions called "headlands". They start out as small, fast, narrow streams in the headlands and end up as large, slow, and broad rivers as they enter the ocean. Sea level is refereed to as "base level", or the level to which the river wants to be even with. This is how and why rivers erode - they are attempting to reach base level. Sometime items such as large lakes or dams can act as a temporary base level. Rivers will try to erode down to this level, then will continue to erode down to the "true" or ultimate base level below this.

Streams in the headlands are called first-order streams as they have no tributaries feeding into them. They join with other first-order streams to form a second-order stream, and so on.

Rivers flowing out of a headland areas form drainage basins and patterns. They are separated by mountains (refereed to as "divides") and can often form intricate patterns.

A River’s Journey: Headland region In the headlands region, rivers down-cut into the mountains (vertical erosion, creating narrow, V-Shaped valleys, such as the Arroyo Seco as it exits the San Gabriel Mountains in La Cañada.
Smaller channel = Smaller discharge = Higher velocity Higher velocity = higher energy = greater competence Riverbeds typically contain large boulders Other features that can be found in these regions are waterfalls, rapid, and river terraces.

A River’s Journey: from mountains to flatlands

As the river moves from the mountains and into the flatlands, the stream velocity decreases.

Decreased velocity = decreased competence = deposition of load

Features such as alluvial fans, braided streams, & stream sinuosity develop as a result of this decrease in stream velocity.

Alluvial fans develop as sediment is deposited due to a change in velocity as the river exits the mountains and enters the valley.	Yukon River, Alaska From: Geomorphology from Space.

A River’s Journey: The Flatlands

Now that the river is in the flatlands, it is closer in elevation to sea level and therefore is doing less down-cutting. By this point the river has had several tributaries join into it, so the river is now much larger that it was before.

Larger channel = Larger discharge = Lower velocity

Lower velocity = lower energy = decreased competence

The load for rivers in flatland areas is typically finer sediments such as silts and clays - the lowered velocity means that larger particles (sands on up) are deposited and not carried downstream with the river. Thus, when the river floods, the silts and clays carried in the suspended load are deposited on the floodplains on either side of the river. Riverbeds typically contain sands and gravels.

Erosion by the river is now lateral instead of vertical. This is due to two reasons: the river is closer to base level and no longer needs to cut down and the bends (or sinuosity) help to slow the river down. Erosion therefore occurs on the outer edges of the bends and sediments are deposited on the inner edges.

Periodic floods deposit rich soils. This lead to agricultural production on floods plains as farmers wanted take advantage of these fertile grounds. This, in turn, was followed by urbanization. Each time the river floods a little bit of silt/clay is deposited on the banks of the river. This forms a natural levee, which helps to constrain the river within it's banks and makes it so the river must reach a higher level before the river will flood again.

A River’s Journey: Into the Ocean

The ocean is not moving compared to the river. Thus, when the river enters the ocean it slams to a stop.

Decreased velocity = decreased competence = deposition of load

In some coastal environments a delta is formed when the river deposits it's sediments when it enters the ocean. The Mississippi River Delta forms as the Mississippi River enters into the Gulf of Mexico. Rivers naturally shift course as they move from the headlands to the coast. Humans, however, do not like it when rivers do this, so we put checks such as dams and man-made levees along it 's banks so the river stays put. The image below shows where the river and delta have moved over the past 5,300 years.

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