Gradational Processes River’s & Valleys

Gradational ProcessesRivers & Valleys

The Power of Running Water

• River’s and running water play a huge part in shaping our landscape.

• From massive canyons, valleys & gorges (Grand Canyon), to delta’s & flood plains

• While it may be a slow process, it’s definitely a constantly changing process as water is one of the key ingredients in defining our landscape

• Rivers have a life cycle that follows a sequence of development. Each stage has distinctive features through this sequence

Life Cycle of a river….

• Youth stage rivers: will cut deep V-shaped valleys. Fast flowing. High erosion with low deposition. Include rapids, waterfalls and various size boulders. Generally flows in a fairly straight direction

Life Cycle of a river cont.

• Mature stage rivers: Flood plains have begun to form on either side of river. Riverway tends to widen. Moderate flow. Sediment deposition taking place. River meanders back and forth through the valleys

Life Cycle of a river cont.

• Old age stage: Extensive meandering, riverway widens and is much more shallow, deposition everywhere. River cuts across some meanders, forming oxbow lakes. Deltas being formed.

Erosional Action of Rivers

1. Hydraulic Action – the force of water flowing in the river will work loose and be carried further downstream & eventually deposited when water flow diminishes

2. Abrasion – debris such as sand can act as a powerful abrasion agent against river banks & river beds. Polished river rocks are a good example of abrasion

3. Attrition – when water moves fast (floods etc). Rocks of varying sizes may be moved along striking other rocks and causing them to break apart

Ways that Rivers Transport Sediments

4. Solution – rivers carry huge amounts of material in solution. Eventually the solution will settle when the river energy dissipates.

• Up to half the sediment carried by rivers is in solution form, yet we can’t see it

5. Suspension – Very light material will be carried in suspension so that it won’t touch the bottom until river slows

6. Saltation – particles that are too heavy will bounce along the river bed

Landforms associated with rivers

• Gorges and Canyons – moving water is responsible for some of the most spectacular gorges, canyons and rivers in the world.

• This picture is of the Grand Canyon with the Colorado River below – estimated it took 6 million years to form

Landforms associated with rivers cont.

• Imagine this as a flat plain when the process first began. That’s a lot of debris to move!

• Meltwater from the last ice age certainly would have helped


• Flood Plains – rivers seasonally overflow to the floodplains, leaving rich alluvial deposits make the area ideal for farming

• Canadian prairies & farming

• This picture of Kakadu National Park in Northern Australia. Massive Flood plain – rich in biodiversity


Levees

• These are narrow ridges of low height on both sides of a river, formed due to deposition action of the stream, appearing as natural embankments.

• These act as a natural protection against floods but a breach in a levee causes sudden floods in adjoining areas


• Meandering River characteristics


Oxbow Lakes

• Meandering rivers eventually straighten their paths by cutting through some meanders and creating oxbow lakes


Deltas

• A delta is a tract of river sediment at the mouth of a river where it deposits more material than can be carried away.

• the suspended sediments and solutions settle and build land formations

• The river gets divided into distributaries which may further divide and rejoin to form a network of channels.

Delta BC – Expands seaward of up to 3 meters per year. Who needs a bridge? The natural bridge is coming to us!

Types of River Deltas…

1. Arcuate (or Fan Shaped) Delta

• many distributaries that carry water & sediment across a very symmetrical delta that is shaped like an inverted cone

• Most common form of delta

• Ex. Nile River Delta


2. Bird’s Foot Delta

• emerges when limestone sediment deposits do not allow downward seepage of water

• Happens when the currents and tides are weak and the number of distributaries lesser as compared to an arcuate delta

• Ex. Mississippi River

New Orleans

Bird’s Foot Delta

Bird’s Foot DeltaMississippi River end point into Gulf of Mexico


3. Estuaries

• Sometimes the mouth of the river appears to be submerged. This may be due to a drowned valley because of a rise in sea level.

• The point where fresh water and saline water get mixed and the river starts ‘filling its mouth’ with sediments, mud bars, marshes and plains seem to be developing in it.

• Ideal locations for fisheries, ports and industries because estuaries provide access to deep water

• Example: Delta, BC.

Fraser River Estuary – this shot looking over where the Massey Tunnel is located

Gradational Processes River’s & Valleys

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