Stages of River Development

1. Youth Stage – A river in youth stage cuts a deep V – shaped valley as the fast moving water transports material downstream Rivers in youth stage are dominated by erosion as the turbulent water allows for very little deposition to occur. Features commonly seen in these rivers include rapids, waterfalls, and various sizes of boulders along the river bed.

2. Mature Stage – As the river enters mature stage a definite drainage pattern is evident in the watershed drained by the river. During this stage of development vertical erosion still continues but downstream lateral erosion of the banks is evident as meanders and a flood plain start to take shape. Here the velocity of the flow slows considerably and deposition of river sediment is common.

3. Old Age Stage – In old age the flood plain which began to form in mature stage continues to widen and extensive meandering occurs. The river even cuts across some meanders to create oxbow lakes. Periodic flooding has deposited rich alluvium throughout the flood plain and natural levees are prominent along the river banks. The river delta is well develop, and in many cases continues to grow by several meters each year as sediment is flushed down from the highland region.

4. Rejuvenated State –in the rejuvenated river stage, the land has undergone a slow uplifting and caused the river to return to a period of vertical erosion such as was the case in youthful stage and a deep v – shaped valley once again takes shape. The river cuts through the flood plain created during old age and continues vertical erosion until its elevation again nears sea level. Once the elevation has been reduced sufficiently, lateral erosion replaces vertical erosion and the creation of a new flood plain begins.

Erosional Action of Rivers

Corrosion – All water is mildly acidic and some rocks are dissolved and transported away.

Ways the Rivers Transport SedimentsSolution – Anything that hs been dissolved by river water and carried in an invisible solution.Suspension – Light material will be carried in suspension and not touch the bottom of the river. Clay for example.Saltation – Particles that are heavy move along the bottom of the river and a dragged, bouncing and rolling. Traction – Heavy rock may drag along the river body. They only move when river velocity is high.

Abrasion

River Deltas

A river delta carries sediment from its drainage basin toward the sea and much of it ends up deposited in the flood plain. Some smaller materials get carried out to the ocean. Rivers with large sediment loads have constructive river deltas (they increase in size each year).

3 Main Types of Deltas

Arcuate Delta – Many distributaries that carry water and sediment across symmetrical delta that has the shape of an inverted cone. Ex. Nile River DeltaBird’s Foot Delta – Many distributary channels that branch out from the main river channel. Ex. Mississippi River DeltaEstuarine Delta – River sediment is deposited in a submerged river mouth.

The formation of a flood plain takes place in lowland areas when the river has reached an elevation that is close to sea level. This happens as the river nears the ocean.

As elevation nears sea level the river begins to meander (curve); this is where the water moves fastest.

Repeated floods carry sediment that is deposited onto the riverbank. As sediment builds it creates natural levees that help hold water in the river channel.

Dikes are manmade levees designed to minimize floods.

Formation of LeveesDikes used for Power Generation