Life Cycle of a Stream

Aim: How does a river shape the land over time?

In the beginning….

• A single heavy rain may form a small valley in loose soil along a hill slope

• When the rain ends, the small valley remains (this is called a gully)

• These will grow in length, and depth and may cut down far enough to become permanent. (Downcutting)

Youthful Stage

• During this stage a river may capture the headwaters of another river through the process of headward erosion.

• This is called stream piracy.

Youthful Rivers

• Youthful rivers have fairly straight channels and steep gradients

Youthful Stage

• V-shaped valley– Water erodes the

sides of the valley walls and the river cuts down into its channel

– The upper valley walls are widened into a v-shape

Youthful Stage

• Rapids• Waterfalls

Maturity

• When sidecutting (lateral erosion) becomes greater than downcutting

• The topography becomes less steep with a smoother gradient

• Floodplains form• The stream begins to meander• Meandering River

Meander

Floodplains

• The land between the stream and the steep walls of the valley that is usually covered when the river overflows its banks during a flood

• Natural levees form along the banks of the river

Manmade levees are built to protect nearby areas when a river overflows its banks

Old Age• Meanders become cut off from the river forming

oxbow lakes• A river officially enters old age when the flood plain

becomes wide enough to accommodate all of the meanders

Oxbow Lakes

Summary of Stages