Alluvial fans/canyons

Alluvial fans:

An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fan.

Alluvial fans are formed when slowing streams and stream branches deposits sediments along the edge of the current. They grow in width as the streams and branches meander. Slowing streams meander because they also deposit sediments in their channels, raising the level of the water, causing sediments at the stream edges to collapse.This frequently occurs at the mouths of narrower canyon,because streams slow where their channels widen and slopes become gentler.

Alluvial fans are subject to flooding and can be even more dangerous than the upstream canyons that feed them. Their slightly convex perpendicular surfaces cause water to spread widely until there is no zone of refuge. If the gradient is steep, active transport of materials down the fan creates a moving substrate that is inhospitable to travel on foot or wheels. But as the gradient diminishes downslope, water comes down from above faster than it can flow away downstream, and may pond to hazardous depths.
In the case of the Koshi River, the huge sediment load and megafan's slightly convex transverse surface conspire against engineering efforts to contain peak flows inside manmade embankments. In August 2008 high monsoon flows breached the embankment, diverting most of the river into an unprotected ancient channel and across surrounding lands with high population density. Over a million people were rendered homeless, about a thousand lost their lives and thousands of hectares of crops were destroyed.

Souce:Wikipedia,yahoo,google images

Canyon:

A canyon means a ravine formed by a river with little rainfall.Canyons are formed by erosion caused by rivers. Cliffs on either side of a canyon are made of harder rock, but the area that has been worn away tends to be of a softer rock which is easily weathered by water, and also by wind. A canyon may also refer to a rift between two mountain peaks such as those in ranges such as the Rocky Mountains, the Alps, the Himalayas or the Andes. Usually a river or stream and erosion carve out such splits between mountains. Examples of mountain type canyons are Provo Canyon in Utah or Yosemite National Park in California's Sierra Nevada. Canyons within mountains or gorges that only have an opening on one side are called box canyons. Slot canyons are very narrow canyons, often with smooth walls.

Canyons are good for buliding dams to generate hydro-electric power.Examples of this is the hoover dam.

-samantha lye