Alluvial fan/Bajada/Bahada. Image credit MediaWiki

Alluvial fan/Bajada/Bahada. Image credit MediaWiki

ZIMSEC O Level Geography Notes: Wadis and Bahadas


  • As already pointed out freak storms occur in deserts characterised by excessive precipitation over short periods of time.
  • Due to infiltration excess flow and even saturation excess flow there is significant run off in the form of ¬†sheet floods,
  • This flow occurs in the form of rills which later join to form gullies which in turn form into wadis,
  • These are deep canyons resulting from sustained erosion, the have steep banks and flat floors.
  • At times the floor may have material deposited by stream floods.
  • Wadis may also be formed by stream floods which erode valley sides although some wadis can be dry and only have water during and in the immediate aftermath of these storms.
  • An example is the Grand Canyon in the United States.

Alluvial fans/Bahadas/Bajadas

  • During the sudden rain storms
  • Flash floods rush down the wadis,
  • They carry large amounts of materials including a lot of silt (all this material is referred alluvium) forming a mud flow
  • As the water dissipates after the end of the storm and when its looses its energy upon reaching less steep slopes the alluvium is deposited to form fan shaped features at the base of the piedmont zone.

Desert piedmont zone

  • Is made up of water landforms formed by erosion, transportation and deposition.
  • The landforms found on the piedmont zone include: mountain front, bahada, peri pediment, playa and rock pediment.
The desert peidmont zone.

The desert peidmont zone.

Features of a piedmont zone.

Mountain Front

  • It is formed as a result of erosion along a steep slope and sometimes as a result of headward erosion.
  • It may also be a made up of large rock boulders resulting from weathering.


  • A depositional feature see above.


  • A sharp change in profile that marks the transition from the mountain front into a pediment.

Rock pediment

  • It is the gentle slope that starts at the knickpoint at the foot of the mountain.
  • This may be made out of bare rock or it can be covered in alluvium deposited during floods.


  • It is made up of deposited material washed across the pediment.
  • The peri pediment is a depositional feature that is formed in the same way as bajadas but it is larger.


To access more topics go to the Geography Notes page.