Formation of Outwash Plains Real

Question 1

Part 1

Answer 1

• An outwash plain (also known as a sandur) is a flat expanse of sediment in the pro-glacial area.
• As meltwater streams gradually lose energy as they enter lowland areas beyond the ice front, they deposit their load.
• The largest material is deposited nearest the ice front and the finest further away. Outwash plains are typically drained by braided streams.
• These are river channels subdivided by numerous islets and channels. Debris-laden braided streams lose water at the end of the melting period and so carry less material.

Question 2

Part 2

Answer 2

• This material is deposited in the channel, causing it to divide. Braiding begins with a mid-channel bar which grows downstream.
• Discharge decreases after a flood or a period of snow melt, causing the coarsest particles in the load to be deposited first.
• As discharge continues to decrease, finer material is then added to the bar, increasing its size. When exposed at times of low discharge, channel bars are stabilised by vegetation and become more permanent features.

Question 3

Part 3

Answer 3

• The river divides around the island and then re-joins. Unvegetated bars lack stability and often move, form and reform with successive flood or high-discharge events. They are very common in outwash areas due to the seasonal fluctuations in the discharge.
• On the south coast of Iceland is an extensive sandur (Figure 2.25) fed by numerous meltwater streams from glaciers such as Gigjökull and Solheimajökull. From the edge of the upland area to the present position of the sea is a distance of some 5 km.