GC Leong Test: Landforms Made By Running Water - 1

• The river moving downstream across a broad, level plain is heavy with debris from the upper course. 

• Vertical corrasion has almost ceased through lateral corrasion still goes on to erode its banks further. 

• The work of the river is mainly deposition, building up its bed and forming extensive flood plains. 

• The volume of water is greatly swelled by the additional tributaries that join the mainstream. 

• Coarse materials are dropped, and the finer silt is carried down towards the mouth of the river

• Rivers in their lower course cry large quantities of sediments. During annual or sporadic floods, these materials are spread over the low-lying adjacent areas.

• A layer of sediment is thus deposited during each flood, gradually building up a fertile flood plain. 

• When the river normally flows its bed is raised through the accumulation of deposits and material, it also drops on the sides, forming raised banks called levees. 

• It will not be long before the water level flows dangerously close to the top of the levees.

i. Active vertical and Lateral erosion in the river's upper course to provide extensive sediments to be eventually deposited as deltas 

ii. The coast should be sheltered preferably tideless. 

iii. The sea adjoining the delta should be shallow or else the load will disappear in the deep waters. 

iv. There should be no large lakes in the river course to 'filter off the sediments. 

v. There should be no strong current running at right angles to the river mouth, washing away the sediments

• A negative movement occurs when there is an uplift of land or a sea-level fall. This will steepen the slope so that active down-cutting is renewed. 

• A fall in sea level leaves the flood-plain at an increased altitude above the sea level. The river with its renewed vigour cuts into the former flood-plain, leaving behind terraces on both sides of the river. 

• There is also a break in the river's graded profile, often marked by a series of rapids.

• The flood plains of large rivers with their thick mantles of fine silt are some of the world's richest agricultural areas. 

• They may support very dense populations, and a chain of large cities may be strung along their banks. 

• Many deltas are equally fertile, e.g. the Ganges delta accounts for almost all the jute grown for world consumption; the Nile delta produces superior quality cotton and several rice crops a year. 

• The productive hinterlands can support ports such as New Orleans for the Mississippi basin, Rotterdam for the Rhineland and Calcutta for the Indo-Gangetic Plain.

• By depositing large quantities of sediments in the lower course, the river silts up ports preventing large steamers from anchoring close to the shores. 

• Deltas are thus less satisfactory sites than estuaries for the siting of large ports. 

• However, this can be overcome by the construction of artificial harbours or by dredging. This is expensive and, in some instances, impracticable.

• If rejuvenation occurs in the upper course, the river valleys are deepened and steep-sided gorges are formed.

• In the middle and lower course vertical corrasion replaces lateral corrasion and the existing meanders are vertically eroded by the rejuvenated stream.

• A distinct new inner trench is cut in the old valley, and the river develops a deep valley with entrenched or incised meanders.