Briefly outline two processes of river erosion.

Explain how irrigation can lead to salinization.

Explain two consequences of salinization for farmers.

Discuss the positive and negative hydrological impacts of dam and reservoir construction.

In each case, award [1] for identifying the process, and [1] for a brief outline.

• Hydraulic action [1] in which the force of water hitting the banks causes erosion [1].
• Abrasion / corrasion [1] – wearing away of river sides and bed by the river’s load [1].
• Attrition [1] – material moved along the bed of a river collides with other material and breaks it up into smaller pieces [1].
• Corrosion / solution [1] – depends on chemical composition with certain rocks dissolving more [1].

Salinization is the build-up of salts in soil to an excessive level. It can occur when there is over-irrigation [1] (ie, where more water is added than can be taken up by the plants). If the groundwater has a high salt content and the
water table rises, the soil in fields may have salt levels that are too high for plant growth [1].

For example: As water passes over/through the soil, it dissolves various minerals and nutrients [1] / in some areas, naturally high levels of evaporation increase the salt content at the surface [1] / capillary action also brings water to
the surface where evaporation leaves the salts [1].

In each case, award [1] for valid consequence and [1] for further explanation/detail.

Possible consequences include:

• may be very costly for farmers [1] as yields of crops may decrease / as may have to add expensive artificial fertilizers [1]
• salinity may affect the structure of the soil [1], resulting in surface soil compaction [1]
• loss of vegetation cover [1] may expose the area to the effects of soil erosion [1].

For example: Crops tend to have low salt tolerance [1] so salinization reduces crop yield/makes it less productive [1].

Responses could cover downstream positive impacts such as more regular discharge, reduced differences in seasonal flow (regime), fewer floods with longer delay and lower peaks. Upstream positive impacts could include lower gradient and change to local base level.

Negative downstream impacts are that lower discharge can cause severe environmental problems, eg, the Nile Delta erosion and red water famine at Aswan, the reduced discharge of the lower River Colorado. Negative impacts
upstream could include increased evaporation and seepage.

There could also be reference to changes in sediment transport immediately downstream or upstream, with effects on fluvial landforms.

Some credit may be given for geographical knowledge of some issues about a recognizable dam/reservoir.

Good answers may discuss in detail the potential positive and negative hydrological impacts of dam construction before arriving at a final evidenced conclusion. An alternative approach would be to provide a structured discussion
of the question from different perspectives, as these may differ on what constitutes positive or negative.

At band D, expect two hydrological impacts to be described and linked to dam construction.

At band E, expect either a more detailed explanation of greater range of hydrological impacts or a structured discussion of both positive and negative hydrological impacts.

At band F, expect both of these elements.

Marks should be allocated according to the markbands.