Define the terms drainage divide and wetlands.

Explain how stream discharge is related to channel size and shape.

Using examples, examine the hydrological impacts that can result from the construction of a dam and reservoir across a river channel.

Drainage divide – the line defining the limit of a drainage basin [1 mark] separating it from neighbouring basins [1 mark]. Also known as a watershed [1 mark]. Credit alternative phrasing.

Wetlands – areas that are regularly saturated [1 mark] by groundwater or surface water [1 mark]. These include freshwater marshes, swamps and bogs [1 mark]. Credit alternative phrasing.

• Award [1 mark] for defining/calculating discharge.
• Award [1 mark] for establishing a link between discharge and channel size (eg higher discharge in a larger/lower course channel).
• Award [1 mark] for establishing a link between discharge and channel shape (lower discharge in wide, shallow or deep, narrow channels, or equivalent point made).

The remaining [3 marks] should be awarded for explanatory points, such as:

• role of hydraulic radius
• importance of wetted perimeter
• idea of friction in relation to stream efficiency
• further development of width/depth or shape explanation.

Responses could cover downstream impacts such as more regular discharge, differences in seasonal flow (regime), fewer floods with longer delay and lower peaks.

Upstream impacts could include lower gradient, local base level, increased evaporation and seepage.

In extreme cases lower discharge can cause severe environmental problems eg, the Nile Delta erosion and red water famine at Aswan, the discharge of the lower River Colorado.

Accept references to changes in sediment transport immediately downstream or upstream, possibly causing clear water erosion and deposition and effects on fluvial landforms.

Impacts that are not hydrological should not be credited; limited credit may be given for geographical knowledge of some issues about a recognizable dam/reservoir.

At band D, responses are likely to be descriptive and at least two hydrological impacts should be related to changes resulting from dam construction.

At band E expect either a greater range of impacts or an evaluation of how impacts may vary eg, positive/negative, short-term/long-term.

At band F, expect both. 

Marks should be allocated according to the markbands.

There were few problems with the two definitions, although the term drainage divide was less successful.

This question was not well understood, and there was some confusion about the meaning of the term stream discharge. Some candidates had a reasonably good idea and could relate discharge to the explanation of friction, hydraulic radius and wetted perimeter. Channel size was dealt with better than shape.

Generally this question was very poorly answered, with the question not being understood by many candidates. The term hydrological impacts caused significant problems. The majority of candidates either tried to relate to the hydrological cycle, or wrote an answer that focused on human and economic, rather than hydrological, impacts.