1. Causes
& Impacts
Case study1: River
Colorado, USA
Case study2:
River Wylye,
Wiltshire

2. What are low flows?
Fall in the level of
water tables with
the resultant drop
in river levels.

3. Causes:
 Drought conditions
 Dam construction which regulates downstream
river discharge and the regime of the river,
affects the migratory routes of some fish 
increase in salts & pollutants
 E.g. in the Russian Federation, dams have
reduced natural discharge from the Dnestr,
Dnepr & Don rivers by 50-60%  accumulation
of salts & pollutants in the Black sea

5. Causes:
 Over abstraction which can lead to:
 water shortages
 Deteriorating water quality
 Increased sedimentation
 Ecological changes

7. Case Study: Colorado river,
USA
 E.g. River Colorado basin area, USA , growing
demand for water due to:
 The aridity of many areas within the catchment
 Extraction of water to irrigate agricultural areas
in the central valley of California
 Expansion of urban areas and the demand for
water supplies such as in the ‘Sansan’
megalopolis area

11. Result:
 Little flow by the time the river reaches the
Gulf of Mexico
 Increase salinity (up to 3000 ppm) due to water
loss through evaporation during the irrigation
process
 Mexico has been forced to spend $200m on a
desalination plant
 Water quotas have been introduced for the use
of water from river, California currently uses
around 59% of the water.

13. Case Study 2: River Wylye,
Wiltshire
 Between 1988 -1992:
 River Wylye & its tributaries registered
particularly low flows.
 People became concerned about the
state of fish stocks in the river ( river
well-known for trout fishery) as well as
aquatic environment in general

14. WILTSHIRE

17. Causes…
 Abstraction of
groundwater
 Climatic –
the amt of precipitation & movement
of water through the drainage basin

19. Impact of abstraction…
 Groundwater abstraction, despite being within its
licensed limit, was having a detrimental effect on
low flows
 Harmful effects on fish stocks
 Growth of algae caused by nutrients from
fertilisers being washed into streams, reduction in
oxygen
 Most serious affected area, the upper Wylye,
abstraction reduced typical summer flows by
50-60%

22. Strategies:
 Sink two new boreholes to augment flows in
Chitterne Brook & the River Till
 Increase river augmentation at Brixton Deveril
 Aim for more constant level of abstraction
 Monitor the effects of abstraction on river
flows esp on the quality of the aquatic
environment

23. Noting activities…
1. What are ‘low flows’?
2.Why do you think people are concerned
about low flows in rivers?
3. Outline two main causes of low flows.
4. What are the impacts of water
abstractions, provide examples to your
explanation.