Causes & ImpactsCase study1: River Colorado, USA Case study2: River Wylye, Wiltshire
What are low flows? Fall in the level of water tables with the resultant drop in river levels.
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
Causes: Over abstraction which can lead to: water shortages Deteriorating water quality Increased sedimentation Ecological changes
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
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.
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
Causes… Abstraction of groundwater Climatic – the amt of precipitation & movement of water through the drainage basin
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%
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
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.