Chalk Rivers, The State of Englands Chalk Rivers

River flows and water quality

River flows

Chalk rivers are reliant on adequate autumn and winter rainfall to recharge groundwater aquifers. These maintain flows throughout the year. Winter flows help to keep river-bed gravels clean. When flows are reduced by abstraction, the duration of flow in winterbournes is shortened; suitable habitat for fish and other animals is lost; and algae frequently choke the channel as a result of increased nutrient levels.

Most chalk rivers are located in the densely populated South East of England, where rainfall is relatively low. In some river catchments, there are no other sources of water for abstraction. For the River Piddle in Dorset, abstractions have caused reduced summer flows and increased siltation. The summer dry period of some winterbournes, such as the River Misbourne in Buckinghamshire, has been extended by months and more of the upper reaches of the rivers have become dry.

The Environment Agency is currently investigating 37 chalk river sites to tackle problems caused by abstractions. There could be more. Of these, 12 have plans for low-flow alleviation schemes. On the River Misbourne, a low-flow alleviation scheme has already been implemented. This has shown that when flows are restored the ecology can soon recover.

Figure 6 Impact of abstraction on juvenile brown trout habitat in the River Piddle, Dorset

Source: Strevens, 1999

River Darent - flowing

River Darent - dry

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Water quality

High-quality chalk rivers have low concentrations of nutrients such as phosphate and nitrate. This limits the growth of algae and improves water clarity.

Water quality can be measured by using biological and chemical indicators. In 2000, 89% of chalk rivers were of good or very good biological quality up from 72% in 1990. Using chemical  measures, 83% were graded as good or very good in 2000, compared with 64% in 1990. These trends are good news, but there is still need for improvement only 37% are very good quality in both biological and chemical terms.

The main threats to water quality are sediment-laden run-off and nutrients from farmland and sewage works. Also a cause for concern are toxic pollutants from industrial effluent, sewage, urban run-off, and pesticides from agriculture and watercress farms.

Plant communities on chalk rivers are greatly affected by the levels of nutrients, especially phosphate. Too much phosphate disrupts the natural balance of plant and animal communities, and unsightly algae may smother underwater plants and coat the river-bed. There is now a guideline phosphate standard (60 g P/l) for designated wildlife sites to protect the natural ecology of chalk rivers. In 2000 only 23% of chalk rivers were below this figure: 10% exceeded it several times over.

In some catchments the majority of phosphate comes from sewage works. Water companies are installing phosphate removal at several sites, which is expected to reduce phosphate levels by as much as 75%. This reduction has already been achieved on the Kennet and Wensum rivers.

(c) Dennis Bright
Very good water quality is important for chalk rivers

Figure 7 Phosphate concentrations in chalk rivers, 1999-2001

Source: Environment Agency

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