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3. Altering the flow of the river

The Thames is a fast flowing and naturally flooding river. This movement of water is important for the animals that live in the river. However, many people would like to build a permanent barrier across the river to stop the flooding. This barrier may make the Thames a difficult place for wildlife to survive.

The tidal river provides a range of habitats for invertebrates, birds and fish. It can be divided into 3 main zones:

Freshwater zone: Teddington to Battersea

Brackish zone (mixture of fresh and salt water): Battersea to Gravesend

Marine zone (salt water): Gravesend to the sea

All the creatures living in the tidal river are adapted to living in fast flowing conditions where water levels rise and fall dramatically. Most are adapted for life in either freshwater or salt water – not many can survive the mix of water in the brackish zone. These zones can move and the wildlife moves with them. When it rains heavily, the freshwater zone shifts downriver. During period of drought, the marine zone creeps upriver.

Some fish need to migrate between zones to complete different parts of their life cycle. Examples are eels, smelt and flounder, which you can easily net at the right time of year. The tide plays an important part in this fish migration. For example, flounder fry spawn in the estuary and move upriver on the rising tide. When the tide starts to fall, they rest in pools on the foreshore until the next tide comes to sweep them further upriver. The foreshore is therefore a vital habitat for migrating fish.

There is currently a strong lobby to build a barrier across the Thames, for example at Wandsworth, in order to dam the river and control the tides. This would make the river easier to use by leisure craft as it would raise the level of the river so that boats could get to the riverside at all times. The proposal is that the barrier would be open around the time of high water to let the tide go in and out for a few hours. There is also strong opposition to this idea.

Impact
A barrier could:

  • Interfere with the tides and hamper fish migration
  • Alter habitats to which river wildlife are adapted
  • Increase pollution as the tide has less time to “flush out” dirty water to the sea
  • Making water levels in London rise by blocking the drains carrying water into the river

What can we do?

  • Find out more about the arguments for and against a barrier and make your own decision as to whether a barrier would improve the Thames.
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