Teddington in west London marks the divide between the tidal and non-tidal Thames. Upstream, the river flows through many urban and rural areas. The main environmental impacts in rural areas tend to come from agriculture, which can pollute the river, and recreational use, for example by walkers and boats.
Don’t forget that the river is all one system so what happens on the river and tributaries upstream can affect the Thames in London. The main impact is from the volume of water that flows over the weir at Teddington. In times of heavy rain, there is lots of flood water pouring through London. This speeds the river up and makes it more difficult for wildlife to survive. In times of drought, there is sometimes hardly any water flowing over Teddington weir – people have used it all before it gets to London. This makes any pollution in the river more concentrated. To find out why, read the information on Flooding and Taking too much water.