The Thames River Basin

The Thames is one of the most iconic rivers in the world and is 346 km long. It is the longest river entirely in England and the second longest in the UK. Its source is at Thames Head, near Kemble in the Cotswolds and its mouth is the Thames Estuary at Southend-on-Sea where it meets the North Sea.
The main tribuaries of the River Thames are:

Lea (68 km)
Leach (30 km)
Churn (37 km)
Coln (52 km)
Windrush (48 km)
Kennet (77 km)
Evenlode (68 km)
Ock (37 km)
Loddon (32 km)

The Thames and its tributaries have many important floodplains and are home to a wide variety of wildlife. They are our water supply and a beautiful part of our landscape to enjoy. The many chalk streams that contribute to the Thames are rich with iconic species of native wildlife including otter, kingfisher and water vole (the River Kennet and Lambourn are SSSIs), but they are suffering due to many pressures; from urbanisation to invasive species. Rivers Trusts are working hard in many different ways to help combat these pressures.

Invasive Species

Once a non – native species has got into the food web it is very difficult to control and the impacts can be devastating.
American mink have played a key role is decimating our water vole populations.
There are now at least 6 species of non-native crayfish in our rivers, which are having a considerable negative effect on our White-clawed crayfish, other freshwater creatures and our riverbanks.  Killer shrimp (Dikerogammarus villosus) can dramatically alter ecosystems, killing native species including young fish.
Plants such as Himalayan balsam have spread along our waterways, growing over 2 metres high in a single season and outcompeting our native flora. For more information on invasive non-native species (INNS) and how trusts are tackling them click here INNS – Plants

Himalayan balsam removal


Urbanisation in concreted cities and towns has left rainfall with less places to slowly infiltrate. This has led to increased pluvial flooding. Human interventions have disconnected floodplains, leading to more flooding and less sustainable fish populations and biodiversity. Trusts are playing a key role in Natural Flood Management and sustainable drainage schemes (SuDS) that are helping to decrease these problems.

Natural Flood Management

Flooding is a natural part of the river cycle and is necessary to maintain wetlands and other riverside habitats and species. Unfortunately, many people are at risk from river flooding in the Thames river basin due to homes, buildings and infrastructure being constructed on floodplains. Many traditional, hard engineering schemes are in place to help protect people and property but they are not guaranteed to prevent all river flooding. Part of the solution is to make more space for flood waters and to slow the flow of rivers at times of high rainfall. These objectives can be helped by the use of Natural Flood Management (NFM) techniques such as creating or restoring floodplains, constructing leaky woody dams, providing overspill areas and planting trees on slopes, especially on the tributary rivers and headwaters. All of these techniques also improve rivers for wildlife. The first pilot catchment for NFM in the Thames region is on the River Evenlode and the Cotswold Rivers Trust is a partner in this. Click here for more details.

Award winning rain garden by Wendy Allen

TRT LWD slow the flow

Slowing the flow with large woody debris (LWD)

Diffuse and Point Source Pollution

Diffuse pollution is the release of potential pollutants from a range of activities that individually may have little effect on the water environment, but at a catchment scale can have a significant effect. The problem is mainly during heavy storms  and results from rainfall carrying runoff pollution from agriculture and roads into our rivers and ground waters.  Point source pollution is from a direct specific source, for example an effluent discharge pipe. Trusts are working to educate the public by raising awareness of the ways pollution enters our rivers, with campaigns such as Yellow Fish. Trusts are working with farmers through catchment sensitive farming schemes to improve soil cover and structure to minimise nutrients leaching into the rivers.

Yellow Fish plaques letting people know not to put anything down storm drains


Unconnected Habitats

Over many years humans have put in mutiple barriers to our rivers and streams, such a weirs. These prevent free movement for fish such as brown trout, salmon and eels that have different requirements throughout their life cycle and need to move between habitats to spawn, decrease chances of predation, isolation and disease.
Water vole are also victims of habitat isolation. Unsympathetic riparian management can leave Britain’s largest vole with nowhere to live and nowhere for young to disperse.
Trusts are working with landowners to open up stretches of river, removing barriers to fish migration. Trusts are surveying for water voles and carrying out river restoration habitat improvements to increase suitable terrain for Britain’s fastest declining mammal.

Fish pass installed to open up free fish movement

Over Abstraction

Public water resources consist of reservoirs, aquifers (which supply 40% of the Thames River Basin District and include the limestone of the Cotswolds) and the River Thames, which provides two thirds of London’s drinking water.
Unsustainable abstraction can lead to reduced flow and rivers being drier for longer periods of time. This has a damaging effect on the biodiversity.
Trusts are lobbying water companies and raising awareness of the importance of water efficiency to the public.

The River Kennet suffering from over abstraction in 2011

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