The Upper Thames catchment includes the entire area that drains into the River Thames from its source south-east of Cirencester at Kemble eastwards to Lechlade. The Upper Thames also includes tributaries that drain from the north in the Cotswolds and the south from the Vale of the White Horse. There are a total of 35 water bodies in the catchment (see map here on the 'Water Bodies' page).

 

 You can download a pdf of the above map by clicking here

 

Governing bodies

The north of the catchment is within the Cotswold District Council and Gloucester County Council, whilst the area to the south of the catchment is within Wiltshire County Council, Swindon Borough Council and the far south-east borders are within the Vale of White Horse District Council.

 

Landscape designation - Areas of Outstanding Natural Beauty

The catchment is predominantly rural in character and has a number of distinctive landscapes. The northern part falls within the Cotswold Area of Outstanding Natural Beauty which is dominated by the extensive limestone plateau and undulating dip slopes to the east. The southern end of the catchment falls partly within rolling chalk downlands of the North Wessex Downs Area of Outstanding Natural Beauty.

 

Living on the Thames

The principal towns are Swindon and Cirencester, however, there are many smaller market towns located throughout the catchment. The waterways and hydrology associated with the catchment have played an important part in the history of the area from market towns based on the woolen industry in Medieval times through to gravel extraction from the Cotswold Water Park area over the last 50 years. The extensive network of modified water meadows has also been very important to agriculture over the centuries for increasing grass growth and helping to control flooding.

The area is important for its recreation provision such as sailing in the disused gravel pits to high-quality game and coarse angling in both rivers and still waters. The limestone streams to the north of the area, including the Rivers Coln, Churn, and the Ampney Brook, contain predominantly wild brown trout and grayling populations, though the lower reaches also contain important coarse fish populations and habitats. Many of the watercourses are stocked by their owners and angling associations, to supplement wild stock levels. The River Thames and its tributaries to the south are predominantly coarse fisheries, as are the still waters that form the Cotswold Water Park.

 

Photo ©Steve Bradley                                   

 

Geology

The catchment has a varied geology which impacts the movement and characteristics of ground and surface waters due to features such as the permeability of bedrock materials. The underlying geology of the central area of the catchment, extending from Purton in the west to Lechlade in the east is characterised by clay. This means that there is a rapid response of surface water to rainfall episodes. Overlaying much of this clay is an area of sand and gravel deposits that provide the base flow for the River Thames and its tributaries in this location. The northern area in the Cotswold hills is characterised by limestone which is a store of groundwater (aquifer) known as the Burford Jurassic. The limestone geology is prone to fissures which are vertical pathways that can allow rapid movement of water and chemicals into the store of groundwater. A line of Kimmeridge clay stretches from the south-east to the north-east of Swindon and out into the Vale of the White Horse hills.

For more detailed information on groundwater management catchments within the Upper Thames see the link to the Environment Agency’s Catchment Data Explorer.

     

The Thames Wildlife

There are many important wildlife habitats and species that are dependent on the hydrology of the area. This includes species-rich floodplain meadows, wet woodlands, a wide range of nationally and internationally important wildfowl, rare stoneworts, water voles, and otters.

The area of Clattinger Farm and North Meadow is designated by the EU as a Special Area of Conservation (SAC). This SAC is made up of two areas, 7km apart which are designated as a single site for their species-rich lowland hay meadows. They are particularly important for their populations of Snakeshead Fritillaries. North Meadow also has a fascinating history as it is Lammas land that has been managed by the local people of Cricklade through the Court Leet for centuries. It is now a National Nature Reserve. The members of the Court Leet still have an active role in managing the grassland.

Other important wildlife sites are designated as Sites of Special Scientific Interest. The Cotswold Water Park is notified for its assemblage of water plants and charophytes, however, is currently undergoing re-notification to include the assemblages of birds which frequent the site. There are also a number of wetland meadow sites whereby maintaining the hydrological regime is vital to maintaining the wildlife interest, for example, Elmlea Meadow which has a population of very rare Downy fruited sedge.

The location and current condition of designated sites can be found on http://www.magic.gov.uk/

Photo ©Corine Bliek

 

Tributaries of the Upper Thames

The largest tributaries of the Upper Thames are the Coln, the Ray (Wilts), the Churn and the Cole.

The River Coln is a groundwater-fed river, which rises to the east of Cheltenham from limestone springs which overlay clays. The predominant land use in the area is farming (a mixture of pasture and arable) and the area is mostly rural. There are no large towns in this catchment but a number of villages which are at risk of flooding. Flow in the Upper Coln is derived mainly from groundwater supplied limestone so the flow is stable with a slow response to winter rainfall events. Summer flow is maintained by the groundwater even in dry years. The Dudgrove Brook and Thornhill Brook join the river downstream of Fairford and contribute surface water runoff to the river flow. They converge with the Thames west of Lechlade.

The River Churn is also a spring-fed river, it rises south of Cheltenham and flows through Cirencester to its confluence with the River Thames at Cricklade. The Upper Churn catchment is mostly rural, the predominant land use being farming. Flows in the upper reaches can fall to very low levels during the summer. This is because the aquifers release water quickly and so in response to little or no rainfall the base flow supplied to the river will be very small.

Its middle and lower reaches are influenced by the Cirencester urban area as it passes through the town. As it passes through Cirencester (12.4km from the Thames confluence), the Churn crosses the geological boundary from limestone to clay so that the river shifts from being groundwater fed to surface water run-off. During winter, groundwater can still be a significant contributor to flows. Throughout the summer months, there are concerns along the Churn over low flows, which is exacerbated by groundwater extraction and low rainfall. A number of mills and weirs have also affected the channel.

The River Cole is a highly modified clay river which has been subject to considerable flood defense and land drainage engineering in the past. Flooding and poor water quality have been exacerbated by the impact of rapid and at times poor quality run-off from the urban area of Swindon at its headwaters. The majority of land management in the catchment is intensive agriculture although there are a number of semi-improved pastures and wetland habitats, the latter on the fringes of Swindon. Perhaps as a result of its flashy nature, the Cole does show some geomorphic features such as berms, earth cliffs and slumps, but in the upstream reaches the river tends to be shaded by bank-side and bank-top vegetation which prevents the development of aquatic and marginal plants. A number of smaller streams of variable ecological quality flow into the Cole, the most interesting of which is the downstream section of the Tuckmill Brook which has a diversity of channel features meandering through marshy floodplain between river terrace features.

The River Ray is a heavily modified clay river that rises at the large conurbation of Swindon. Unlike the Cole, the River Ray receives effluent from a large sewage treatment works (STW) which serves to sustain flows. Until recent improvements, the STW effluent was a significant factor in limiting the ecological value of the watercourse. All the main river tributary streams of the Ray are small watercourses flowing in part or wholly through urban areas and in the most part are heavily shaded by bankside vegetation. The Ray, downstream of Swindon, flows through a mainly pastoral floodplain landscape with improved hay meadow grasslands. There is moderate diversity of channel and marginal vegetation in the lower Ray, and the river does display several geomorphic features despite the highly modified nature.

 

 

 


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