The South Chilterns Catchment - Overview

The River Thames flows from the limestone of the Cotswolds through Gloucestershire, Oxfordshire and onwards via Berkshire and Buckinghamshire, then to London, after which it finally enters the North Sea at the Thames Estuary. The underlying geology of the catchment consists of Upper Greensand, Chalk and younger Eocene sands and clays, dipping in a south-easterly direction. To the north of the catchment lie the Upper Greensand deposits which are replaced by chalk deposits (Lower, Middle and Upper Chalk) just south of Wallingford. Towards the south of the catchment around Reading and Pangbourne lies the transition between the chalk and younger sands, gravels and clays. Within the Thames floodplain and beyond, there are extensive drift deposits comprising gravels, clays with flints and head deposits.

The South Chilterns catchment is the southern half of the Thames and South Chilterns Managed Catchment that in turn forms part of the Thames River Basin District. The catchment includes parts of both the Chilterns Area of Outstanding Natural Beauty (AONB) and the North Wessex Downs (AONB) . The boundary between the two areas is marked by the River Thames. Though both areas are chalk uplands, their character differs. The North Wessex Downs AONB includes both uplands and lower land towards the Thames, while the Chilterns AONB is generally of a more enclosed and wooded nature. It has three distinct geographical areas: in the West is the River Pang, a chalk stream rising in a rural area flowing through West Berkshire to enter the River Thames at Pangbourne; the navigable River Thames flowing through the middle of the catchment between Wallingford and Cookham; and in the east is the River Wye, an urban chalk stream rising near High Wycombe and entering the Thames at Bourne End. A characteristic of the area is the large number of chalk streams that flow into the Thames. Chalk only occurs in northern Europe, North America and New Zealand. It absorbs rainfall, releasing it slowly at a constant temperature of 7 -10oC, adding calcium, which together with steep gradients, oxygenate the water resulting in a rich wildlife habitat.

The land is mostly medium grade agricultural land, a mixture of arable and livestock farming. There are also areas of chalk grassland and beech woodland which are characteristic of the Chiltern Hills and North Wessex Downs. Though predominately rural in nature the catchment contains some extensive urban areas – High Wycombe, Henley-on-Thames, Marlow, Reading, and Wallingford. As with most areas within the London commuter belt there is a significant amount of development pressure in the area.

Like many catchments, the South Chilterns and its tributaries face numerous pressures, including Agricultural and Land Management practices, industrial pollution, water abstraction, past channel modifications, invasive non-native plant species, and the impacts of a changing climate. River Basin Management Plans (RBMPs) are designed to protect and improve the state of rivers, lakes and other waters in each of the 11 river basin districts throughout England and Wales. Being updated every six years, the new 2015 RBMPs describe how improvements will be made. The Thames RBMP provides a basis on which a good quality water environment can be achieved across the whole region. The Environment Agency’s Thame and South Chilterns Catchment Summary is based on the Thames River Basin Management Plan. It identifies the Water Framework Directive failures of each water body in the catchment and sets out actions that can address them.

The Catchment Summary and updated Thames RBMP have been essential in helping the partnership identify priorities for action and forming ideas for projects detailed in our Catchment Action Plan.

In addition, the Environment Agency has produced the Thames River Basin District Flood Risk Management Plan 2015- 2021 PART B – sub areas in the Thames river basin district, that gives details of the sub-areas and flooding statistics within the Thames RBD. The management of flood risk is influenced by the diverse physical features of the Thames catchment. Towards the west in Gloucestershire, Oxfordshire and Berkshire, the Thames and its tributaries flow through a rural landscape with rolling hills and wide, flat river floodplains. The rivers generally flow in a natural channel and there are extensive areas of rich floodplain habitat. Some areas of the floodplain are internationally designated environmental sites. Some of the most notable are the Oxford meadows in the floodplain of the River Thames. Winter flooding of the undeveloped floodplain is a regular occurrence and this floodplain provides a large area to store water that reduces the risk of flooding to people and property further downstream. It is crucial that this existing undeveloped floodplain is safeguarded from development. On the Pang, flooding from rising groundwater is more likely to occur, particularly around Stanford Dingley.

There are over forty Sites of Special Scientific Interest (SSSI) within the catchment though the majority of these are located outside the actual river corridor. These are discussed in greater detail within the individual catchments. There are 13 water bodies within the South Chilterns catchment. Select the button to take you to that location.

South Chilterns Catchment Information

Number of water bodies in the South Chilterns catchment

Water body category Natural Artificial Heavily modified Total
Rivers, canals and surface waters 8 0 4 12
Lakes 0 1 0 1
Coastal 0 0 0 0
Estuarine 0 0 0 0
Groundwater 0 0 0 0
Total 8 1 4 13

Ecological and chemical classification for the catchment surface waters - 2015 Cycle 2

  Ecological status or potential Chemical status
Number of water bodies Bad Poor Moderate Good High Fail Good
13 1 4 5 3 0 0 13

An explanation of these classifications can be found in The Water Framework Directive and the Catchment Based Approach.

Berrick Stream
Cholsey Brook
Ewelme Stream
Hamble Brook
Mill Brook
Sonning Eye
Upper Middle Thames
Lower Middle Thames
Upper Wye
Whole Catchment