Tidal Thames habitat audit

The Thames and its tidal creeks encompass the entire length of the river in London and the tidal limit of its tributaries. In many cases this tidal limit is artificially restricted by the operation of various barriers and weirs. The River Thames runs 42 miles through Greater London from Hampton in the west to Dartford Creek in the east. For much of its length it is tidal, the tidal influence reaching as far upriver as Teddington Lock. There are several tributaries of the Thames which enter the river within Greater London, a number of which (notably the Wandle, Ravensbourne, Lea, Roding, Darent and Ingrebourne) have tidal creeks.

The Thames in London covers an area of approximately 2400 ha, about 1.5% of London's surface area. At low tide the river comprises c2050 ha of open water (85% of the river's surface area), 310 ha of intertidal mud, sand or shingle (13% of the surface area) and 17 ha of saltmarsh (0.5%). The remaining area comprises patches of neutral grassland, woodland and scrub associated with the islands in the Thames, and remains of former river walls that are within the existing flood defence. Several areas of tidal reedbed have developed in recent years, particularly in areas such as Barking Creek and Bow Creek (see Reedbed Audit).

The flood defences (river walls) on the Thames vary in nature and characterise the different reaches of the river. Upstream of Putney Bridge much of the flood defence is sloping revetment, often vegetated, which softens the river's edge and riverbank. Between Wandsworth Bridge and the Greenwich Peninsula the river is largely constrained between vertical concrete and sheet metal piled walls (although areas of mud, sands and gravel are exposed at low tide). Downstream of the Greenwich Peninsula, despite much of the flood defence still consisting of vertical concrete walls and sheet-metal piling, it is set further back from the main river channel thus exposing extensive areas of intertidal mud at low tide.

Management of the Thames rests primarily with two organisations; the Port of London Authority (PLA) and the Environment Agency (EA). The PLA is concerned primarily with navigation, pollution control and land-use planning issues related to the river; the EA has responsibilities covering flood defence, pollution control, fisheries, water quality, environmental protection and nature conservation.

The Thames represents the largest continuous natural habitat in Greater London. The whole of the Thames and its tidal tributaries has been identified by the London Ecology Unit as a Site of Metropolitan Importance for nature conservation. More than 100 fish species have been recorded in the Thames estuary over the past 30 years, many of these in the river within London.

Although there is very little natural riverbank along the Thames and its tidal tributaries (the only significant stretch being the riverbank at Syon Park), several quite large stretches of riverbank consist of earth embankment set back from the river. These sites have allowed saltmarsh, tidal reedbeds and other intertidal habitats to develop. Furthermore, the sloping revetment that forms the flood defences in certain stretches of the river provides an opportunity for aquatic vegetation to become established along the river's edge. Downstream of Tower Bridge, sloping revetment provides an opportunity for the establishment of saltmarsh.

The two most significant threats to the biodiversity of the Thames in London are pollution and the loss of intertidal habitat by the encroachment of built development. Although the severe pollution of the river in the 19th and early 20th centuries is now a thing of the past, because it flows through the largest conurbation in Europe the potential for pollution of the Thames is ever present.

Encroachment of built development on the river corridor is a major threat to biodiversity in the Thames. The river, particularly in the central London reaches, has already been severely constricted so that at low tide only a very narrow fringe of foreshore is exposed. Further encroachment is likely to prevent or hinder fish movements and restrict opportunities for diversifying riverside habitats.

Opportunities exist for retreat from the river as riverside sites are redeveloped, enabling the establishment of sloping embankments. With appropriate design riverside walks can enable people to enjoy the river without undue disturbance of birdlife.

There is a significant potential for restoring and recreating some of the habitats along the Thames which were lost when flood defences were installed without due regard to biodiversity.

Innovative approaches to enhancing the value of the river corridor for wildlife include installing timber cladding on concrete and sheet-steel flood defences to provide niches for plants and invertebrates and stepping back (or otherwise adapting) flood defences to enable habitat enhancement.

The Thames, as a familiar feature of London, provides great potential for raising awareness of the biodiversity of the river and beyond. Illustrating the value of the Thames and its tributaries as a nationally important corridor for migrant birds, for example, will be an important element of an Action Plan. Hundreds of thousands of people a day cross the river or travel along its banks. Some of London's major areas of open space (Kew Gardens, Battersea Park and Greenwich Park) and some of its major attractions (The Millennium Dome and the Wetland Centre - both opening in 2000 - and the Tower of London) adjoin, or lie adjacent to the river. Furthermore, the seats of both central government and the new local government for London are, or will be located alongside the Thames in central London.