Thames the River Thames

Sea level

Sediment cores up to 10 m deep collected by the British Geological Survey from the banks of the tidal River Thames contain geochemical information and fossils which provide a 10,000-year record of sea-level change.^[16] Combined, this and other studies suggest that the Thames sea-level has risen more than 30 m during the Holocene at a rate of around 5–6 mm per year from 10,000 to 6,000 years ago.^[16] The rise of sea level dramatically reduced when the ice melt nearly concluded over the past 4,000 years. Since the beginning of the 20th century, rates of sea level rise range from 1.22 mm per year to 2.14 mm per year.

Catchment area and discharge

The Thames River Basin District, including the Medway catchment, covers an area of 6,229 square miles (16,130 km²).^[17] The river basin is a mixture of urban through to rural in the east and northern parts while the western parts of the catchment are predominantly rural with many towns to a suburban layout such as, archetypically Swindon. The area is among the driest in the United Kingdom. Water resources consist of groundwater from aquifers and water taken from the Thames and its tributaries, much of it stored in large bank-side reservoirs.^[17]
The Thames itself provides two-thirds of London's drinking water while groundwater supplies about 40 percent of public water supplies in the total catchment area. Groundwater is an important water source, especially in the drier months, so maintaining its quality and quantity is extremely important. Groundwater is vulnerable to surface pollution, especially in highly urbanised areas.

The non-tidal section

Brooks, canals and rivers, within an area of 3,842 square miles (9,951 km²),^[18] combine to form 38 main tributaries feeding the Thames between its source and Teddington Lock. This is the usual tidal limit; however, high spring tides can raise the head water level in the reach above Teddington and can occasionally reverse the river flow for a short time. In these circumstances, tidal effects can be observed upstream to the next lock beside Molesey weir,^[18] which is visible from the towpath and bridge beside Hampton Court Palace. Before Teddington Lock was built in 1810–12, the river was tidal at peak spring tides as far as Staines upon Thames.
In descending order, non-related tributaries of the non-tidal Thames, with river status, are the Churn, Leach, Cole, Ray, Coln, Windrush, Evenlode, Cherwell, Ock, Thame, Pang, Kennet, Loddon, Colne, Wey and Mole. In addition, there are occasional backwaters and artificial cuts that form islands, distributaries (most numerous in the case of the Colne), and man-made distributaries such as the Longford River. Three canals intersect this stretch: the Oxford Canal, Kennet and Avon Canal and Wey Navigation.
Its longest artificial secondary channel (cut), the Jubilee River, was built between Maidenhead and Windsor for flood relief and completed in 2002.^[19][20]
The non-tidal section of the river is managed by the Environment Agency, which is responsible for managing the flow of water to help prevent and mitigate flooding, and providing for navigation: the volume and speed of water downstream is managed by adjusting the sluices at each of the weirs and, at peak high water, levels are generally dissipated over preferred flood plains adjacent to the river. Occasionally, flooding of inhabited areas is unavoidable and the agency issues flood warnings. Due to stiff penalties applicable on the non-tidal river, which is a drinking water source before treatment, sanitary sewer overflow from the many sewage treatment plants covering the upper Thames basin should be rare in the non-tidal Thames. However, storm sewage overflows are still common in almost all the main tributaries of the Thames^[21][22] despite claims by Thames Water to the contrary.