answers: River Severn
context: the whole of Greater London. Its tidal section, reaching up to Teddington Lock, includes most of its London stretch and has a rise and fall of . Running through some of the driest parts of mainland Britain and heavily abstracted for drinking water, the Thames' discharge is low considering its length and breadth: the Severn has a discharge almost twice as large on average despite having a smaller drainage basin. In Scotland, the Tay achieves more than double the Thames' average discharge from a drainage basin that is 60% smaller. Along its course are 45 navigation locks with accompanying weirs. </s> in the UK include: The lowest point of the UK is in the Fens of East Anglia, in England, parts of which lie up to 4 metres below sea level. The longest river in the UK is the River Severn () which flows through both Wales and England. The longest rivers in the UK contained wholly within each of its constituent nations are: The largest lakes (by surface area) in the UK by country are: The deepest lake in the UK is Loch Morar with a maximum depth of 309 metres (Loch Ness is second at 228 metres deep). The </s> much of it stored in large bank-side reservoirs. The Thames itself provides two-thirds of London's drinking water while groundwater supplies about 40 per cent 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. Brooks, canals and rivers, within an area of , 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
question: what is the longest river in great britain