There is a wide network of rivers in the British Isles, though generally short in length and navigable but in their lower reaches, especially during high tides. Mild maritime climate keeps them free of ice throughout the winter months.

In the Middle Ages, river transport played a major role in the British internal transport system, and all the large towns of the time were situated on navigable rivers. But since the beginning of the nineteenth century the waterways, including numerous canals, have steadily declined in importance, and many have fallen into disuse.

The drainage map of the British Isles seems to contain no very clear pattern. The largest river of Great Britain, the Severn (390 km), for example, follows a particularly puzzling course. After rising on the slopes of Plynlimmon, in central Wales, it flows at first north-eastwards, but later turns sharply through the Iron bridge gorge and then runs southwards and southwestwards to the Bristol Channel. The courses of the Trent (274 km) and the upper Thames (332 km) also show many changes of direction. Many of the largest rivers in Scotland, such as the Tweed, Forth, Dee and Spey, drain directly to the North Sea. Scotland’s longest river, the River Tay, some 170 km long, also follows this course. Among other important rivers, which flow eastwards, to the North Sea, are the rivers Trent, Tyne, Tees, Humber, Ouse, in England.

A number of streams flow down to the west coast, to the Irish Sea, including the Clyde in Scotland, the Eden, Ribble, Mersey and the Severn. A few small rivers flow to the English Channel.

There are many rivers in Ireland, They are short but navigable due to an abundant and even distribution of precipitation throughout the year. The longest river in Ireland is the River Shannon (384 km), flowing from north to south of Ireland. Among other more or less important rivers are the Foyle, flowing to the north, the Lagan, Boyne, Liffey, Slaney — to the east, the Barrow and the Blackwater — to the south.

Most of the British lakes are in part the result of glacial erosion and in part due to chemical solution of the underlying limestone. There is a host of small winding lakes in Scotland, in Cumbria and in Ireland.

The largest lake in Great Britain and the biggest inland loch in Scotland is Loch Lomond, covering a surface area of 70 square km, although the longest lake is Loch Ness (56 square km) which also has the greatest volume of water. In England the largest lake is Lake Windermere (the Lake District) with a surface area of 15 square km.

The largest fresh water lake in the British Isles is Lough Neagh in Northern Ireland (381 square km).

The Quaternary glaciation has further modified the river patterns in many areas. This is especially true of central Ireland, where the uneven surface of the drift cover has led, as in the basin of the Shannon, to much bad drainage, many peat bogs and numerous large lakes, such as Loughs Ree and Derg.