To the north of the Polden Hills the wetland is supplied by the rivers Axe, Sheppey and Brue, while to the south, the rivers are the Cary, Yeo, Tone and Parrett, the last of which is tidal up to Oath Lock near Langport. Each area is separated from its neighbour by well-wooded, low Jurassic clay and limestone ridges. If fields were too large, then the flood waters that used to cover the Levels and Moors, could not be drained fast enough from a field, conversely, too much of the land converted to ditches meant land lost to production. Each moor in the Levels and Moors has a drainage system that has evolved over a considerable period. Each field in turn has a ditch system that carries water away to the main collecting channels.
Most moors have main collecting pumps that lift the drained water into embanked rivers.
If pumps were not used then the Levels and Moors would stay flooded for much longer per year, because much of the land, especially the peat moors, is below the level of high tide and the river discharges are tide-locked. The pumps are owned and managed by the Environment Agency, which is primarily responsible for flood defence, while 16 Internal Drainage Boards, run by farmers and landowners, control the drainage and water levels of each of the moors and their sub-catchments. In winter, ditch water levels are reduced. The balance between drainage and the maintenance of adequate wetland water levels has always been debatable. The debate centres upon the need to retain the traditional landscape and its historic heritage, promote soil, water and wildlife conservation, while protecting farming and its social fabric.