It tells the story of centuries of relatively ineffective efforts to drain the Levels and to put an end to flooding. The ineffectiveness has been partly due to bureaucracy, partly due to conflicts of interest, and partly due to a belief that draining the Levels by means of gravitational flow is possible - which it isn't - so that the only way to do it is bound to require modern pumping equipment.
The bureaucracy goes back to the medieval 'Court of Sewers,' but the problem essentially is that until the second world war there has never been a central authority able and willing to provide the finance and to make decisions which were not possible for locally-based authorities. In 1939, spurred on by the need to supply the new Ordnance Factory at Puriton with 4.5 million gallons of water a day for the production of high explosives, considerable progress was suddenly made. Since then, the maintenance of the Somerset Levels drainage system has remained the responsibility of central government.
The conflicts of interest have been between the needs of river transport (which requires reasonably deep water) and drainage (which means getting rid of water as efficiently as possible); between the rich (who have wanted to 'improve' the land, which really meant to improve their income from it) and the poor (whose rights of commonage on the moors - summer grazing, peat cutting etc - were gradually eroded by the enclosure and attempted drainage of the levels); between the peat lands (which are only much use for agriculture if drained completely) and the clay lands near the Somerset coast (where there was a need to retain water for irrigation in summer); between different areas of the Levels, which would often drain their land into each others'; between drainage and the construction of fishing weirs, mills and other such installations that would tend to restrict the river's flow and encourage flooding; and more recently between farmers and conservationists. The issue is anything but simple.
And regarding gravity, which is how water normally drains off the land and ultimately into the sea: during times of flood the main river channels through the levels, for reasons that are part-natural and part-man-made, have water levels that are higher than the surrounding land; in addition to which the coastal belt has a layer of peat that is over-laid with clay (alluvial deposits from ancient sea-flooding), so that the ground level is actually several feet higher than further inland. With the fall from, say, Bruton to Highbridge being only a few inches per mile anyway, 'drainage' per se is not a feasible method of flood prevention. Pumping began with steam power in the nineteenth century, and these days the whole system relies on sophisticated electrically-powered pumping stations which operate more or less the whole time.
After all this Williams, who was writing in 1970, finishes with a warning concerning "the ever-present prospect of the occurrence of a particularly unfavourable combination of high tides, adverse wind and barometric pressure conditions, and heavy and prolonged rainfall. This combination would make short work of man's effort to control the adverse physical environment of the Levels."
In the winter of 2013/14, this is precisely what has happened.