With a wet, undrained subsoil and a large population of Indians and half-breeds living in crowded quarters, the death-rate has been notoriously high, though the completion of the Valley drainage works in 1900, supplemented by underground sewers in the better parts of the city, and by better sanitation, have recently improved matters.
Decoys were once numerous in the undrained state of the Fens.
The marshy districts of Sologne, Brenne, Landes and Dombes still contain large undrained tracts.
Nearer the Great Lakes the till sheets are trenched only by the narrow valleys of the large streams; marshy sloughs still occupy the faint depressions in the till plains, and the associated moraines have abundant small lakes in their undrained hollows: hence these drift sheets are of late Pleistocene origin.
The streets as originally laid out were wide and spacious, but being unpaved and undrained they were no better than mud tracks diversified by piles of garbage and foul-smelling stagnant pools.
As a whole the system forms the watershed between rivers flowing to the Indian Ocean - the Indus and its tributaries, Brahmaputra and its tributaries, and Salweenand the streams flowing into the undrained salt lakes to the north.