The marshy districts of Sologne, Brenne, Landes and Dombes still contain large undrained tracts.
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.
There are undrained, swampy districts in Campeche, in the vicinity of the Terminos Lagoon, where malarial diseases are prevalent, and the same conditions prevail along the coast where mangrove swamps are found.
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.
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.