Before the drainage, the fens were a fresh water marshland studded with islands. The marsh received the drainage water of a vast area of higher ground surrounding the fenland basin carried by the main rivers, the Welland, Nene and Ouse and the many tributaries flowing into them.
The marsh was usually flooded in the winter but varying areas would dry in the summer, subject to the rainfall.
Human settlements were confined to the land around the edge of the marsh and on the islands. There is ample evidence of occupation dating back at least to the bronze age. There are many places in the fens with the suffixes "ey", "ea"or simply "y" (all pronounced "ee").
This sound is the Anglo-Saxon word for island and indicates land that was never flooded. and this word ending is found today on many of the place names for villages which were originally islands in the marsh.
The best known is of course Ely (eel island) but also Stuntney, Coveney, Hainey, Norney, Quanea, Manea and Shippea Hill.

The fen provided a lot of resources to the early settlers :
FOOD (in the form of fish, eels and wildfowl).
BUILDING MATERIALS (in the form of turf for house walls, reeds and sedge for thatching, willow for wattle and daub construction).
FUEL (in the form of peat which was dug and stacked to dry ).
SUMMER GRAZING for farm animals.

During the 7th Century AD a number of religious settlements were established in the area, most notably at Ely, Crowland and Ramsey. It was thought that the monks were attracted by the relative solitude and the ability to be self-sufficient. The Ely settlement in particular became very rich and powerful as evidenced by the ability to build the magnificent cathedral ("the ship of the fens").

A lot of work was carried out to water courses before the 17th Century. Some parts of rivers were straightened and some new channels were constructed. Major river works include the course of the main Ouse from Littleport, running northwards to Stowbridge and its present course past Ely. Other works include Soham, Wicken and Reach Lodes. For most of these works, we have few clues as to who carried them out. It could have been the Romans, the Anglo Saxons or the Normans and it is probable that they all contributed.
Most of the early works though, were probably more to do with transport than with drainage, as boats were the only practical way to carry goods across the area, right up until the coming of the railways in the 19th Century. As an example, the stone used to build Ely Cathedral was carried by boat from a monastic quarry at Barnack, near Peterborough.

There seems to have been no systematic attempt to drain the whole area until the arrival of the "Adventurers" in the 17th Century.