Within the San Rossore Estate - near the River Arno Estuary - lies a vast wetland covering approximately 618 hectares, of fundamental importance as an overwintering and stop-over site.

The complex of retrodunal pools of the Reserve is directly related to the tidal movements which initiated the formation of the dunes.

Under favourable conditions, as a tombolo begins to form, a consolidated sandbank builds up in front of the dunes. The new, growing beach tends to join the pre-existing shore and causes a sheet of water to accumulate between the new dune and the old coastline.

Finally, further sediments are deposited which close any connections with the sea and a proper retrodunal lagoon is formed.

Over the centuries, these basins, at first lagoons, then retro-dunal and coastal pools, gradually turned into depressions ("lame") adjacent to the tomboli and so became a suitable habitat for the development of mesohygrophilous forests.