[3][4] Three areas are local nature reserves, Boldre Foreshore,[5] Sturt Pond[6] and Lymington and Keyhaven Marshes;[7] the latter site is managed by the Hampshire and Isle of Wight Wildlife Trust.

On the seaward side of the seawall the SSSI encompasses three estuaries of substantial streams and their associated intertidal mudflats, marshes dominated by Spartina anglica as well as higher level saltmarsh.

It is geologically important as the basis of a seminal paper explaining the relationship of beach alignment to the direction of dominant waves.

Flocks of brent geese, Eurasian wigeon and Northern pintail use the estuary in the winter while waders such as dunlin, black-tailed godwit and grey plover feed on the mud and roost in the marshes and shingle ridges.

Birds of prey, such as Western marsh harriers, peregrine falcons, and merlins are attracted by flocks of wildfowl and waders.