|Lagoons are expanses of shallow coastal salt water, of varying salinity and water volume, wholly or partially cut off from the sea by sandbanks or shingle, or, less frequently, by rocks. Salinity may vary from brackish water to hypersalinity (very salty) depending on rainfall, evaporation and through the addition of fresh seawater from storms, temporary flooding of the sea in winter or tidal exchange. Salt-marshes form part of this complex system.
Three small coastal lagoons are located in the upper extremities of tributary estuaries in the upper, middle and lower Milford Haven Waterway: Pickleridge Lagoon on the Gann estuary (established as saline lagoon between 1950s and 1980s); Neyland Weir Pool at Westfield Pill (established as saline pool mid-1980s); and Carew Mill Pond on the Carew River (date of establishment at least the early 1600s, history of flushing via sluices variable over time). All are semi-natural habitats having been created by the construction of artificial structures behind which they develop.
Carew Mill Pond is of historical significance; the Mill itself and tidal causeway are Listed Buildings, and the nearby Carew bridge, Castle green, and walled garden comprise a Scheduled Ancient Monument.
Pickleridge Lagoon has a maximum depth of 1.5m. It has a boulder/cobble/shingle embankment with one end mostly open water and the other a series of drying sandy islets. It has possibly the largest population of lagoon cockle (Cerastoderma glaucum) in Wales.