It then flows south through a deep valley past Llawhaden and becomes tidal at Canaston Bridge, the lowest crossing point.

Historically, the estuary gave seaborne access to castles such as Pembroke and Carew, allowing these to be used as depots in the Norman invasion of Ireland.

It was important in the early Industrial Revolution, shipping anthracite from Llangwm, Landshipping and Crescelly, and limestone from Lawrenny and West Williamston.

A small fishing industry operated from harbours such as Pill, Angle and Dale,[4] but in 1790 the building of the new town of Milford commenced,[5] and a large herring fishery grew up based on its docks.

Milford was originally built for a naval dockyard, but this project was transferred in 1814 to Pembroke Dock on the opposite side of the estuary,[7] where it operated until closure in 1926.

The town of Neyland, originally known as New Milford, was also purpose-built, this time by the Great Western Railway as a transatlantic shipping terminal.

[12] The Cleddau rivers are also a Special Area of Conservation designated for the European bullhead (Cottus gobio), European river lamprey (Lampetra fluviatilis), Brook lamprey (Lampetra planeri), Otter (Lutra lutra), Sea lamprey (Petromyzon marinus); rivers with floating vegetation often dominated by water-crowfoot (watercourses of plain to montane levels with the Ranunculion fluitantis and Callitricho-Batrachionvegetation); active raised bogs; and Alder woodlands on floodplains (Alluvial forests with Alnus glutinosa and Fraxinus excelsior (Alno-Padion, Alnion incanae, Salicion albae)).

[13] Additionally, land around a north-eastern tributary of the Eastern Cleddau, Gweunydd Blaencleddau,[14] is designated as a Special Area of Conservation for habitats including: calcium-rich springwater-fed fens - Alkaline fens; the southern damselfly (Coenagrion mercuriale), marsh fritillary butterfly Euphydryas (Eurodryas, Hypodryas) aurinia; purple moor-grass meadows - molinia meadows on calcareous, peaty or clayey-silt-laden soils (Molinion caeruleae); wet heathland with cross-leaved heath Rhostiroedd gwlyb – Northern Atlantic wet heaths with Erica tetralix; very wet mires often identified by an unstable 'quaking' surface – transition mires and quaking bogs; and blanket bogs.