The mouth of the River Adur has been greatly affected by the drifting of shoals of sand and gravel from west to east, and by major attempts by man to maintain it as a usable harbour.
The original port on the estuary of the Adur was at Old Shoreham 1.5km north of the present coastline near the old church of Old Shoreham, and the river was navigable up to Bramber (1). By the eleventh century, silting of the river resulted in the port being moved 1km downstream to what is now the town of Shoreham but was then called New Shoreham (2). Silting continued to affect the port as a result of an eastward growing spit across the estuary mouth.
At the time of the Armada Survey in 1587 (Fig 4a) the river ran eastwards from New Shoreham for a distance of about 1km to discharge at a point between New Shoreham and Kingston-by-Sea. The coast west of the mouth was fronted by a shingle beach which attained a width of 1km opposite New Shoreham, tapering away towards Lancing. Behind the beach, and along the line of the original coast was a depression occupied by an arm of water flowing into the river and directly across the river from New Shoreham there were a series of lagoons. Large sand banks lay in the mouth of the Arun which must have hampered entrance and exit by boats.