Complex sentence: The river Adur was navigable to Bramber, and Botolphs was apparently the site of the "Old Bridge"; but the sea began to recede in about 1350, and the river silted up, after which the bridge fell into disuse and the village population declined.
Split into simpler sentences: The river Adur was navigable to Bramber, and Botolphs was the site of a river crossing. The sea began to recede in about 1350, and the river to silt up, after which the village population declined.