The evidence is outlined in Nayling and Caseldine*. Essentially they find that the broad marine area that existed in the Mesolithic moved to freshwater channels in the Neolithic and early Bronze Age (~4000BCE onwards), before the channels broadened once more and became tidal in the late Bronze Age (possibly as early as 1500BCE, but more definitely by 1000BCE) and early Iron Age (~500BCE). These changes reflect the changing relationships between rising sea level and sediment inputs, which, in turn, reflect changes in climate modulated by local agricultural changes*.