Boreades wrote: Yes, any river that is being used for streaming will be flushing huge amounts of sediment downstream. There's clear historical evidence for that in many rivers in Devon and Cornwall that have been heavily silted up. As a result, the ports and docks had to move downstream closer to the sea.
The port of Plymouth or Sutton as it used to be benefited from Plympton being silted up by tin-streaming, so I'm told. The river at whose mouth it sat was the Lurie or Lary. Tin-streaming on the upper reaches of the river was a widespread practice on Dartmoor in the Middle Ages and Sutton harbour was eclipsed by the more important port upriver at Plympton, whose name apparently derived from an early farmstead (or ton) where plums were grown. At some point the river Lurie became known as the Plym* - through back-naming - but by this time the tin-streaming had so silted up the river that Plympton was losing its viability as a port. Eventually all shipping was forced to dock at the port at the mouth of the Plym, and although the old harbour there is still called Sutton Harbour, the much larger town that grew up around it became Plymouth.
* the tidal stretch that ends at Plympton is still called the Laira.
I can only see it making economic sense to our Veneti shipping magnates (to ship raw ores) if there were huge amounts of wood/charcoal in Gaul/Brittany. Or more solar henges?
It would make economic sense if there were good quality and plentiful metals. Apparently Basques and Iberians were obtaining tin from Britain and Ireland for centuries, if not millennia. And not just tin: there was an abundance of silver and copper and useful minerals. Including gold. No wonder the Romans invaded Britain.