Complex sentence: The settlement of Plympton, further up the River Plym than the current Plymouth, was also an early trading port, but the river silted up in the early 11th century and forced the mariners and merchants to settle at the current day Barbican near the river mouth.
Split into simpler sentences: The settlement of Plympton, further up the River Plym than the current Plymouth, was also an early trading port. As the river silted up in the early 11th century, mariners and merchants were forced to settle downriver at the current day Barbican near the river mouth.