Complex sentence: It is known that when villages were built since the Yuan Dynasty (14th century) and situated next to a river or stream which flowed towards the ancient sea of Shiqi, the region then was named after the river stream.
Split into simpler sentences: It is known that villages were built since the Yuan Dynasty (14th century). Since they were situated next to a river stream which flowed towards the ancient sea of Shiqi, the region then was named after the river stream.