name of a river in western Hunan that flows into Dongting lake
沅 is a proper noun referring to a specific river in western Hunan, China, that flows into Dongting Lake. Its meaning is tied to that geographic name rather than a general concept.
沅 is a phono-semantic compound: the water radical 氵 indicates a river, and 元 likely provides the sound. The character was created specifically for this river name.
The water radical 氵 flows beside 元, like the Yuan River flowing through Hunan. Picture the river's source (元 can mean origin) as a spring feeding the lake.
For ゲン, imagine the Gen River: the English word 'gen' sounds like ゲン, and the river's name starts with that sound.