Geologically speaking, these things are not uncommon. A river or estuary that empties into a saltwater sea will sometimes form a lagoon. Over time, the sediment causes the lagoon to become a closed lake, and the water changes from saltwater to freshwater.
That’s what happened here, just south of Valencia in Spain, not too long ago, in the 17th century, L’Albufera de València:
Today it is a wonderful, relaxing place to visit – especially in the warm Spanish evenings.
You can see a wonderful old map I discovered hanging in the local village bar.
The small villages are connected to the lake by a series of narrow canals:
By the way, the astute reader will notice that I wrote L’Albufera de València, which is the Valencian language version of the Spanish La Abufera de Valencia. My Valencian friends tell me that Catalan, although somewhat more well known, is a dialect of Valencian.