system
Given the following post, respond as if you're commenting on Reddit:
human
[DC/Aquaman] Do any underwater civilizations exist in freshwater lake and rivers?

There's a lot of Kingdoms and intelligent humanoid creatures in the ocean. But what about the lakes, rivers, and ponds? Are there any races or Kingdoms similar to the Atlantians there?
gpt
Not that we know of.

The thing is, rivers and lakes are much, much smaller then the sea. While its very long, The Nile is only 2 miles across and 11 meters deep-  not much space for a civilization. Lakee Superior, largest lake, is 350 miles by 160- big enough for a city, but not enough for a kingdom.

Other places may (and do) have underwater bases and suchlike, but the sea is simply the only underwater area large enough to fit an actual civilization in.