I don't wanna reveal too much about my world, however, the basic idea is that there is a town of people who live deep underwater (they are biologically capable of doing underwater gas exchange and withstanding the crushing pressure) and mine this lava river for its crystals that can be fashioned into tools and weapons.
These crystals are a fictional material, however, they don't affect the way the lava behaves.
This lava river is pouring out of some sort of volcano or vent, and it is continually harvested for its crystals as it cools. It looks like this. My main question is whether or not this lava river could exist.
I've seen this question, which details the feasibility of red hot lava existing underwater, but could a river of flowing lava form? Would the underwater environment prevent the formation of such a river?