As glow squids swim around blocks of flowing water, they convert them to full water sources, and increase the size of the water streams without any blocks being placed. This has happened multiple times in my world, and especially affects waterfalls in larger caves.

The image attached below was a simple waterfall  when I first found it in survival. I went into spectator and turned on NV to show this better. I also have a screen recording I can send but it's too big to upload, have attached some screenshots from it below

glowsquid