The sinkhole that was created in the lake was exclusively caused by Eris' attempts to seize control. Her darkness wanted to find a way to make their connection stronger, and the best method it discovered was to remove all obstructions that stood between them.
The darkness spread throughout the water, so it wasn't as if Eris could only control what was in that small area of the lake, but that was where she directed her will. Through the sinkhole, which contained a darkness she was more familiar with, she was able to connect to the more elusive form of the element that existed in the depths.
It was August's water. That much never changed. But, swimming within it were the shadows she controlled.
Neither of them moved. They stopped attacking each other, because the nature of their battle had changed.