From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer rules that a pool filled from a spring whose flow has since ceased should lechatchilah be treated stringently following the Ra'avad, though the Rambam would permit it. He rejects the Maharik's additional stringency regarding a still-flowing spring-fed mikvah as unwarranted, since the Rambam's permissive ruling is sufficient authority to avoid imposing a new gezeirah. As long as the flow openings are visibly open, one may immerse without hesitation.
Source: Shu"t Chasam Sofer YD §209