From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer distinguishes between 'nizchalin' (water flowing through and past a pool) and 'katfares' (water flowing on an incline toward a final resting place), arguing that only water destined to settle in the lower pool can serve as a valid connection. He rejects the Rivash's equation of the two terms and concludes that, given the potential kares violation involved, a mikvah must satisfy all interpretive opinions without relying on combined leniencies.
Source: Shu"t Chasam Sofer YD §210