From: Rabbi Moshe Sofer
To: Rabbi Meir
The city of Ungvar is bisected by a water channel, raising whether a tzuras hapesach may span the water and whether the channel's depth is measured by the riverbed or the water level. The Chasam Sofer rules that a tzuras hapesach over water is valid and that only the fixed ground depth determines domain status, thereby permitting the eruv.
Source: Shu"t Chasam Sofer OC §87