Which preposition to use with debouchments

at Occurrences 1%

Every acre of the Connecticut, from the northernmost bridge that spans it in Vermont to its debouchment at Saybrook, might be made productive of as great a value as any onion-garden acre at Wethersfield.

of Occurrences 1%

The old town of Marietta, the cities of Louisville and Cincinnati, the villages huddled at mouths of such rivers as came down from the Virginia hills, or the larger settlements marking points near the debouchments of slower streams like the Muskingum and Wabash, which crossed the flatter lands beyond, made the chief points of traffic and of interest in those days of west bound travel.

Which preposition to use with  debouchments