Hi Zoe
There is always a possibility of predation (human or wildlife). However, the presence of three new swans may be a better answer. If the three new swans are juveniles (less than a year), there would possibly be a fight, but usually the established pair would win. Seeing there are three, this would be the theory that they are juveniles dispatched from their parents and they found your area. If they were adults, there would usually only be two and the third dispatched from the area due to breeding. In the event of adult swans, there would also possibly be a fight and either pair (old or new) could win the territory.
More likely is that the parents have moved the cygnets to another area away from the nest. This usually happens two weeks after hatching. Nature moves families and hastens offspring growth to protect them from predation. Since you have been getting rain, there may be better food resources in outer areas and the family has moved to take advantage of better food and habitat ( shelter) resources.
The last possibility would be predation. If something happened to the cygnets, the parents would carry on as if nothing occurred. Hormones would dissipate along with the usual territorialism that occurs during mating season. Your two swans may now be with a new swan. Unfortunately, the killing of Mute Swans is not to be occurring in NY by the NYDEC, but there is always the possibility that the killings are still occurring. Hopefully, this is not the case.
Check the area around the canal for other reed/wetland habitats and watch that mother swan is not carrying the cygnets under her wings. The swan family may still be nearby. FYI- the adult swans can move cygnets very far in a short period of time if the need for more food resources or safety is required. The Regal Swan