Yes, the swans are in danger if the pond dries to the point of no water. Additionally, if there is no aerator (fountain) on the pond and the conditions get worse (dry) or there is a heavy rainstorm which can dredge up anerobic bacteria at the bottom of the muck, the swans can die from botulism.

The best advice would be to capture the swans and move them to indoor pens with fresh water flow (lined ponds or kiddie pools sunk into the ground to form a zero entrance).
The small ponds need to have a way to drain and flush the water as well as fill the ponds with fresh water.

If you cannot place them on another pond that you know is safe from toxins, predators or other hazards, then the best thing would be to temporarily house them in indoor pens, at a wildlife (waterfowl) rescue facility if they will allow you to house them temporarily, or you might contact Bob Knox at Knox Swans. Bob is in the Illinois area and he may be able to advise you regarding possible alternatives in your area. We hope this helps. Please let us know how this situation progresses. The Regal Swan