Ponds can be a big problem, particularly artificial ones with straight, slippery sides that make it impossible for hedgehogs to climb out. Although they can swim fairly well, they cannot tread water indefinitely. Without anything for them to climb onto in order to exit a pond so they will eventually tire and drown. Please keep ponds topped up and provide some form of a ramp, e.g. a plank, brick steps or chicken wire ladders or a towel to enable them to climb out. If you are considering adding a pond to your garden, why not make it Grace-friendly with sloping sides to make it safe for hedgehogs and other wildlife to drink the water?