In Maryland, the climate doesn’t usually get cold enough for long enough to cause any issues. The only time you need to worry is if ice dams start forming. If ice damming is a worry, all you need to do is unplug and disconnect the pump from the plumbing.
Ice may form over the waterfall/stream, but water will still be running underneath the ice. At this point, the ice actually acts as an insulator from the cold. The water will keep running underneath.
Ice dams happen when debris or rocks begin building up ice that dams the waterway. Eventually, this forces water out of the sides of the stream. This will run the pump dry and cause damage. You can pour warm water over ice dams to melt them.
If you unplug the pump, make sure to drain the tube and store the pump in water. This way it doesn’t dry out and crack.