The pilot stated he was flying about 700 ft above ground level (AGL), then noticed that the airplane had descended “well below 500 ft.” He could see a pump station ahead with powerlines crossing the waterway and stated, “I’m too low.” He was traveling about 170 kts when he pulled back to climb and heard a “thud.” The airplane continued flying, but the pilot had limited rudder control. He returned to the departure airport and landed uneventfully; however, he noticed that the rudder was missing from the airplane. He stated that he looked up the pump station address and drove over to find the rudder laying in the grass near the powerlines. The rudder had impacted the powerline and separated from the airplane, resulting in substantial damage. The pilot reported that the airplane had no preimpact mechanical malfunctions or failures that would have precluded normal operation.