The pilot reported that when he arrived at night, there were snow showers at the airport. While turning final, he turned his landing lights on. However, he reported that he could see better with the landing lights off, so he turned them off. After touching down on the grass runway, he elected to slow down and make a 180 degree turn to taxi back down the runway to the terminal building. During the landing roll, he moved over to the left side of the runway toward a ditch that protruded 26 ft inside the runway lights. The ditch was located about 1654 ft from the approach end of the runway where a stream crossed through a culvert, under the runway. There were indications that the ditch had eroded inside the runway lights after the airport was certified. Six yellow halves of 55 gallon drums were used to mark the obstruction, but they were not lighted. As the pilot was slowing near the left side of the runway, the left main gear and nosewheel dropped into the ditch.