An account in Crawford County, Missouri

Two friends and myself were driving back roads in our home town out of boredom in the evening. We stopped at a small conservation area known as Riverside off a dirt road that shares the same name. There is a boat ramp off the Meremac River and a picnic table about 50 yards off the road and obstructed by woods and brush. There are trash barrels at the location and fishermen empty their coolers and discard fishing waste when they take their boats out of the water. There are farms and residences in the area, but nothing for about half a mile due to the area's proclivity to flood. There have also been incidents of violence in this location in the past (person on person) so there's a local aversion to the area, which makes it particularly absent of people at night. The three of us were talking and walking down the boat ramp to the edge of the river (the trees and brush make the ramp the only place that the river is visible in this area). There is a small gravel bar/island that is heavily wooded about 200 feet from the edge of the boat ramp where the river forks and splits. There was a decent amount of moon light and we were watching the water. We heard some noise in the brush on the island or maybe the opposite bank. A few moments passed and what sounded likes a rock hit the bank on our side (maybe fifteen or twenty feet down river) followed by what sounded like another rock landing in the river (presumably in the same vicinity of the first rock). We froze for maybe ten seconds, then we heard what we all collectively determined to sound like an ape yowling from about the same direction of the noises and rocks. I would say the sound was more like a one tone, moderately deep exhale or grunt that had a shrill undertone that gave it an aggressive feeling. We left immediately. We've all lived in rural areas for most of our lives and know the sounds of local wildlife. This sound wasn't one any of us had encountered before. We never saw anything.