Wood can last a long time in fresh water.
In the Adirondack Mts. of upstate NY I have seen dams constructed out of logs that are still in good shape after 100+- years. The timber companies would build those “log dams” to form a small lake, then via horse power would drag the timbers/logs down to the edge and roll them into the water. When they accumulated enough logs they would blow out or tear out the log dam …… and KERWOOSH, ….. down the river went all the dammed up water and the logs.