The first definition of log jam in the dictionary is pretty straightforward: “a situation in which a large number of logs floating down a river become tangled with each other so that further movement is not possible.”
Mother Nature made the earliest log jams. Hundreds of years before Europeans and commercial logging arrived in North America, river flooding would tear loose trees from the forests lining the water’s banks. Entire trees would fill the river and form jams. If the jams didn’t clear quickly, the timber got locked into place when the water receded. New streams and lakes were created as the water diverted.