Logjams are areas on a river that has been clogged by floating logs, trees, or other plant debris. These trees may have been knocked down by storm, landslide, or other natural event or they may have been cut down. Historically, felled trees were often transported by being floated down a river (a process called log driving), so logjams may be downstream of a logging camp and upstream of a sawmill.  Log jams can be crossed by moving from log to log but are extremely dangerous – a person that falls in the water within a logjam may be crushed or drowned by the heavy logs, especially if the water beneath is fast and turbulent.

Logjams may occur at sites where the river bends, narrows, or contains obstacles like rocks or bridges. A set of rapids or waterfall may be a particularly treacherous portion of river for a logjam to occur.

Lumber workers may risk their lives to break apart logjams.

If your world has tree-like fictional plants, consider if they may form logjams.