A log bridge is a bridge which is made by felling a log across a river. There are a number of variations on the basic log bridge, including bridges which are actually quite complex and designed for long-term use. The log bridge is probably the oldest bridge known to humans, with minimal effort being required to install the most basic of log bridges.
Humans aren't the only ones to build log bridges. Occasionally, nature makes log bridges without any help. Trees which grow near streams can fall naturally, especially during heavy rains when the soil becomes too boggy for the roots of the tree to grip. In this case, the tree may fall across the river, creating a natural bridge which can endure for years before eventually rotting away. Such bridges are sometimes used by animals as a natural crossing, and humans have been known to do likewise.
In a basic log bridge, a tree near a river is felled to cross it, or a felled tree is dragged to a site where a bridge is desired. The top of the bridge may be shaped to create a flat walkway, making the bridge safer and more comfortable to use, and some people also install rails which can be gripped for safety, or overhead lines gripped by people walking across the bridge. Because log bridges can become slick, especially in the winter, a grab rail can be a very good idea for safety.
More advanced log bridges are made with multiple logs felled together to create a broad bridge. The tops of the logs can be shaped, or planks can be laid across them to make a flat surface for walking or driving. These types of bridges can accommodate pack animals and vehicles as well as people. If the bridge is up high, buttressed supports may be created to ensure that the bridge is secure.
People who want a log bridge to last will install footings before putting the bridge in. Gravel or rock footings create drainage at either side of the bridge, reducing the risk of rot in the area where the bridge is in contact with the ground. The bridge can also be strapped for reinforcement, supported with pillars and scaffolding, and otherwise secured so that it will be as stable as possible. For extra durability, treated wood can be used, with the wood resisting mold, mildew, rot, and insects which could compromise the integrity of the bridge.