Unnavigable means impossible or extremely difficult to travel through, usually describing water routes but also other kinds of passages. When a river becomes unnavigable, boats can't safely move through it anymore, perhaps because it's too shallow, filled with rocks, frozen over, or choked with fallen trees.

Rivers that are unnavigable might have dangerous rapids, waterfalls, or sections that simply don't have enough water. Some rivers are unnavigable during certain seasons but perfectly fine at other times of year.

The word also describes anything that's nearly impossible to get through. A website with confusing menus and broken links might be called unnavigable because users can't find what they need. A book with terrible organization could be unnavigable for readers trying to locate specific information. When your room is so messy you can barely reach your bed, your parents might joke that it's become unnavigable.

The opposite is navigable, meaning you can successfully travel through or use something. Explorers throughout history tried to determine which rivers were navigable, since water routes were often the fastest way to transport people and goods before roads and railways existed.