A body of water, such as a river, canal or lake, is navigable if it is deep, wide and slow enough for a vessel to pass. Preferably there are few obstructions such as rocks or trees to avoid. Bridges must have sufficient clearance. High water speed may make a channel unnavigable. Waters may be unnavigable because of ice, particularly in winter. Navigability depends on context: A small river may be navigable by smaller craft, such as a motor boat or a kayak, but unnavigable by a cruise ship. Shallow rivers may be made navigable by the installation of locks that increase and regulate water depth, or by dredging.

Inland Water Transport (IWT) Systems have been used for centuries in countries including India, China, Egypt, the Netherlands, the United States, Germany, China, and Bangladesh. In the Netherlands, IWT handles 46% of the nation's inland freight; 32% in Bangladesh, 14% in the United States, and 9% in China.

What constitutes 'navigable' waters can not be separated from the context in which the question is asked. Numerous federal, and some states, define navigability for various purposes from admiralty jurisdiction to pollution control,and from property boundaries to the licencing of dams. The numerous definitions and jurisdictional statutes have created an array of case law specific to which context the question of navigability arises. Some of the most commonly discussed definitions are listed here.