Head of navigation is a term used to describe the farthest point above the mouth of a river that can be navigated by ships. "Riverine" redirects here For the use of that term in Maritime geography, see there A ship /ʃɪp/ is a large vessel that floats on water Ships are generally distinguished from Boats based on size This term can be somewhat subjective on many streams, as this point may vary greatly with the size of the ship being contemplated for navigation. On others it is quite objective, being caused by a waterfall or a dam without navigation locks. A waterfall is usually a geological formation resulting from water often in the form of a Stream, flowing over an Erosion -resistant rock A dam is a barrier that divides waters. Dams generally serve the primary purpose of retaining water while other structures such as Floodgates, Levees A lock is a device for raising and lowering boats between stretches of water of different levels on river and canal Waterways.