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 or walk. 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.