These vessels are usually less sturdy than ships built for the open seas, with limited navigational and rescue equipment, as they do not have to survive the high winds or large waves characteristic to large lakes, seas or oceans. They can thus be built from light composite materials. They are limited in size by width and depth of the river as well as the height of bridges spanning the river. They can be designed with shallow drafts, as were the paddle wheel steamers on the Mississippi River that could operate in water under two metres deep.