Shallow areas: Shallow areas are large expanses of shoals or of shallow water where the changes in depth are relatively slight. Some bays fed by river systems are shallow throughout.
Shoals, banks and bars: Shoals are shallows that constitute offshore hazards to navigation. They are defined as having a depth of 10 fathoms or less and may be composed of any material except rock or coral. A shoal may be an isolated feature or part of a shoal area composed of two or more shoals. A bank is an area of relatively shallow water which is, however, of sufficient depth for safe navigation. Bars are ridges of sand or gravel, often at the mouth of a river, which may obstruct navigation. Note that shallow areas of rock and coral are charted as ledges and reefs and labeled, rather than being delineated solely by depth indicators.
Irregular bottoms: Irregular bottom areas may consist of shoals, shallows, passages, deeps, etc., and are characterized by relatively large and abrupt depth differences throughout the region.
Smooth bottoms: Smooth bottom areas are expanses where variations in depth are gradual and are relatively small compared to the size and depth of the area as a whole. Smooth areas in relatively deep water are the least important feature shown on charts. Generally, they pose no navigational hazard. These are depicted to provide “bottom detail” to navigators, rather than to enhance boaters’ “safety.”
Navigable channels and passages: A channel or passage is a relatively deeper navigable route through an otherwise shallow area. Natural channels or passages are important features which contribute to the navigational value of a chart. Natural channels may constitute routes from deep water into shore or harbor areas, or routes between deep water areas through shoals or bordering shallow areas.
Deeps: Deeps are local deformations in the bottom configuration characterized by a significant increase in depth when compared to the surrounding areas. The boundary of a deep is the zone which separates the deep area from the surrounding shallower water. The size of the zone depends on how well the deep can be distinguished from the surrounding area.
SOURCE: Nautical Chart Manual.