A swamp is a wetland that is forested. Many swamps occur along large rivers, where they are critically dependent upon natural water level fluctuations. Other swamps occur on the shores of large lakes. Some swamps have hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp forests and "transitional" or shrub swamps. In the boreal regions of Canada, the word swamp is colloquially used for what is more correctly termed a bog or muskeg. The water of a swamp may be fresh water, brackish water or seawater. Some of the world's largest swamps are found along major rivers such as the Amazon, the Mississippi, and the Congo. Swamps can be found on all continents except Antarctica with the largest swamp in the world being the Amazon River floodplain, which is particularly significant for its large number of fish and tree species.