Marshes form along slow-moving rivers, at lake edges, or in depressions of land. They are often a transition between dry and wet ecosystems. Grasses and reeds dominate them (as opposed to trees) and they can be home to fish, birds, amphibians, aquatic mammals, and dangerous water-dwelling predators of our design. The water level is shallower than other wetlands. Saltwater marshes are found farther from the equator along coastlines where the tide floods them. They must be protected by lagoons or estuaries.
A wetland with a forest is called a swamp. They occur near large rivers and lakes. A hammock is a dry land area within one, albeit with aquatic vegetation that can survive occasional flooding. The difference between a swamp forest and a brush swamp is the amount of tree cover, as the shrub version has far fewer, and shorter, trees and is instead largely bushes. The shrub version is also drier and the shrubs are mostly on the hammocks.
While elves are known for their love of forests, they don’t appear much interested in swamps, but maybe world builders can change this, creating a new variant on elves. How might these elves utilize the water and animals within it?