Freshwater wetlands are found in many kinds of physical regimes, and include such diverse ecosystems as the extensive raised and blanket bogs of the northern Canadian shield, sedge meadows in Minnesota, freshwater tidal marshes on the Atlantic and Gulf coasts, and marshes of high-energy rivers such as the Mississippi and the Atchafalaya. Much of the Everglades is a marshy sea and the shores of many lakes are laced with pocket marshes and fringe marshes. All these wetlands have at least one thing in common: they are flooded frequently enough so that the roots of the emergent vegetation exist in an anaerobic environment. The term "emergent vegetation" is used to include mosses and other nonvascular plants such as sphagnum, vascular grasses and herbs, and woody trees such as black spruce, cypress, and tupelo.

On the other hand, different wetlands may vary widely in other ecosystem attributes, including characteristic biota, species richness, productivity, the amount of organic matter accumulating in the sediments, and in the degree of coupling with adjacent ecosystems.

One can consider any specific wetland type to occupy a certain niche. This niche is one instance of the environmental mosaic that results from the interaction of climatic, geologic, hydrologic, and biologic processes. It supports certain species at a particular level of productivity. To the extent that the hydrologic regime distinguishes emergent wetlands from aquatic and terrestrial systems, it is the primary determinant of all wetland systems. Yet solid quantitative information about the hydrodynamic characteristics of different wetlands is surprisingly difficult to find. In this paper we discuss, in a conceptual way, the relationship between hydrodynamics and wetland ecosystem characteristics. Specifically we identify several attributes of the hydrologic regime that seem to be important variables, we show that these attributes directly influence or modify a range of other secondary abiotic parameters (pH, nutrient flux, dissolved 02, etc.), and that these secondary parameters in turn determine the biotic response. Finally, we complete the loop by relating changes in the hydrologic regime to environmental changes resulting from the biota.

The insights and examples used draw heavily from European and Canadian literature about inland bogs, mires, and fens. These are, in general, fairly low- energy examples (in the sense of water velocity, flooding frequency) of the broad range of wetlands that exist, and we attempt to extend and amplify concepts from these systems to wetlands in general.