The following diagram is a simple model that defines the components of a wetland. Climatic influences such as temperature and radiation may have an overriding influence on the type of species that develop in a wetland, but because the hydrologic interactions with wetland ecosystems are our primary interest, we limit the discussion to local effects.

The WATER SOURCE determines ionic composition, oxygen saturation, and toxin load. The FLOW VELOCITY affects turbulence and the ability of the water to carry suspended particulate matter. The RENEWAL RATE describes the frequency of replacement of the waterIt depends on water depth (volume) frequency of inundation, and velocity, and it is one of the most difficult hydrodynamic parameters to measure. The TIMING, that is, the frequency of inundation (daily, seasonal) and its regularity or predictability, influences the potential for system succession and maturation.

FFour chemical and physical properties of the substrate (2) that are strongly influenced by the hydrologic regime, and which appear to limit ecosystem development in flooded systems are:

Under most conditions of plant growth water is a limiting commodity. However, freshwater emergent plants must often cope with long periods of flooding. For this reason, the major effect of water on wetland plants is secondary; that is, it does not itself directly limit plant growth. Rather its influence can be traced through secondary responses related to 02 availability in the root zone.

The necessity for, role of, and limitations placed on plant growth by inorganic nutrients are too well documented to require amplification.

Toxins have a controlling role in ecosystems. They can be natural toxins, as salt or H2S, or human artifacts such as pesticides.

Availability The flooded condition of wetland soils results in an anoxic environment, and this in turn leads to a whole set of chemical differences compared to oxidized soils.  For example there is a tendency for micronutrients to be more readily available; toxic  materials such as H2S accumulate in the substrate; 02 is limiting to the roots and plants must develop alternate methods of respiring or of acquiring 02. Surprisingly, there has been relatively little experimental study of the effects of anoxic soil conditions on marsh plant growth, but the analyses of flooded rice and marsh soils, for example, show that the anoxic environment is a stress factor and that growth in general is reduced by highly anaerobic conditions.

The resultant spatial heterogeneity appears to be a major determinant of species richness in ecosystems.