2.1 Hydrogeomorphic Setting and the Categorization of Peatland Systems

The combination of the hydrological and geomorphic settings of a peatland controls the rates and directions of flow, type and amount of dissolved minerals and nutrients from groundwater, and its plant and microbiological ecology. Peatlands in early stages of development commonly occur in valley bottoms and hollows/depressions in the landscape and remain saturated by groundwater discharge from local or regional aquifers in addition to precipitation and surface water.

For a given climate, the strength of groundwater interaction relies on the difference in hydraulic head (e.g., water table elevation) between the wetland and the adjacent upland or regional aquifer, as well as the permeability of the basal materials under the valley that limit the rates of groundwater exchange. Where inflow and precipitation rates are sufficient to cause persistent saturation of the ground surface so that hydrophytic (water-loving) vegetation grows, peat will form and accumulate. Groundwater interaction with peatland systems evolves with time because of strong ecohydrological feedbacks. For example, peat decomposition creates zones of lower hydraulic conductivity that reduce groundwater discharge rates and alters water -table fluctuations and hydraulic gradients. Because of this, we can broadly distinguish a range of peatland types based mostly on their groundwater relations: swamps, fens, and bogs as shown in Figure 2 (Zoltai and Vitt, 1995).

Figure 2 – Transition from a) an incipient rich fen peatland with relatively strong groundwater inflow, to b) poor fen where peat accumulation accompanied by a rise in water table diminishes groundwater inflow, to c) a raised bog where the groundwater flow direction is reversed. Swamps have a hydrogeological configuration similar to fens, although groundwater flow may be weaker compared to surface water inflow.