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"Hydrology is probably the single most important determinant of the establishment and maintenance of specific types of wetlands and wetland processes." -Mitsch and Gosselink (1993)

Wetland hydrology is perhaps the most key element in all wetland restoration project. Many wetland scientists have referred to wetland hydrology as the most important component of wetland ecosystems (Mitsch and Gosselink, 1993; Erwin, 1990 in Kusler and Kentula, 1990). It is ironic however that restorationists often times know very little about the hydrology of the wetland that they are trying to restore. It is difficult to tackle the restoration of wetland hydrology because there are still so many variables unknown to scientists.

• The exact hydrological function of each wetland type is still undetermined. In addition, because each wetland is uniquely situated within the landscape, wetland hydrology will be somewhat unique to all wetland types.
• The hydrological regime of each wetland differs in frequency and magnitude of high water, duration, timing, and temporal sequences of high and low water (Zedler, 2000). We still do not know how fluctuations in the hydroperiod affect plant and animal communities (Zedler, 2000).
• Pre-degradation long-term hydrological data is unknown for a lot of wetlands. Typically, one of the main goals of wetland restoration is to restore the hydrology of a particular wetland. This is often a difficult goal when there are no previous records of hydrological data.
• It is unclear to scientists exactly how much of the natural hydrological regime has to be restored; we need to know if partial improvements can restore the biota and biological functions of wetlands (Zedler, 2000).

Wetland Hydrology

Because wetland hydrology is such a broad topic in wetland restoration, this next paragraph will provide you with a very basic description of the importance of hydrology and its functions in wetland ecosystems. For a more descriptive overview of wetland hydrology, water quality, and associated functions, click here. To see a case study of wetland hydrology manipulation, click here.

Small changes is wetland hydrology can significantly affect the chemical and physical properties of a wetland such as nutrient availability, degree of substrate anoxia, soil salinity, sediment properties, and pH (Mitsch and Gosselink, 1993). When hydrologic conditions in wetlands change, plant species composition can shift and ecosystem productivity can decline (Mitsch and Gosselink, 1993). Because hydrology plays a vital role in the structure of a wetland's ecosystem, particularly by acting as the main pathway in which nutrients are transported in and out of the system, the vegetation and species composition are significantly effected when natural or man made hydrologic alterations occur. Mitsch and Gosselink (1993) list several principles underscoring the importance of hydrology in wetlands, (click here). Wetland restoration is often centralized around establishing hydrological equivalence (Bedford) because of the important role it plays in wetland function. In part, the hydrological regime of a wetland is what defines certain areas of land as wetland ecosystems. Without certain hydrological criteria and hydric soils, an area of land cannot be defined as a "wetland". (See wetland definitions)

Hydrological measurements are often difficult and expensive to take but often can be done with a minimal amount of equipment and expense. Because of the important role that wetland hydrology plays in ecosystem function, hydrological measurements should not be overlooked. In fact, they should be one of the key variables assessed and measured in all restoration projects. To see examples of some hydrological measurements that can be taken for wetland restoration monitoring purposes, click here.

Wetland Hydrology Today

Humans have altered wetland hydrology through a variety of activities: drainage, filling, damn construction, water diversions, groundwater pumping, and dredging (Zedler, 2000). All of these activities alter the timing, amplitude, frequency, and duration of high water (Zedler, 2000). Today, wetland scientists are aware of the importance of hydrology. Despite all the "hype" that wetland hydrology has received, we still know very little about how to manipulate or simulate certain hydrological conditions. However, the future is getting brighter. Many restorationists are manipulating hydrological variables in their restoration restoration projects (see Case studies). It is important that long term monitoring of hydrologic variables including, groundwater flow, surface water recharge, water level fluctuation, and precipitation input, and others, are recorded. After long-term hydrological data is available, restoring wetland hydrology will become "easier" and a more attainable goal.

Mini-Case study
Restoration of a south Florida forested wetland (Weller, 1995)

Restoration methods consisted of the removal of undesirable vegetation and the addition of water.
The restoration objectives of this project included (reprinted from Weller, 1995):

• Enhance the few remaining wetland areas
• Re-hydrate the drained wetlands
• Promote the re-colonization of native wetland plants
• Reestablish several vital functions including: wild life habitat, fishery habitat, water storage, biodiversity, food chain support, water filtering and water quality, and groundwater recharge

To improve the probability of hydrological "success", the following procedures were used (reprinted from Weller, 1995):

• The addition of pumped water was only to occur in natural amounts.
• The amount of water pumped would follow the south Florida wet and dry seasons.
• The introduced water was contained entirely within the delineated wetland areas.
• Pumping was interrupted when rain was forecast, so that precipitation could hydrate the system.

Results:

• Since the addition of water, thirty seven new plant species have been found within wetland areas.
• 16 aquatic birds, 21 aquatic animal species, and 8 fish species have been sighted.

Conclusion:
The authors have suggested that because of the increase in plant and animal species composition in a relatively short amount of time (2-4 years), it is possible to restore forested wetlands and their functions sooner than the previous suggested 15-20 years. Long-term monitoring of this site is needed to determine if the successful results of the re-watering techniques are self-sustainable.