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Wetland Words and What They Mean:  Hydrology

As in any area of endeavor, wetland science has developed its own particular vernacular. This is the first in a series of articles that will focus on some of the most commonly used terms in the wetland field, what they mean, and how they fit into wetland science.

Much wetland lingo developed from efforts to describe and define wetland boundaries for regulatory purposes. Under most regulatory programs, wetlands are defined by their vegetative, soil, and hydrological characteristics. Obviously, delineating wetland boundaries is predestined to be a difficult ask. Delineation requires that a boundary, described by regulatory standards, be placed around a type of ecosystem which, by its nature, is transitional between a deep water aquatic ecosystem and a drier upland ecosystem. The delineation process is complicated by evolving knowledge about wetlands, how they are formed and how they work. In addition, various wetland regulatory programs set different criteria to define wetlands.

Regardless of these differences and difficulties, there is a foundation of knowledge about wetlands that is commonly used in the wetland community. This article will explore the area of hydrology, specifically wetland hydrology and how it is related to wetland definition.

Hydrology and Wetland Hydrology

Hydrology is the study of water in an environment, including its properties, distribution and circulation. The water cycle (remember that picture from earth science class) deals with the circulation of water, where it falls out of the clouds, runs into the rivers and then into the ocean where it evaporates and is recirculated back into the atmosphere.

A wise wetland scientist once said that before one decides an area is a wetland, one should be able to identify where the water is coming from, how much water there is, how long the water stays in the area and how it leaves. Water is the key factor driving any wetland ecosystem, and its importance is reflected in most definitions of wetlands. For example, from the federal regulations: "Wetlands are areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal conditions do support, a prevalence of vegetation typically adapted to life in saturated soil conditions" (Federal Register, July 19, 1977, July 22, 1992).

Wetland hydrology is the study of the movement of water in and out of the wetland ecosystem. In wetlands the presence of water is the overwhelming characteristic of the ecosystem. In wetlands the continuous wet condition causes the soil to become inundated (covered with water) and/or saturated. The pore spaces in the soil fill with water, causing the oxygen in the soil to be either forced out by the water or used up by the plants and microbes during metabolic processes. In a short period of time, all available oxygen is used from the soil and the plants and microbes extract energy from other minerals in the soil. Soon, all minerals are reduced and metabolic activity by microbes and plants is conducted through anaerobic pathways, which are less energy efficient. Wetland plants have developed mechanisms which allow them to survive in these stressful environments for extended periods of time; this is shy they are called adapted "to life in saturated soil conditions."

Water can find its way into an area through any combination of: direct precipitation, flooding, tidal influence, and groundwater. The first three mechanisms involve surface water flows, whereas the last involves groundwater. Direct precipitation may result in rain or snow falling into an area. If the landscape is shaped in a manner (concave, rather than convex) which causes it to retain the precipitation, the area can become saturated or inundated. Flooding is the condition when the soil surface becomes temporarily covered with flowing water from any single or a combination of sources, such as streams overflowing their banks, runoff from adjacent or surrounding slopes, or tidal flows (Environmental Laboratory, 1987). Groundwater, which is water underground at pressure greater than atmospheric pressure (Environmental Laboratory, 1987) may be present at the surface of the ground in an area due to a depressional landscape feature which intersects the groundwater table, or from groundwater seeps exiting from the sideslope of a hillside.


Inundation is the condition when land is covered with water; this can be either a permanent or temporary condition (Environmental Laboratory, 1987). Soils are saturated when all easily drained pores between soil particles in the root zone are temporarily or permanently filled with water to the soil surface at greater than atmospheric pressure (Environmental Laboratory, 1987). Soil texture greatly influences whether a shallow groundwater table will cause the root zone to be saturated. Soil with smaller grain sizes, such as clay, will have a wider zone of saturation above the groundwater table than a soil with larger grains such as sand. This is due to capillary action, which causes water to rise higher above the water surface in a narrow tube than in a wider tube. The smaller pore spaces in clay soils act as tiny capillary straws, pulling water up out of the groundwater and into the soil layers above.

The presence of water, even standing water, or strong debris lines or water-stained leaves on a site does not necessarily mean that the site is a wetland. For example, in higher portions of a floodplain area after a 100-year flood, it is quite possible that there will be water present, or lingering indicators of the water, such as debris lines or water stains. However, a 100-year flood does not make a wetland. For an area to abe a wetland, water must be present at a sufficient "frequency and duration."

Frequency is the number of times an area is saturated or inundated in any give time period, and duration is the length of time the area is saturated or inundated after each event (Environmental Laboratory, 1987). According to the U.S. Army Corps of Engineers 1987 Wetlands Delineation Manual, it appears that the duration of soil saturation or inundation is more important than frequency in the development of anaerobic soils and, therefore, wetland plant communities. Studies conducted in development of the Manual found that 7 days of continuous soil saturation during the growing season are more likely to create anaerobic conditions within the soil profile.

From these studies, the U.S. Army Corps of Engineers determined that an area must have saturated soils or be inundated for a duration of between 5 and 12.5 percent of the growing season to support the growth of a wetland plant community. The growing season is the portion of the year when the soil temperatures are above biological zero, or 5 degrees Celsius. Much of New York State is in a mesic growing season, which is March through October, or a period of 244 days. Therefore, an area would need to be saturated or inundated for 12 to 29 days or into April for the soils to become anaerobic and take on wetland characteristics. A continuous saturated or inundated condition is more likely to create wetland conditions than intermittent saturation or inundation for the same number of days spread over a longer period. This condition should also be present on a relatively regular basis year after year (frequency).

There are numerous features that influence the frequency and duration of the wet condition, including amount of rain or snowfall, soil type and permeability, topography, and plant cover. For example, a low area in a floodplain will be wetter more frequently and for longer durations than an area higher in elevation within the same floodplain. The same amount of precipitation will cause a soil with low permeability, such as a clayey soil, to be wetter longer than a soil which can drain more easily, such as a sandy soil. Plant cover can cause water to flow more slowly over the area, which can increase the duration of the wet condition, or increase transpiration rates, reducing the duration of soil saturation.

A wetland scientist looks for clues in the field and through data gathering to determine what type of hydrology is present on a site and whether there are strong enough indicators to say that the area has wetland hydrology. The indicators include (Environmental Laboratory, 1987):

1. Aerial photographs taken during the early part of the growing season. A series of aerial photographs that show a pattern of soil saturation or inundation year after year is a strong historical indicator of wetland hydrology.

2. Visual observation of inundation or soil saturation in the major portion of the root zone, especially when present well into the growing season.

3. Watermarks on trees or structures.

4. Drift and debris lines, water-stained leaves, and sediment deposits, all indicating past inundation.

5. Oxidized rhizospheres on living roots. These are bright orange colors found in the soil immediately adjacent to a living root in a plant, when the surrounding soil has grey or dark (low chroma) background colors. This indicates that a plant is living in an anaerobic atmosphere and is pumping oxygen out of its roots into the surrounding root canal in order to live. The oxygen pumped out by the plant oxidizes the surrounding soil, causing it to turn orange.

6. Drainage patterns.

A wetland scientist observes landscape features and patterns to see where water enters and exits the site. The scientist looks for evidence of past and present human and animal (ie. beaver) activity on the site to determine if the area has been drained or flooded recently, and how permanent those conditions are. The scientist also examines the wetland hydrology findings to see if they are consistent with the soil and vegetative conditions, and determines the reasons for any discrepancies observed. And, in the end, answers should have been found for the questions "where is the water coming from, how much is there, how often and how long does it stay, and where does it go?"


Environmental Laboratory. 1987. Corps of Engineers Delineation Manual. Technical Report Y-87-1, U.S. Army Corps of Engineer Waterways Experiment Station.

Massachusetts Department of Environmental Protection, Division of Wetlands and Waterways. 1995. Delineating Bordering Vegetated Wetlands Under the Massachusetts Wetland Protection Act. ?William Francis Galvin, Secretary of the Commonwealth.

Tiner, R.W., Jr. 1988. Field Guide to Nontidal Wetland Identification. Maryland Department of Natural Resources, Annapolis, Maryland and U.S. Fish and Wildlife Service, Newton Corners, Massachusetts. Cooperative publications.

Submitted by Barbara B. Beall, PWS



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