By Daniel Collins
The simple answer is, of course, water. But that says little about the natural history of wetlands, or what physical conditions are necessary to maintain, restore or even engineer them. For that, we need to take a closer look at wetland hydrology.
Wetlands are tracts of land that are water-logged at least seasonally. They may be spongy bogs, mountain tarns, verdant swamps, or many other types. They remain wet because the inputs of water from rain, rivers or groundwater compensate any losses.
The various types can be distinguished based on their hydrology. In their book on wetland restoration, Bev Clarkson and Monica Peters (2010) quantify this continuum with the “gumboot test”. Short “red bands” are usually okay for keeping you dry in bogs, taller gumboots are needed for fens, thigh waders for swamps, and waist waders for marshes.
Controlling this water balance are climate, geomorphology and even the vegetation itself. Wetlands typically form in gently sloping or topographically convergent portions of a landscape, where surface and ground waters meet. The vegetation plays several roles here, including the build-up of peat, changing evaporation and water flows, and by controlling erosion and hence the shape of the local landscape to some degree. Kettle holes, such as those in the Ashburton Lakes, are an example of the climate and glacial geomorphology controlling the hydrology, which in turn controls the ecology.
Each plant species is adapted to a particular range of wetting and drying. Too dry for too long, and terrestrial plants can invade. This is particularly important to bear in mind when conserving, restoring or engineering wetlands. It’s not enough to simply add water – the hydrological regime must match the desired ecosystem’s needs.
Some of the hydrological effects of wetlands are in essence also ecosystem services – benefits conferred to society by the wetlands. Reducing flooding and augmenting low flows are two services often cited, though they are not true for all wetlands (Bullock and Acreman, 2004). Science is actually a little in the dark as to which biophysical features of wetlands confer or degrade the various hydrological services.
And as we consider the hydrological origin of wetlands and differences between wetland types, it is also interesting to consider the etymological origin of wetland words. The word “swamp”, for example, can be traced back to the Old Norse word for “sponge”, sharing a common ancestry with “sump”. “Marsh”, “morass” and possibly “moor” have origins in the Proto-Germanic word for “sea”, and are in turn related to the words “marine” and “maritime”. “Mire” comes to us from the Proto-Indoeuropean (PIE) word for “damp”, and shares this root with “moss” and “must” (as in “musty”). “Bog” came to us via Gaelic, with a meaning of “soft” or “moist”, and earlier still from the PIE word for “bend” (as does “bow”). And lastly “fen”, which remains truest to its roots, goes back to the PIE word for “swamp”.
Ordering these words on a scale from less to more wet, in terms of their etymological roots, we get: bog – fen/swamp – mire – marsh. As a testament to biophysical basis of words, this aligns nicely with the order of wetland types illustrated above.
Much, however, remains to be learned about the hydrological origins, needs and impacts of wetlands. Continued research in this front will assist in conservation and restoration, the use of ecosystem services, and in the broader understanding of the water cycle – all very useful as NZ seeks to balance resource use with environmental protection.
Bullock A. and Acreman M. (2003). The role of wetlands in the hydrological cycle. Hydrol. Earth Syst. Sci., 7, 358-389, doi:10.5194/hess-7-358-2003.
Clarkson B. and Peters M. (2010). Wetland types. In B. Clarkson and M. Peters, eds., Wetland restoration: A handbook for New Zealand freshwater systems. Manaaki Whenua Press, Lincoln. Pp. 273.
Dr Daniel Collins is a hydrologist and water resource scientist at NIWA.