Hydrology and Physiography of Groundwater and Wetland Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 62

wetlands have been created to allow for rice culture (Grist, 1986), and these
rice paddies have fed billions of people over the centuries. The decline in
wetlands is global; for example, a large percentage of wetlands have been
lost in United States (54%), Cameroon (80%), New Zealand (90%), Italy
(94%), Australia (95%), Thailand (96%), and Vietnam (

99%).
Wetlands are distributed worldwide (Fig. 4.12), with large areal cov-
erage in northern Europe, northern North America, and South America.
The processes that form these wetlands vary (Table 4.4). A classification
system for wetlands has been proposed to allow assessment of wetland
functions (Brinson et al., 1994). The wetlands can be classified by geo-
morphology, hydrology, climate, nutrient input, and vegetation. (Table
4.5). Four broad geomorphic classifications
that can be used are riverine, depressional,
coastal, and peatland. Depressional forma-
tion processes will be described more fully
in Chapter 6, and formation of riverine wet-
lands is discussed in Chapter 5.
Hydrologic regimes of wetlands can be
highly variable or fairly constant. Hydro-
logic regime forms probably the most im-
portant abiotic template that influences wet-
land ecology (Wissinger, 1999). Important
characteristics include permanence, pre-
dictability, and seasonality. For example,
permanence controls the ability of large
aquatic predators to inhabit a wetland. The
presence or absence of these predators then
structures the invertebrate and vertebrate
community.
Wetlands can receive any of three
sources of water: precipitation, surface wa-
ter, or groundwater. Hydrodynamic charac-
teristics include fluctuations in water level
and direction of water flow. When rivers
flow through wetlands, water moves unidi-
rectionally. Tidal wetlands have bidirec-
tional flow. Hydrologic regimes of riparian
wetlands are characterized by sporadic
flooding with unidirectional flow, followed
by extended periods of stagnation. Conser-
vation of wetlands clearly requires under-
standing of hydrology. For example, in river-
ine wetlands hydrodynamic characteristics
related to links to the river channels and ge-
omorphology are important components of
conservation (Bornette et al., 1998a; Galat
et al., 1998).
Some wetlands have high hydrologic
throughput (minerotrophic) and others are
mainly fed by precipitation and have

Sidebar 4.2.
Definitions of Wetlands

Several definitions of wetlands have been
chronicled by Mitsch and Gosselink (1993) and
by the Committee on Characterization of Wet-
lands (1995). The definition used often depends
on the requirements of the user.

United States Fish and Wildlife Service: Wet-
lands are lands transitional between terrestrial
and aquatic systems where the water table is
usually at or near the surface or the land is
covered by shallow water. Wetlands must have
one or more of the following three attributes:
(i) at least periodically, the land supports pri-
marily hydrophytes; (ii) the substrate is pre-
dominantly undrained hydric soil; and (iii) the
substrate is nonsoil and saturated with water
or covered by shallow water at some time.
Canadian National Wetlands Working
Group: Wetland is defined as land having the
water table at, near, or above the land surface
or which is saturated for a long enough period
to promote wetland or aquatic processes as
indicated by hydric soils, hydrophytic vegeta-
tion, and various kinds of biological activity
which are adapted to the wet environment.
Section 404 of the 1977 United States Clean
Water Act: The term "wetlands" means those
areas that are inundated or saturated by sur-
face or groundwater at a frequency and dura-
tion sufficient to support, and that under normal
circumstances do support, a prevalence of veg-
etation typically adapted for life in saturated
soil conditions. Wetlands generally include
swamps, marshes, bogs, and similar areas.