Geoscience Reference
In-Depth Information
11.3 Hydrological services
systems such as wetlands, which could store
large quantities of water and reduce the height
of l ood peaks (Zedler and Kercher 2005). A
similar major l ood episode took place on the
lower Mississippi River in 2011, which required
opening the Morganza spillway and consequent
l ooding of the Atchafalaya region in southern
Louisiana. This l ood has renewed calls for
signii cant changes in river and wetland
management.
Water comprises an essential element of all wet-
lands. Besides providing an obvious source for
fresh water, numerous additional hydrological
services make wetlands some of the most func-
tionally productive ecosystems in the world.
Wetlands improve water quality through toxin
removal, enhance ground-water recharge, and
protect against erosion from coastal storms and
l ood events.
11.3.2 Water quality
11.3.1 Flood abatement
Most experts agree that wetlands serve as
buffers or i lters, which trap excess nutrients,
heavy metals, particulate matter, suspended sed-
iments, and toxic contaminants (DeBusk and
DeBusk 2001; Verhoeven et al. 2006; Rodriguez
and Lougheed 2010). In recent decades, con-
structed and natural wetlands have been used
globally to treat waste water effectively and aid
in the incorporation of toxic metals like lead
(Pb), copper (Cu), zinc (Zn), mercury (Hg) and
cadmium (Cd), among others (Vymazal 2008).
As DeBusk and DeBusk (2001, p. 255) identi-
i ed, a variety of processes active in wetland
environments improve water quality:
Beyond coastal regions, wetlands also provide
l ood abatement services along l ood-prone
river plains. Wetlands with large storage capaci-
ties act as sponges holding excess water, gradu-
ally releasing the water over extended time
periods, and minimizing the damage caused by
sudden snow melts and extreme precipitation
events. Flood-control services also reduce runoff
velocity and prevent stream- and river-bank
erosion during peak water discharges. Runoff is,
however, inl uenced by climatic factors, season-
ality and soil characteristics. In some cases, such
as upper latitude wetlands in Alaska and Siberia,
runoff velocity may be high due to permafrost
conditions which hinder storage and percola-
tion (Carter 1997).
With urban, industrial and agricultural devel-
opment and wetland conversions, the economic
costs associated with l ood events may rise
astronomically. The catastrophic 1993 l oods of
the upper Mississippi River basin due to exces-
sive precipitation resulted in over 40 deaths and
close to US$20 billion in economic costs (Perry
2000). More than 150 rivers and tributaries were
in full l ood, affecting vast sections of the
central United States (see Color Plate 4-16;
Johnson, Holmes and Waite 2010). At the time,
it was the most expensive l ood ever, devastat-
ing towns, inundating millions of farmland hec-
tares, and destroying transportation links and
urban services across nine states. While not all
of these costs were related directly to the con-
version of l oodplains and wetlands to other
land uses, l ood impacts are exacerbated by the
lack of naturally occurring l ood abatement
• Biological - wetland plants, some types of
algae, and microbes remove excessive nutri-
ents and toxic metals through uptake and
incorporate them into their tissues. Nutrients
such as nitrates and phosphates are readily
taken up and stored by hydrophytic vegeta-
tion including
Typha
,
Phragmites
, and
other reeds before eventually being released
through the process of decomposition or
leaching. In some cases, nutrients may
remain stored for the long term through the
deposition of partially decomposed material
and the formation of peat.
• Physical - the slow movement of water
through wetlands, the physical structure of
plant roots and stems, and the presence of
emergent and submerged plants all promote
the trapping of suspended solids and par-
ticulate matter or even, in some cases, larger
non-plant litter items. This matter is often
deposited on the surface of the soil or
