Dedicated in Memoriam to Professor Gernot Bretschko
SUMMARY
Natural flood plains are among the most biologically
productive and diverse ecosystems on earth. Globally,
riverine flood plains cover 2 10
6
km
2
, however,
they are among the most threatened ecosystems.
Floodplain degradation is closely linked to the rapid
decline in freshwater biodiversity; the main reasons
for the latter being habitat alteration, flow and flood
control, species invasion and pollution. In Europe and
North America, up to 90% of flood plains are already
‘cultivated’ and therefore functionally extinct. In the
developing world, the remaining natural flood plains
are disappearing at an accelerating rate, primarily as
a result of changing hydrology. Up to the 2025 time
horizon, the future increase of human population will
lead to further degradation of riparian areas, intensifi-
cation of the hydrological cycle, increase in the
discharge of pollutants, and further proliferation of
species invasions. In the near future, the most threat-
ened flood plains will be those in south-east Asia,
Sahelian Africa and North America. There is an
urgent need to preserve existing, intact flood plain
rivers as strategic global resources and to begin to
restore hydrologic dynamics, sediment transport and
riparian vegetation to those rivers that retain some
level of ecological integrity. Otherwise, dramatic
extinctions of aquatic and riparian species and of
ecosystem services are faced within the next few
decades.
Keywords: conservation, restoration, catchment, biodiversity,
connectivity, wetland, climate change
INTRODUCTION
Riverine flood plains are among the Earth’s most distinctive
landscape features. In the natural state they are characterized
by high biodiversity and productivity, and corresponding
recreational and aesthetic values. Flood plains are of great
cultural and economic importance; early civilizations arose in
fertile flood plains and throughout history people have
learned to cultivate and use their rich resources. Riverine
flood plains have also served as focal points for urban devel-
opment and exploitation of their natural functions.
Several comprehensive books on flood plains have been
published during the last 20 years, including monographs on
the Pongolo flood plain (South Africa; Heeg & Bren 1982),
the Hadejia-Nguru flood plain (Nigeria; Hollis et al. 1993),
the Gearagh (a small anastomosing river in Ireland; Brown
et al. 1995), the Luznice (a small meandering river-floodplain
system in the Czech Republic; Prach et al. 1996), the Amazon
(South America; Junk 1997), the Pantanal (South America;
Heckman 1998), Czech flood plains and the effects of water
management (Penka et al. 1985, 1991) and British flood plains
(Bailey et al. 1998). Further, there are textbooks on floodplain
fisheries (Welcomme 1975, 1979), on forested wetlands
including flood plains (Lugo et al. 1990) on floodplain
processes (Anderson et al. 1996), on the geomorphology of
lowland rivers (Carling & Petts 1992), on macroinvertebrates
in North American flood plains and wetlands (Batzer et al.
1999), on biodiversity in wetlands and flood plains (Gopal
et al. 2000, 2002), on wetland ecology and management
(Mitsch & Gosselink 2000), on European floodplain forests
(Klimo & Hager 2001), and on the restoration of river-
floodplain systems (Middleton 1999; Smits et al. 2000).
Flood plains, with an estimated global extent ranging from
0.8 10
6
km
2
to 2 10
6
km
2
(Mitsch & Gosselink 2000;
Ramsar & IUCN [World Conservation Union] 1999) repre-
sent a primary wetland type that will deserve increased
attention as a key global resource in the near future. This
review is seen therefore as a necessary and overdue summary
on riverine flood plains that complements reviews on other
continental wetlands. Brinson and Malvaréz (2002) review
the status of temperate wetlands and Junk (2002) treats
subtropical and tropical wetlands in a more general way.
Moore (2002) focuses on cool temperate bogs, while
Malmqvist and Rundle (2002) review the state of rivers, and
Williams (2002) summarizes the present and future state of
inland saline water bodies.
The present review provides an overview on the actual
extent of riverine flood plains, the major economic and
ecological services they provide, and the multifaceted threats
that make them to one of the most endangered landscapes
worldwide. Further, we discuss their changing ecological
status and finally we predict the conditions of flood plains
Riverine flood plains: present state and future trends
KLEMENT TOCKNER
1
* AND JACK A. STANFORD
2
1
Department of Limnology, EAWAG/ETH, 8600 Dübendorf, Switzerland and
2
Flathead Lake Biological Station, The University of Montana,
311 Bio Station Lane, Polson, MT, USA
Date submitted: 19 October 2001 Date accepted: 10 April 2002
Environmental Conservation 29 (3): 308–330 © 2002 Foundation for Environmental Conservation DOI:10.1017/S037689290200022X
* Correspondence: Dr Klement Tockner e-mail: tockner@eawag.ch
in the year 2025 (see Foundation for Environmental
Conservation 2001) under different scenarios of conserva-
tion/exploitation. Since flood plains, wetlands and
freshwater systems are not always clearly distinguished in the
published literature, general examples on fresh waters or
wetlands are included in the present review if necessary.
Definition and classification of flood plains
Flood plains are defined as ‘areas of low lying land that are
subject to inundation by lateral overflow water from rivers or
lakes with which they are associated’ ( Junk & Welcomme
1990). This definition includes fringing flood plains of lakes
and rivers, internal deltas and the deltaic flood plains of estu-
aries. Brinson (1990) and Brinson and Malvaréz (2002)
proposed a hydrogeomorphic classification of wetlands that is
based on the (1) geomorphic setting, (2) water source, and (3)
hydrodynamics. Considering these components, riverine
flood plains are located on low-gradient alluvial ‘shelves’;
water sources are primarily from lateral overspill of river
water, although other sources may also contribute to flood-
plain inundation; and, although primarily unidirectional,
water flow is characterized by highly complex, multidimen-
sional exchange pathways. Four sources of water are
recognized as contributing to floodplain inundation: lateral
overflow, groundwater, upland sources and direct precipi-
tation. Flood plains can be solely fed by rainfall such as the
Flooding Pampa grasslands in Argentina (Perelmann et al.
2001); however, several sources normally contribute to inun-
dation (Tockner et al. 2000a).
As riparian zones, flood plains are usually defined as
ecotones between terrestrial and aquatic realms (Gregory
et al. 1991; Malanson 1993) that extend from the low-water
mark to the high-water line and also include the terrestrial
vegetation influenced by elevated groundwater tables or
extreme floods (Nilsson & Berggren 2000; Naiman et al.
2000). In practice, there are several transition zones, and
riverine flood plains may contain a complex of different
wetland types.
Flood plains develop in all geographic regions and at
different locations along river corridors (e.g. Tockner et al.
2000a,b). For streams in the USA, the estimated average
floodplain width ranges from 3 m for small rivers to about
1 km for the largest rivers (Table 1). Total floodplain area (all
stream segments combined) is similar across different stream
orders (Table 1). Riverine corridors are often composed of
flood plains sequentially arrayed between canyons or bedrock
constrained segments from the headwaters to the ocean. The
floodplain segments can be as large as 90 000 km
2
as for the
Nile River (Sudd, Sudan), although they range mostly from
tens to hundreds of ha in small and medium-sized rivers
(Stanford & Ward 1993). Along large tropical rivers such as
the Amazon, the Orinoco or the Magdalena, average flood-
plain widths are 32 km, 9 km and 35 km, respectively
(Hamilton & Lewis 1990). The immense area that natural
flood plains may cover is demonstrated by the Fly River
(Papua New Guinea), one of the largest intact floodplain
rivers in Australasia; 60% of the 76 500 km
2
catchment area
becomes seasonally inundated, and the average floodplain
width of the lower 800 km is more than 40 km (Swales et al.
1999).
Flood plains are centres of biodiversity and
bioproduction
Flood plains are considered as centres of biocomplexity and
bioproduction although this has never been rigorously tested
in a regional landscape context (e.g. Megonigal et al. 1997).
Indeed, more species of plants and animals by far occur on
flood plains than in any other landscape unit in most regions
of the world. In the Pacific coastal ecoregion (USA), for
example, approximately 29% of wildlife species found in
riparian forests are riparian obligates (ranging from 12% of
mammals to 60% of amphibians; Kelsey & West 1998).
Although 1% of the landscape of the western USA
supports riparian vegetation, this vegetation provides habitat
for more species of breeding birds than any other vegetation
association. For example, of all bird species breeding in
northern Colorado, 82% occur in riparian vegetation, and
about half of south-western species depend upon riparian
vegetation (Knopf & Samson 1994). Riparian areas in semi-
arid zones are also critical in providing stopover areas for en
route migrants (acting as ‘dispersal filters’), and therefore
affect the breeding success of northern bird populations
(Skagen et al. 1998). In tropical Asia, many nominally terres-
trial mammals are associated with riverine wetlands for part
of their life cycle, including important representatives of the
‘charismatic megafauna’ (e.g. Malayan tapir, Indian rhino;
Dudgeon 2000a,b). However, the biodiversity associated with
rivers and streams has been neglected in most areas of the
world such as Asia or Africa. For example, the Mekong River
contains 500 known fish species, although perhaps 1200 are
expected (Dudgeon 2000a,b). A significant proportion of this
The future of riverine flood plains 309
Table 1 Stream order, estimated number of streams, average and
total length of rivers and streams, average riparian width and total
floodplain surface area in the USA (modified from Leopold et al.
1964).
Stream Number Average Total Estimated Floodplain
order length length floodplain surface
(km) (km) width (m) area (km
2
)
1 1 570 000 1.6 2 526 130 3 7578
2 350 000 3.7 1 295 245 6 7771
3 80 000 8.5 682 216 12 8187
4 18 000 19.3 347 544 24 8341
5 4200 45.1 189 218 48 9082
6 950 103.0 97 827 96 9391
7 200 236.5 47 305 192 9082
8 41 543.8 22 298 384 8562
9 8 1250.2 10 002 768 7681
10 1 2896.2 2896 1536 4449
310 K. Tockner and J.A. Stanford
diverse fish fauna depends on the rich resources provided by
intact flood plains.
In Europe, 30% of threatened bird species are inland
wetland-dependent species and 69% of the important
breeding areas for birds contain wetland habitats, primarily
flood plains (Tiker & Evans 1997). In Switzerland, 10% of
the entire fauna is restricted in its occurrence to riverine flood
plains, although flood plains only cover 0.26% of the
country’s surface. Moreover, 28% of the fauna frequently
uses flood plains and about 44% is occasionally found in flood
plains (Table 2). In total, about 80% of the fauna occurs in
riverine flood plains. A high proportion of the riparian oblig-
ates (47%) is listed as endangered, compared to 28% for the
entire fauna (Walter et al. 1998).
Flood plains are important centres of biological diversifi-
cation. Fittkau and Reiss (1983) assumed that riverine flood
plains belong to those aquatic ecosystems where biota of
lentic areas (standing water bodies) started their evolution.
The temporal continuity of riverine systems and their associ-
ated disturbance regime allowed the permanent presence of
lentic and semi-lentic water bodies throughout time. The
speciation of groundwater crustaceans is also supposed to be
favoured by the lateral shifting of river channels that lead to
the isolation of former connected channels (e.g. cyclopoid
copepods in the alluvial aquifer of the Danube; P. Pospisil,
personal communication 1996).
Flood plains are among the most productive landscapes on
Earth, owing to continual enrichment by import and reten-
tion of nutrient-rich sediments from the headwaters and
from lateral sources, and they are more productive than the
parent river and adjacent uplands. Net primary production in
riparian forests ranges between 750 and 1370 g m
2
yr
1
(mean: c. 1000 g m
2
yr
1
; Mitsch & Gosselink 2000). The
production of wetland/floodplain animals is probably
9.0gm
2
yr
1
, which is 3.5 times the value for terrestrial
ecosystems (Turner 1982, cited in Keddy 2000). Production
depends on hydrology. In Virginia flood plains (USA),
aquatic invertebrate production ranges between 1.1 and
6.12 g m
2
yr
1
, with highest values in the most dynamic
segments (Gladden & Smock 1990). Rivers derive nearly all
their fish productivity from flood plains (e.g. Welcomme
1979; Bayley 1988). There is a positive correlation between
fish catch and the maximum inundated floodplain area in
African rivers, with fish yield being most influenced by the
flood state in previous years (Welcomme 1975, 1979). The so
called ‘flood-pulse advantage’ (sensu Bayley 1995) was recog-
nized by the ancient Egyptians, since taxes were based on the
extent of the annual flood of the Nile. On a worldwide basis,
there is a quantitative relationship expressed as: fish catch
(kg) 5.46 floodplain area (ha). Care has to be taken,
however, as these studies are somehow conjecture since fish
catch does not equate to fish production. Fish catch is usually
adults, which may have achieved their production elsewhere,
and not juveniles. Fish concentrate in flood plains and are
often easier to catch there (Galat & Zweimüller 2001).
Economic importance of flood plains
The estimated worldwide value of the services provided by
flood plains is US$ 3920 10
9
yr
1
, assuming that total
floodplain area is about 2 10
6
km
2
and area-based value is
US$ 19 580 ha yr
1
compared to US$ 969 ha yr
1
for forests
and US$ 92 ha yr
1
for cropland (Constanza et al. 1997). In
total, flood plains contribute 25% of all terrestrial
ecosystem services, although they cover only 1.4% of the land
surface area (for discussion on floodplain area see Aselman &
Crutzen 1989; Mitsch & Gosselink 2000). The major services
of flood plains include disturbance regulation (37% of their
total value), water supply (39%) and waste treatment (9%).
Value of floodplain land in Illinois (USA) was quantified as
high as US$ 7500 ha yr
1
, with 86% based on regional flood
water storage (Schaeffer et al. 2002). Nitrogen removal,
an important floodplain service, varies from 0.5 to
2.6 kg N ha
1
day
1
(e.g. 2.6 kg NO
3
–N ha
1
day
1
in a
Danubian flood plain; Tockner et al. 1999). Flood plains
along the Danube are valued at EUR 384 ha
1
yr
1
for
recreation and nutrient removal (Andréasson-Gren & Groth
1995; 1 EUR 0.88 US$, March 2001). Similarly, the
nitrogen reduction capacity of Estonian coastal and flood-
plain wetlands is worth EUR 510 ha
1
yr
1
. Barbier and
Thompson (1998) valued the weighted aggregate of agricul-
tural, fishing and fuelwood benefits of a Sahelian flood plain
at US$ 34–51 ha
1
yr
1
. The natural value of the flood plain
would be even higher if other important benefits such as the
role in pastoral grazing and recharging groundwater were
included. Agricultural benefits of a planned irrigation project
would, however, be only in the range of US$ 20–31 ha
1
yr
1
.
Table 2 Species pool of selected faunal groups in Switzerland and
the number of species that are floodplain obligates (K1), that are
found frequently in flood plains (K2 K3) and that occur
occasionally in flood plains (K4), and the relative proportion (%) of
species within each category (K1–K4) in the total fauna (data from
Walter et al. 1998; Tockner & Ward 1999).
Group Total Obligatory Frequently Occasionally
number (K1) (K2–K3) (K4)
of species
Mollusca 211 12 17 87
(terrestrial)
Odonata 82 10 48 19
Heteroptera 750 47 170 530
Saltatoria 117 12 30 68
Rhopalocera, 204 6 29 120
Hesperidae
Carabidae 523 139 132 159
Apoidea 585 24 246 125
Amphibia 24 7 16 1
Reptilia 15 3 8 4
Aves 391 33 123 158
Mammalia 83 7 21 57
Total 2985 300 (10%) 840 (28%) 1328 (44%)
Firewood, recession agriculture, fishing, and pastoralism
generate
US$32
per 1000m
3
flood water, compared to
US$
0.15 per 1000m
3
water for irrigation. In the Inner Delta
of
the Niger River over 550 000 people with about 1 mill ion
sheep
and
1 million goats use the flood plain
for
post-flood
dry season grazing (Dugan 1990).
There
are
many other
examples
of
how local communities make use
of
the diversity
and productivity
of
flood plains and wetlands. Especially in
drylands, the benefits
of
natural flood plains are very high
and multifaceted.
Extensive development in flood plains has increased flood
damages
at
unprecedented rates over the past years (e.g.
Burby 2002).
For
example, there
is
a 26% chance
of
a prop-
erty in the 100-year flood plain being damaged by flooding
over the 30-year life
of
a standard mortgage (compared
to
1%
chance
of
fire
damage; Burby 2002).
In
the USA, with 6
million buildings located within the boundaries
of
a 100-year
flood plain, flood losses are widespread
and
losses from flood
hazards have increased dramatically over the last decades
(averaging
US$
115
million per week) and will continue
to
do
so in the next decades (Congressional Natural Hazard Caucus
Work Group 2001).
101
0
1
o9
1 08
1
o7
i
1 06
...
Qi
.s
1
o5
Q)
'°
0
Vi
·~
1 o
4
<ti
0.
1
o3
(/)
1 02
Q
Geomorphic
units
,.-
....
)
Vegetation
units
,
__
c:J Ecosystem processes
....
,.
I
Herbaceous
1
. I
....
,
' \
\
\
vegetation I
& seedlings I
of
early :
suoaessiona/ I
tree species I
I
\
\
'
Floodplain '
grass/ands
1
I
I
I
I
The
future
of
riverine
flood
plains
311
ENVIRONMENT AL FORCING FACTORS
Natural
influences
Flood plains are disturbance-dominated ecosystems charac-
terized by a high level
of
habitat heterogeneity
and
diverse
biota adapted
to
the high spatio-temporal heterogeneity.
The
fo1mation
and
maintenance
of
flood plains
is
closely tied
to
fluvial dynamics (Hughes
1997;
Ward
et
al.
1999a;
cf. Fig.
1).
Fluvial dynamics, including the expansion/ contraction
of
surface waters ('flood and
flow
pulses'),
is
also the driving
force that sustains connectivity in flood plains
(Junk
et
al.
1989;
Petts 1990;
Tockner
et
al.
2000a;
Ward et
al.
2002;
Table 3). Hydrologic connectivity, a
key
process in riverine
flood plains, refers
to
water-mediated transfer
of
energy,
matter and organisms within
or
among elements
of
riverine
corridors. Inundation
of
flood plains
is
a complex phenom-
enon caused by different water sources via multiple
pathways. Small changes in the relative contribution
of
indi-
vidual water sources may drastically alter species com position
and species diversity.
For
example, local groundwater
upwelling
is
often associated with a higher standing crop
of
algae, higher zoobenthos biomass, faster growth rates
of
Postglacial
flood
plain
Point
bars,
channel
edge
sediments
&
recently
cut-off
meanders
........
..
··
.....
,
Flood
plain,\
not
i
previously
j
glaciated
i
1
/
..
..
..
....
10
0
'--~~~
-"-
~~~~~~
---
B~
_._
C~~~~~~~~~
-"-
-D~~~~~
_._
E~
10-1 100 1
01
102
Turnover
time
(yr)
1 in 1 0 1 in 1
00
1 in 1
000
Annual
flood
frequency
Figure
1
The
organization offloodplain oomponents
and
processes as a spatiotempora1 hierarchy (after Hughes 1997). A = primary
succession
of
herbaceous vegetation and early suocessional woody species, associated with annual flood; B = primary
and
seoondary
floodplain suocession, associated with medium-magnitude/ frequency floods; C = long-term floodplain suocession, widespread erosion
and
reworking
of
sediment, associated with high magnitude/low-frequency floods; D
=species
migration upstream/downstream, local species
extinction, long-term succession
on
terraces, and life-history strategies, associated with climate
and
base-level change, and the influence
of
postglacial relaxation phenomena
on
hydrological
and
sediment inputs to flood plains;
and
E = species evolution,
and
changes in
biogeographical range, associated with tectonic change, eustatic uplift and climate change.
312
K.
Tockner and
J.A.
Stanford
Tab
le 3
The
estimated relative importance
of
environmental
factors that determine the properties
of
wetlands
in
general (after
Keddy 2000) and flood plains specifically (empirical values).
En
vi
ronmental factor
Wetland(%)
Flood
plain(%)
Hydrology
50
60
Fertility
15
<JO
Salinity
15
<5
Disturbance
15
30
Competition
Grazing
Burial
<5
<5
<5
<5
<5
<5
cottonwood trees and a higher species richness
of
woody and
herbaceous plants (J.A. Stanford, personal communication
2001). Despite its overwhelming imp
or
tance
in
flood plains,
hydrology is often given only cursory attention in restoration
and mitigation projects (e.g. Bedford
1996).
Human
influences
Flood plains are among the most altered landscapes world-
wide and they continue to disappear at an alarming rate, since
floodplain 'reclamation' (i.e. elimination) is much higher than
for most
ot
her landscape types (Vitousek et al.
1997;
Olson &
Dinerstein 1998; Ravenga et al. 200
0)
.
The
net result is vast
constriction
of
flood plains, sometimes by more than 50%
of
the historic expanse (Snyder et al. 2002).
As
a consequence,
the decline
of
freshwater biodiversity, including the rich
floodplain diversity, is much greater than in terrestrial
systems. For example, 47%
of
all animals federally endan-
gered
in
the USA are freshwater species (Stein 2001).
Although no specific data are available,
we
may expect a
disproportional contribution by floodplain species (see
Tab
le
2).
T he major factors responsible for the decline
of
fresh-
water biodiversity are ha
bi
tat alteration, pollution,
competition for water, invasive species and overharvest
(Abramovitz
1996)
. Among these
fac
to
rs, land transform-
ation is the single most imp
or
ta
nt
cause
of
species extinction
(Vitousek
et al.
1997)
. Habitat degradation and loss
contribute to the endangerment
of
85% of the imperiled
species in the USA (Wilcove
et al.
1998;
Ta
ble 4). For fresh-
water groups, water development projects account for 91
%
of
th
reats to endangered
fis
h and
63
% to endangered
amphibians.
Habitat alteration
Habitat alteration includes both the degradation
of
the
natural landscape and the modification
of
the hydrologic
regime. Worldwide, more than
500
000
km
of waterways have
been altered for navigation and more than
63
000
km of canals
have been constructed (e.g. Abramovitz 1996).
In the USA,
only
2%
(about
100
000
km)
of rivers have sufficiently high
quality features to be worthy
of
federal protection status
(Benke
1990).
T hese free-flowing sections are often single-
thread rivers that lack extensive flood plains.
In
Austrian
Tab
le 4 Information on the relative importance (percentage)
of
different threats for 1880 (75%)
of
the 2490 imperiled species in
the USA, and for amphibians and fish separately. Categories are
non-exclusive and therefore
do
not sum
up
to
JOO
(Wilcove et
al.
1998).
Cause
All
species
Amphibians Fish
n
= 1880
n =
60
n = 213
Habitat degradation/loss 85 87
94
Alien species 49
27
53
Pollution
24
45
66
Overexploitation
17
17 13
Disease 3 5 I
rivers with catchment areas >
500
km
2
,
it is the floodplain
segments that have been most severely impacted.
To
day, less
than
2%
of
former braided, anastomosing and meandering
Austrian rivers are in a semi-pristine state, compared to 25%
of
single-thread headwater streams (Muhar et al. 1998).
To
evaluate the effect
of
river regulation on selected
Central European river-floodplain systems, we used shoreline
length (the interface between the terrestrial and the aquatic
compartments
of
the
floo
d plain)
as
an index
of
habitat quality
(Schiemer
et al.
2001;
K.
Tockner, personal communication
2001).
In dynamic systems (e.g. the Tagliamento River,
Italy), shoreline length can be
up
to
25
km
per river
km
and
remains high throughout the annual cycle, except during
major flood events.
In channelized rivers, however, shoreline
length drops to about 2
km
per river km (Fig.
2).
Reduction in
shoreline length not only affects habitat availability
of
already
endangered communities but also impedes the exchange
of
matter and organisms between the river and its
ri
parian area
(Naiman
& Decamps
1997).
Hydrology is by far the single most important driving
variable in flood plains (see Table 3). Changes in river
flow
alter the extent, duration and frequency of floodplain inun-
dation. After dam closure the Nile showed a reduced annual
:::--
30
I
Tagliamento
=
..i2
25
=
g
20
t
15
=
10
~
Q)
5
.5
~
0
0
..c:
0
Cl)
......... ....... ........ ..... .... .................. ... ;
..
.
...
. . .
200
1
00
300
Time
(days)
Figure
2 Shoreline length (km per river-km)
in
natural
(Tagliamento, north-east Italy), constrained (Danube, Alluvial
National Park, Austria) and channelized (Rhone, Switzerland)
river-floodplain systems. All flood plains are characterized
by
a
dynamic hydrology (Van der
Nat
et
al.
2002; K. Tockner, personal
communication 2001).
The
Rhone and the
Tag
liamento
Ri
ve
r are
comparable in discharge and catchment area. In its pristine state,
the Rhone was morphologically similar to the present Tagliamento.