Frontiers in Ecology
and the
Environment
When the river runs dry: human and
ecological values of dry riverbeds
Alisha L Steward, Daniel von Schiller, Klement Tockner, Jonathan C Marshall, and Stuart E Bunn
Front Ecol Environ 2012; doi:10.1890/110136
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R
ivers that intermittently cease to flow and “run dry”
have been described as being more representative of
the world’s river systems than those with perennial flows
(Williams 1988). These temporary rivers are a truly global
phenomenon (Larned et al. 2010), and their spatial and
temporal extent is likely to further increase resulting from
the combined effects of altered land-use patterns, climate
change, and increased water extraction for human uses
(Meehl et al. 2007; Palmer et al. 2008). Dry riverbeds are
defined as the channels (the area between river banks) of
temporary rivers during the dry (flow cessation) phase that
can be exposed during periods of drought. They are habitats
in their own right and differ from adjacent riparian and
other terrestrial habitats in their substrate composition,
topography, microclimate, vegetation cover, inundation fre-
quency, and biota (Kassas and Imam 1954; Coetzee 1969;
Steward et al. 2011). Often considered to be harsh environ-
ments, dry riverbeds are subject to flow disturbances that
mobilize, deposit, and scour bed sediments. They can also
be exposed to intense solar radiation, wind, and extreme
temperatures (Steward et al. 2011). Dry riverbeds may be
devoid of vegetation; however, in arid regions, they can be
where the greatest diversity and density of vegetation is
found (Figure 1; Kassas and Imam 1954).
Although often linked with negative connotations, dry
riverbeds are associated with a range of important societal
and ecological values. Unfortunately, dry riverbeds have
been largely ignored by aquatic and terrestrial ecologists,
probably because they are perceived to be outside the
domain of their respective disciplines. A temporary river-
bed can be dry for much of the time and may only be
“aquatic” for a brief period after a flood or a period of
heavy rainfall. The role of dry riverbeds as habitats is
“only beginning to be understood and is an exciting fron-
tier, albeit it is still terra incognita” (Datry et al. 2011).
This paper aims to advance the traditional view of tem-
porary rivers by (1) recognizing dry riverbeds as important
features in the landscape and (2) highlighting their eco-
logical values and their importance to humans.
n
Dry riverbeds and landscape connectivity
Rivers expand and contract – longitudinally, laterally,
and vertically – over time in response to their flow
regimes (Stanley et al. 1997; Döring et al. 2007), and the
greatest contraction is seen when the entire riverbed
becomes dry. Headwater streams in temperate, subtropi-
cal, and tropical zones can cease to flow on a seasonal
basis, leaving behind perennial pools in amongst dry sec-
tions of riverbed (Figure 2). Water in these systems can
continue to flow beneath the riverbed, along subsurface
routes. Dry riverbeds are not restricted to headwaters,
however, and can also be found in the mid-reaches and
lowlands of river networks (Figure 2). Many arid and
semi-arid rivers can be dry along most of their length for
CONCEPTS AND QUESTIONS
When the river runs dry: human and
ecological values of dry riverbeds
Alisha L Steward
1,2*
, Daniel von Schiller
3
, Klement Tockner
4
, Jonathan C Marshall
1
, and Stuart E Bunn
2
Temporary rivers and streams that naturally cease to flow and dry up can be found on every continent.
Many other water courses that were once perennial now also have temporary flow regimes due to the effects
of water extraction for human use or as a result of changes in land use and climate. The dry beds of these
temporary rivers are an integral part of river landscapes. We discuss their importance in human culture and
their unique diversity of aquatic, amphibious, and terrestrial biota. We also describe their role as seed and
egg banks for aquatic biota, as dispersal corridors and temporal ecotones linking wet and dry phases, and as
sites for the storage and processing of organic matter and nutrients. In light of these valuable functions, dry
riverbeds need to be fully integrated into river management policies and monitoring programs. We also
identify key knowledge gaps and suggest research questions concerning the values of dry riverbeds.
Front Ecol Environ 2012; doi:10.1890/110136
1
Queensland Department of Environment and Resource Manage-
ment, Ecosciences Precinct, Brisbane, Queensland, Australia;
2
Australian Rivers Institute, Griffith University, Brisbane,
Queensland, Australia
*
(a.steward@griffith.edu.au);
3
Catalan
Institute for Water Research, Scientific and Technological Park of the
University of Girona, Girona, Spain;
4
Leibniz-Institute of Fresh-
water Ecology and Inland Fisheries, and Institute of Biology, Freie
Universität Berlin, Berlin, Germany
In a nutshell:
• Most river systems have reaches with temporary flow regimes
and riverbeds that can remain dry for days to years at a time
• Dry riverbeds have important human and ecological values
that are often overlooked by river and catchment managers
• Conceptual models of riverine landscapes that do not include
dry riverbeds are incomplete, and thus lack relevance in many
parts of the world
Human and ecological values of dry riverbeds AL Steward et al.
www.frontiersinecology.org © The Ecological Society of America
most of the time, except for the presence of isolated
perennial pools (Figure 2). Although common in desert
environments, dry riverbeds can be found in a wide range
of ecosystems. For example, almost 50% of the network of
the 2700-km-long Tagliamento River, an alpine river in
northeast Italy, is temporary (Döring et al. 2007), whereas
streams in Antarctica flow for several months and are dry
for the remainder of the year (McKnight et
al. 1999).
Dry riverbeds can be created or inundated
by anthropogenic influences (Figure 2).
Dams and weirs can intercept flow, drying
riverbeds downstream. Alternatively, natural
temporary rivers can become perennial as a
result of constant flow releases from dams or
weirs, or when water is discharged from min-
ing operations or sewage-treatment plants
(Hassan and Egozi 2001). Water extraction
from rivers and groundwater during droughts
can reduce river flows, causing riverbeds to
dry (Holmes 1999; Palmer et al. 2008).
Future climate warming is predicted to
increase the frequency of droughts in many
regions (Meehl et al. 2007), increasing the
temporal and spatial extent of dry riverbeds.
The drying of a riverbed represents a loss of
longitudinal connectivity for aquatic biota as
well as for physical aquatic processes
throughout the river network (Figure 2).
During a flow event, previously isolated pop-
ulations can be reconnected through both drift and the
active dispersal of aquatic biota. Organic matter and
nutrients are transported and processed downstream dur-
ing this time. Although dry river reaches are barriers to
aquatic downstream movement and processing, they are
connected laterally to the riparian zone, floodplain, and
adjacent terrestrial ecosystems. These surrounding areas
provide dry riverbeds with inputs of organic
matter and nutrients, and can allow for the
movement of terrestrial biota between
them. Dry riverbeds are connected to sub-
surface waters and sediments below; they are
also connected to the airspace above and
can act as a corridor for aerial biota. A key
knowledge gap concerning dry riverbeds in
landscape ecology concerns how the spatial
configuration and extent of dry riverbeds
determine catchment-scale processes, such
as the distribution of biota and the transfer
of energy through food webs. Further know-
ledge gaps and research questions are pre-
sented in Table 1.
n
Values of dry riverbeds
Value to humans
Temporary rivers, streams, and dry riverbeds
are widely recognized in human culture and
language (Table 2), and feature in stories
told by indigenous peoples around the
world. In the Dreamtime stories of
Australian Aboriginal people, Tiddalik the
Frog drank all of the water, leaving the
Figure 1. In arid regions, dry riverbeds may be where the most diverse and
most dense vegetation is found, as shown in this aerial photograph of a dry river
channel in the Lake Eyre Basin, Australia.
Table 1. Knowledge gaps and questions regarding dry riverbed
research
Value Knowledge gap/research question
Value to humans Which communities of people rely on dry riverbeds?
What is the distribution of dry riverbeds at risk of
degradation?
Unique biodiversity During extreme conditions, do dry riverbeds serve as
a refuge for upland terrestrial biota? Do dry riverbeds
trigger the (rapid) evolution of life-history traits, such
as higher dispersal capability and dormancy?
Studies are needed to investigate the traits that allow
terrestrial invertebrates of dry riverbeds to survive
both wet and dry phases.
Refuge for specialized How long can quiescent stages of aquatic biota remain
aquatic biota viable in dry riverbeds, and how will changes in hydrol-
ogy influence these taxa?
Corridors for Is rafting during flood events an important dispersal
terrestrial biota mode for maintaining the viability of populations of
terrestrial invertebrates?
Temporary ecotones Are there critical thresholds in the duration, spatial
linking wet and dry extent, and severity of drying in temporary river
phases systems that may lead to fundamental shifts in com-
munity structure, ecosystem processes, and services?
Storage and processing What is the extent to which ecosystem processes
of organic matter and during the wet phase control those during the dry
nutrients phase, and vice versa?
AL Steward et al. Human and ecological values of dry riverbeds
© The Ecological Society of America www.frontiersinecology.org
rivers dry. Dry riverbeds have also been popularized in
modern Australian culture; for example, the annual
Henley-on-Todd Regatta, which takes place in the arid
zone of Australia’s Northern Territory, is the world’s only
dry riverboat race, in which teams of “rowers” race each
other along a dry riverbed (Figure 3a).
Dry riverbeds are a source of food and water. In
Botswana, people “fish” for catfish aestivating in dry
riverbeds. Water may be found by digging in dry water
courses, and wells are often constructed within them
(Jacobson et al. 1995). In Egypt, they are grazed by cattle
and camels, medicinal plants are collected from them,
and woody vegetation growing along the edges of the
riverbed is harvested for fuel (Kassas and Imam 1954).
Dry riverbeds can provide fertile substrates for agricul-
ture. Fruit and vegetables are grown in the dry beds of the
Ganges River in India (Hans et al. 1999) and in Egypt’s
Wadi Allaqi (Briggs et al. 1993); in Mediterranean Spain,
it is common to find citrus orchards and other crops grow-
ing within dry riverbeds (Gómez et al. 2005). Gravel and
sand are often extracted from dry riverbeds for building
materials, and they are also places of recreation where
people can camp, hunt, hike, ride, and enjoy nature.
Dry riverbeds are used as walking trails and vehicle
tracks (Figure 3b), as car parks (Gómez et al. 2005), and as
animal transportation routes. In Spain, shepherds once
used dry riverbeds as migration corridors, and in 1993 it
was estimated that more than 100 000 camels were
herded along dry riverbeds from Sudan to Egypt to be sold
at market (Briggs et al. 1993).
Unique biodiversity
Temporary rivers are characterized by frequent and
intense disturbances and extreme environmental condi-
tions. These features place strong selective pressure for
the evolution of traits for the resistance and resilience of
the biota to survive both wet and dry phases (Robson et al.
Figure 2. Dry riverbeds in a landscape context, showing examples of natural and human-altered temporary river networks. Hydrologic
connectivity (flowing sections) is represented by solid lines; dashed lines represent disconnected (dry) sections. Perennial pools are
indicated by ellipses. Conceptual model symbols are courtesy of the Integration and Application Network (ian.umces.edu/symbols/).
NATURAL TEMPORARY
RIVER NETWORKS
WET SEASON
TEMPORARY RIVER
NETWORK
HUMAN-ALTERED
TEMPORARY RIVER NETWORKS
Aquatic biota
and processes
are connected
longitudinally
DRY HEADWATERS
eg some temperate rivers,
sub-tropical rivers, tropical rivers
DRY MID-REACHES
eg some alpine rivers,
temperate rivers, tropical rivers
DRY NETWORK
eg arid rivers,
semi-arid rivers
DAMS AND WEIRS can create
“man-made” dry riverbeds
downstream by restricting flow,
and may inundate dry riverbeds
upstream
WATER ADDITION
(eg waste water) can
inundate dry riverbeds
WATER EXTRACTION
can create “man-made”
dry riverbeds
+ –
CLIMATE-CHANGE predictions
of higher temperatures, higher
evaporation, and more
frequent droughts will cause
dry riverbeds to be
encountered more frequently
and in more places
Perennial
pool
Perennial
pools
Dam or weir
Human and ecological values of dry riverbeds AL Steward et al.
www.frontiersinecology.org © The Ecological Society of America
2011). Indeed, the drying of pools in temporary river net-
works has been postulated to have led to the evolution of
traits that first allowed aquatic vertebrates to leave the
water and colonize the land (Romer 1958), and may have
been the driving force in the evolution of desiccation
resistance (Williams 2006). Temporary rivers host a
unique combination of aquatic, amphibious, and terres-
trial assemblages as a result of their wet and dry phases
(Figure 3, c and d). Desiccation-resistant stages of aquatic
biota are present in riverbed sediments during the dry
phase and, conversely, inundation-resistant stages of ter-
restrial biota may be present during the wet phase.
Amphibious and semi-terrestrial biota may inhabit tem-
porary rivers (Gibbs 1998), and a succession of biota can
be observed during the transition from wet to dry phase.
An initial “clean-up crew” of amphibious and terrestrial
biota may consume any stranded aquatic matter, including
dead and dying fish and aquatic invertebrates (Williams
2006). The terrestrial assemblages,
such as invertebrates, that follow can
be highly diverse, and differ from adja-
cent riparian and other terrestrial
communities (Wishart 2000; Steward
et al. 2011).
Dry riverbeds have been described as
linear oases, containing vegetation
that is richer than other types of desert
habitat (Figure 1; Kassas and Imam
1954; Fossati et al. 1999). They also
provide important habitat for verte-
brates; for example, riverbeds are the
most heavily utilized vertebrate habi-
tat in the southern Kalahari Desert in
Africa, with ungulates moving in and
out according to food availability
(Mills and Retief 1984). Dry riverbeds
can also provide abundant prey for
mammals (Geffen et al. 1992), such
that some predatory mammals are now
regarded as semi-permanent inhabi-
tants (Coetzee 1969). There is even
fossil evidence that they once served
as nesting grounds for sauropod
dinosaurs (Kim et al. 2009).
Refuge for specialized aquatic
biota
Dry riverbeds often act as egg banks
for aquatic invertebrates and seed
banks for aquatic plant, algal, fungal,
and bacterial propagules (Williams
2006; Lake 2011). Some aquatic crus-
taceans live exclusively in temporary
waters and require, or benefit from, a
desiccation phase in order for their
eggs or cysts to hatch (Figure 3c;
Brendonck 1996). Other aquatic invertebrates take
refuge in moist depressions, under woody debris and leaf
litter, or in crevices under rocks, or they burrow into the
riverbed itself (Chester and Robson 2011). Some fish
species aestivate in dry riverbeds until they are rewetted
(Berra and Allen 1989). Such a strategy may provide
these fish with a competitive advantage over other fish
species that recolonize from upstream, downstream, or
lateral refugial pools when flow resumes.
Aquatic plants can have desiccation-resistant frag-
ments – for example, tubers or seeds that persist during
the dry phase and then grow or germinate when rewetted
(Brock et al. 2003). Some algae have physiological attrib-
utes that allow them to resist desiccation for years, before
reactivating and growing when the waters return.
Cyanobacterial and algal taxa can survive within dried
microbial biofilms that establish on hard substrates during
the wet phase (Robson et al. 2008), or as freeze-dried mats
Figure 3. Values of dry riverbeds: (a) cultural significance – the Henley-on-Todd Regatta
(Todd River, Northern Territory, Australia); (b) vehicle transport route (Mitchell River,
Queensland, Australia); (c) egg banks for aquatic biota, such as clam shrimp
(Branchiopoda: Spinicaudata) (Northern Territory, Australia); (d) habitat for terrestrial
biota, such as wolf spiders (Araneae: Lycosidae) (Northern Territory, Australia); (e)
wildlife corridors (Tagliamento River, Friuli-Venezia Giulia, Italy); and (f) storage sites
for organic matter, such as leaf litter (Riera de Fuirosos, Catalonia, Spain).
D Logan/www.henleyontodd.com.au
