When is a river not a river? Challenges for managing temporary waterways

Dry river bed near Mt. Seoraksan in Korea. Image: Wikimedia

Not all rivers and streams plot a constant course towards the sea.  Some naturally dry up when there is little rain, leaving behind a dry stream bed which floods the next time there is a heavy storm.  In fact, most river systems have areas where at least some of the river bed will dry up, usually for days, sometimes for months or years.

For example, at least half of the 2700km long Tagliamento River in Italy is temporary (according to this journal article).  At the other end of the world in a far colder climate, most of the streams formed from melting ice on Antarctica last only for a few months every year.  Other rivers and streams around the world have become temporary due to the fragmentation of water flows through dam and weir construction, or through the removal of water for crop irrigation and industry.  It is thought that changes to rainfall and air temperatures in future climates will cause even more rivers to become temporary, especially in arid regions.

A new journal article in Science by Vicenç Acuña and colleagues including BioFresh leader Klement Tockner argues whilst temporary rivers and streams are extremely important, both ecologically and culturally, they are not adequately managed and protected by current environmental policy.

A tangle of braided wet and dry channels on the River Tagliamento at Cornino in Italy. Image: Wikipedia

Dry river beds create surprisingly diverse ecosystems

Despite their barren appearance, dry river beds can be very important ecologically.  They can support a unique set of plants and animals that are adapted to huge fluctuations in water availability (what ecologists term ‘wet’ and ‘dry’ phases), and massive disturbances when high rainfall floods a dry channel. The dropping  of nutrient and mineral rich silt by receding waters (often termed ‘deposition’) often creates fertile strips of land for vegetation to grow, even in a desert, and so can provide unusually diverse habitat for many mammals, birds and insects, even when there is no water flow.

Similarly, dry river beds can act as egg and seed banks for many aquatic insects and plants which lie dormant in dry periods, then hatch or germinate when water returns.  Dry river beds can also provide important migration corridors through the landscape for some animals, often guiding the way to where there may be temporary waterholes.

Ostriches feeding on a dry river bed in the Kgalagadi Transfrontier Park, Botswana. Image: Wikimedia

Connecting the wider environment

Temporary rivers and streams are important providing in what ecologists term ‘connectivity’, in the way they connect different ecological communities and flows of water, energy and nutrients across the wider landscape.   A river channel that appears to be dry may still have water flowing underneath the surface, connecting more permanent flows elsewhere.  Temporary rivers remind us that a river ecosystem is not only made up of the water you can see flowing on the surface, but of the often invisible flows and interactions of underground water, nutrients, plants and animals.

The importance of flux and change

Stone bridge across a dry river bed in Agia Roumeli, Crete, Greece. Image: Wikimedia

Temporary rivers are an example of an ecosystem in constant flux, regularly altered by environmental change and disturbance.  A key point here – especially when considering the work of the MARS project – is that intermittent and sometimes unpredictable water flows should not be seen as an unnatural stress on the ecosystem.  Instead, these changing flows are an important part of these distinctive environments, shaping a unique habitat for a number of different plants and animals.

As a result, temporary rivers are increasingly studied by ecologists (e.g. Larned et al, 2010), the results of which will help work out how best to manage and conserve these fluctuating ecosystems.  This research is particularly important given predictions of changing climates where some areas of the world become more arid and dry and others become wetter and more flood prone.

Dry river beds as spaces for human subsistence, storytelling and culture

Aboriginal spot painting of Tiddalik the frog. Image: balyat.blogspot.com

Dry river beds are important to humans, too, providing migration routes and fertile places for animal grazing and agriculture.  In the Dreamtime stories of Australian Aborigines, dry river beds were formed when a giant frog named Tiddalik drank all the rivers dry.  The story of Tiddalik has been compared to that of the water-holding frog, which burrows underground during dry periods, reemerging during heavy rainfall to feed, breed and take on water in special skin pockets.  When in search of water in dry periods, one Aboriginal trick is to dig up water-holding frogs from dry riverbeds in the Outback and give the animals a gentle squeeze, which releases water from the frog’s skin – a bit like a natural water bottle.

The Boteti River in Botswana. Note how the river appears to stop flowing at the bottom of the image. Image: NASA / Wikimedia

In Botswana, there are similar unexpected natural riches to be found beneath the partially dry Okavango delta.  Local people ‘fish’ for catfish which lie dormant for weeks on end beneath dry river beds – a process known by biologists as aestivation.  As seasonal flood waters shrink on the delta in the autumn, shoals of catfish rampage up shrinking stream channels to breed and feast on small prey fish trapped by the receding water.  This ‘catfish run’ is a well-known phenomenon to locals and tourists alike, and people gather to see shoals of catfish turn the shrinking streams into boiling brown masses of water as they feed.

When is a river not a river? Challenges for managing temporary waterways

Cattle using a dry river bed in Mozambique as a migration corridor. Image: Stevie Mann / Wikimedia

So, temporary rivers are important to humans as well as to plants and animals.  But how can we best manage and protect rivers that are not always there?  As the new paper by Acuña et al states, “widespread degradation [of temporary rivers] stems from lack of recognition, poor understanding, and inadequate management”.

For example, as this paper by Alisha Steward and colleagues points out, seasonally dry riverbeds have been completely covered up by roads in many cities, such as Las Ramblas in Barcelona.  Other temporary rivers have been inundated by water from dams or wastewater from industry, completely changing their natural flow regimes and encouraging the growth of populations of invasive species.

This lack of visibility for temporary rivers is also evident in global environmental policy.  As Acuña et al outline:  “The legal status of intermittently flowing streams and rivers and the extent to which they are incorporated into policy, management, and regulatory decisions vary widely depending on how temporary waters are defined by the authorities, as well as what kinds of protection are given to temporary waterways. Even where flow intermittency is prevalent, temporary waterways may not be legally recognised as part of the river network.” 

Examples of current management from the EU, United States and Australia

Cars on the dry bed of the Leichhardt River in Australia, 1912. Image: Wikimedia

Definitions are a key challenge here: if you can define what needs managing, then it makes that task a lot easier.  For example, in the European Union, a temporary stream or river may not be protected under the Water Framework Directive, depending on the different ‘typology’ systems that different areas of Europe use to classify their waterways.  As the Acuña et al paper describes, this means that protection for temporary waterways in Europe is patchy and insufficient.

In the United States, rivers and streams which flow year round are given ‘jurisdictional’ legal protection against pollution and alteration.  The definition, and need for protection, of temporary streams is considered on a case-by-case basis, meaning that many are not protected under this same jurisdictional basis.  In contrast, in Australia temporary waterways are managed as part of wider river management plans in most areas, perhaps as a result of the wider awareness of these intermittent flows in Australian culture.

Recommendations for better management and protection of temporary rivers and streams

The Allt nan Uamh in Scotland. The river flows mostly underground through limestone caves, although the size of the boulders in the channel show how powerful spate flows can be. Image: Wikimedia

Broadly, it’s the inconsistency in definition, awareness and management of temporary rivers and streams that is key here.  As Acuña et al argue, for environmental policy to be consistent with the current state of science, temporary waterways should be legally defined and managed as part of the river network they connect to – however irregularly.  Even if flows are so rare that the river channel is usually dry, the authors argue that this definition as part of the wider system should still apply where the dry channels are important habitats for plants and animals.

How would stronger policies for managing and protecting temporary waterways be put into practice?  Acuña et al state that an important step is to better map temporary rivers and streams, their flows and their biodiversity.  The ecology of temporary streams will need to be monitored using biological indicator systems to understand the diversity of life they support, and how populations of plants and animals change in response to different water levels.

The intermittency of flow in these ecosystems isn’t a negative ecological stress, instead an important factor in shaping the form and diversity of these unique environments.  How environmental policy can respond to such variation in flow and ecology of temporary rivers, particularly under future climate change, is an important challenge for global water management.  In this context, the Acuña et al paper is important in bringing together the growing set of available research, and publishing these clear recommendations in such a high-profile journal.

Reference: Acuña et al (2014) ‘Why Should We Care About Temporary Waterways?’ Science, 343, 1080-1081