February 2018. Local media circulated information about the appearance in the village of Kaczory (Wielkopolska voivodeship) of a river that is not usually there. Thanks to heavy rainfall and rising water levels in the local Kopcze Lake, a river flowed down the road toward Pila. This phenomenon is not new, as it repeats itself every few or several years.
The last was recorded 17 years earlier (in 2001), the still previous one in 1987. You never know when the river will appear, but it can be expected after periods of long, heavy rainfall. As a result, it has been hailed (probably a bit exaggeratedly) as “the only periodic river in Poland.”
Although intermittent rivers occur all over the world, the phenomenon is more likely to be associated with subtropical areas of Australia or Africa rather than Greater China. A periodic watercourse is called a watercourse in which water flows only at certain times of the year, when groundwater is high enough to be its main source of supply, and precipitation is an additional source of water.
In dry periods, such intermittent rivers disappear. In contrast, a watercourse in which water flows only during precipitation or for a short time afterwards is called ephemeral. In many cases, the term refers to streams whose beds are above the groundwater table year-round, so they are not fed by it and depend solely on precipitation.
Several classifications of such perishable watercourses are available in the literature, but they are most commonly referred to as ” ephemeral streams and intermittent rivers,” so-called IRES (from intermittent rivers and ephemeral streams). The term IRES collectively captures the entire range of ecosystems characterized by high flow variability, alternately flowing and drying up.
Where do ephemeral streams and periodic rivers occur?
Ephemeral streams and periodic rivers are found on all continents, including Antarctica. They are most common in extremely arid, arid and Mediterranean regions, being the dominant form of flowing water ecosystems there. For example, more than 70% of Australia’s 3.5 million km of rivers are intermittent, and in southwestern American states such as Arizona and New Mexico, IRES account for up to 94% of total river length. However, such ecosystems can occur in large numbers even in areas with temperate and humid climates, especially in the upper reaches of rivers.
Although ephemeral streams and periodic rivers have been present on Earth since time immemorial, they have only recently received interest, with an increase in the number of scientific publications on them generally observed in the last decade[3-5], as well as the first monograph on ERES from 2017. This is likely related to the fact that intermittent rivers and ephemeral streams are becoming more common around the world due to climate change and increasing anthropogenic pressures.
How are ephemeral streams and periodic rivers formed?
The causes of periodic rivers and the factors shaping their hydrological dynamics and ecological processes are plentiful, but they primarily include climatic, hydrological and morphological conditions. In different regions, some factors may be more important than others.
In arid and semiarid regions, flow decay is largely due to low precipitation, high evaporation and high evapotranspiration. In alpine and Arctic regions, where watercourses are formed from melting snow and glaciers, low temperatures are the primary determinant. During colder periods, the snow stops melting and the water surface freezes, resulting in the flow slowing down or stopping. In such regions, ephemeral streams and periodic rivers flow only for a few months, during the summer.
In addition to climatic factors, the physical characteristics of the channel, such as the porosity of the substrate and the depth of the water table, are also important, affecting the rate of infiltration. In IRES with coarse bed substrate (e.g., gravel rivers) and a groundwater table well below the bed, water easily infiltrates downward, recharging aquifers. A decrease in groundwater levels, whether as a result of natural seasonal fluctuations or human activity, can contribute to slowing down the flow, interrupting it, or even drying up the watercourse along its entire length. Lakes’ drainage streams usually stop flowing when the mirror level drops.
In addition to the many natural factors that condition ephemeral streams and intermittent rivers, flow disturbance or interruption can be the result of human activities, mainly land use changes, watercourse regulation, surface and groundwater abstraction, and reduced precipitation and increased evaporation due to climate change. Today, even large rivers such as the Colorado River in the United States and the Yellow River in China[8, 9] show flow disruptions and periodic breaks in hydrological continuity due to the construction of dams and dams and increased water withdrawals for drinking and agricultural use.
In addition to transforming permanent streams and rivers into periodic and ephemeral ones, human activities can also modify the functioning of natural IRES by extending the duration and extent of zero-flow periods and altering their frequency. The result is the creation of entirely new ecosystems (novel ecosystems) with structures and functioning completely unrecognized by us.
Ecological specificity of IRES
The characteristic and most interesting feature of all IRES is their instability and high dynamics. Due to cyclical fluctuations in flow and hydration levels, they form a spatially and temporally variable mosaic of lotic (flowing water), lentic (standing water) and terrestrial (dry) habitats. All of these habitats, or any combination of them, can occur simultaneously, and as their areas expand or contract in response to changes in rainwater availability and groundwater levels, their spatial pattern, temporal dynamics and hydrological connectivity change.
These ecosystems are colonized by a wide range of species showing adaptations to different types of habitats. During wet periods, flowing water (lentic ecosystem) and non-flowing water (volatile ecosystem) phases in IRES troughs are dominated by aquatic plants and animals. On the other hand, during dry phases, exposed and dried river bottom sediments are colonized by terrestrial and aquatic organisms typical of the riparian zone and flood plain habitats.
The regime of alternating flow, alternating flooding and drying of the land, poses a challenge to both aquatic and terrestrial organisms. Aquatic organisms have to contend with long periods of slowed flow or even lack of water. The land-based ones, on the other hand, experience periods of flooding and inundation (sometimes prolonged), which they must avoid or learn to tolerate.
Some aquatic species have developed the ability to produce survival forms and can survive for months or years in dried river sediments in the form of cysts, cocoons or diapausing juveniles or adults. Other organisms have the ability to rapidly re-colonize a habitat after a period of disturbance due to their ability to spread rapidly, short life cycle and high fecundity.
Terrestrial organisms, on the other hand, adapt to flood conditions, for example, through their ability to store air or float. In addition, trapped aquatic plants and animals in drying pools and along retreating stream banks provide an attractive food base for terrestrial and semi-terrestrial animals inhabiting ephemeral streams and intermittent rivers.
While the strategies of aquatic species have been fairly well recognized and studied for decades, in the case of terrestrial assemblages inhabiting the IRES, recognition of flood resilience strategies is very sparse and requires further research. The temporal dynamics and variability of habitat patterns in alternately flooded and drained IRES make these ecosystems ideal arenas for studying metapopulations and metacommunities and analyzing the complex ecological relationships between abiotic conditions, biogeochemical processes and biotic interactions.
Will our rivers become intermittent watercourses?
Natural intermittent aquatic ecosystems are populated by unique and highly diverse biological communities, are characterized by specific hydrological and biogeochemical processes, and provide many important ecosystem services. As a result of increasing anthropogenic pressure and ongoing climate change, they can be expected to become more widespread and more vulnerable in many regions.
Given that temporary streams mainly occur in regions where there is a shortage of water for human use and demand is increasing, intermittent river ecosystems are increasingly being modified. Over the past few decades, increasing anthropogenic pressures, combined with strong seasonal and inter-annual flow variability exacerbated by climate change, have led to a decline in the ecological quality of many periodic rivers.
The nameless river in the village of Kaczory appears so rarely that it does not appear on maps and has no name. Locals make bets on whether it will swim in a given year, and if so, what length it will reach. In 1987. It managed to reach a length of 9 km and flowed all the way to the Gwda River. In 2001. She has now swum only 2.5 kilometers. Whether and when it reappears depends on the amount of rainfall. Its next “visit” is expected around 2030. Given the hydrological problems associated with climate change, it is possible that we may never see it again.
Although periodic rivers are not formally distinguished in Poland, it seems that the recognition of the nameless river in Kaczory as “the only periodic one” is, however, a tad exaggerated. The hydrographic network in our country is full of such periodically drying up watercourses. However, as long as these are tiny ciurls with a catchment area of no more than a few square kilometers, it is of little concern.
The problem begins when regular, continuously flowing rivers begin to be threatened with drying up. Every summer, the media alerts about more rivers drying up. According to data from the Chief Inspectorate for Environmental Protection, already several percent of Poland’s rivers flow only in winter and spring, drying up completely in summer, making it impossible for inspectors to take water samples for testing.
A lowering of the water level or an incidental drying up does not yet make a river periodic or ephemeral, because these have their own definitions, regime and ecological characteristics. However, who knows whether, as a result of progressive changes, these phenomena will not take on a regular character, and the images we know from the tropical zone, typical of Africa or Australia, will not become common here as well.
In the article, I used, among others. From the works:
 Sheldon, F., Bunn, S.E., Hughes, J.M., Arthington, A.H., Balcombe, S.R., Fellows, C.S., 2010. Ecological roles and threats to aquatic refugia in arid landscapes: dryland river waterholes. Mar. Freshw. Res. 61, 885-895.
Levick, L.R., Goodrich, D.C., Hernandez, M., Fonseca, J., Semmens, D.J., Stromberg, J.C., et al, 2008. The Ecological and Hydrological Significance of Ephemeral and Intermittent Streams in the Arid and Semi-Arid American Southwest. US Environmental Protection Agency, Office of Research and Development, Washington, DC.
 Datry, T., Arscott, D. B., & Sabater, S. (2011). Recent perspectives on temporary river ecology. Aquatic Sciences, 73(4), 453-457. https://doi.org/10.1007/s00027-011-0236-1.
 Arthington, A. H., Bernardo, J. M., & Ilhéu, M. (2014). Temporary rivers: Linking ecohydrology, ecological quality and reconciliation ecology. River Research and Applications, 30(10), 1209-1215. https://doi.org/10.1002/rra.2831.
 Datry, T., Boulton, A. J., Bonada, N., Fritz, K., Leigh, C., Sauquet, E., … Dahm, C. N., 2018. Flow intermittence and ecosystem services in rivers of the Anthropocene. Journal of Applied Ecology, 55(1), 353-364. https://doi.org/10.1111/1365-2664.12941.
 Datry, T., Bonada, N., & Boulton, A. J. (Eds.), 2017. Intermittent Rivers and ephemeral streams (pp. 1-20). Academic Press. https://doi.org/10.1016/B978-0-12-803835-2.00001-2.
 Gleick, P.H., 2003. Global freshwater resources: soft-path solutions for the 21st century. Science 302, 1524-1528.
 Changming, L., Shifeng, Z., 2002. Drying up of the Yellow River: its impacts and counter-measures. Mitig. Adapt.Strat. Globe. Chang. 7, 203-214.
 Fu, G., Chen, S., Liu, C., Shepard, D., 2004. Hydro-climatic trends of the Yellow River basin for the last 50 years. Clim. Change 65, 149-178.