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7. RIVERS WITH HIGH SELF-PURIFICATION CAPABILITY

(good ecological status and self-purification capability of water systems)
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  • How can a river clean itself?
  • 7. RIVERS WITH HIGH SELF-PURIFICATION CAPABILITY

The focus of this chapter is to present the physical characteristics of rivers with a high level of self-purification ability, and to explain what good and bad practices in river management are.

Background information

WHAT DOES A RIVER WITH A HIGH LEVEL OF SELF-PURIFICATION CAPACITY LOOK LIKE?

Life conditions in naturally formed river beds are more favorable for aquatic organisms than in regulated or dammed rivers. In natural watercourses, the diversity and number of habitats are far greater. There are more rapids and greater turbulence of water current, allowing better entry of oxygen, and better straining and filtering of water via sandy deposits at the bottom of the watercourse. Another characteristic of natural watercourses are riverbanks with abundant aquatic and terrestrial plants. These plants increase the ability of assimilation and protect watercourses from surface runoff of nutrients from agricultural land, and from excessive sunshine (radiation) that enhances water temperature and plant growth through primary production.

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Figure 1: Natural river bed, Danube upstream Iron gate, Serbia. (Foto J. Kostić)

artificialriverbank edietd

Figure 2: Modified river bed. Danube in Belgrade, Serbia. (Foto J. Kostić)

WATER PURIFICATION IS ENHANCED WHEN WATER IS NATURALLY RETAINED AND FILTERED

In natural landscapes, woodlands, wetlands, and grasslands absorb rainwater and act as sponges that slow down the movement of water through landscapes to the streams and rivers, therefore mitigating flood peaks. Much of that rainwater never reaches the streams since it is incorporated into the physical bodies of plants or evaporates. Water that reaches the streams is clean and filtered through natural systems. In contrast, in urban areas, water usually quickly drains through city rainwater sewers into the channeled rivers or streams, taking away pollution from streets that end up in the river water.

In nature, streams form dams from natural woody debris and sediments, like stones and gravel, when trees near the stream die and fall into the water. The water is forced to slow down, creating bends and making the stream more favorable for periphyton organisms to grow and give them more time to use the nutrients in the water. Furthermore, such dams provide more possibilities for good aeration of stream water. They also provide habitats for benthos and fish (hiding places against predators).

Biological processes are one of the most critical processes in the self-purification of rivers, and these processes take time. Therefore, the longer water takes to make its way across the landscape, the greater the chance that biological processes will clean it.

WHAT DECREASES THE SELF-PURIFICATION CAPACITIES OF RIVERS?

With interventions in river basins, such as draining lands for agriculture (hydro-meliorations), spreading of settlements on floodplains and wetlands (urbanization), straightening of watercourses and destroying the vegetation cover of river banks (regulation), we increase the flow rate and, hence, reduce the self-purification abilities of surface waters. Also, dam construction is a major problem, since it slows down the river, leading to the accumulation of waste and loss of aquatic life. At the same time, we are increasing the ecological sensitivity of rivers with the increase of drinking water consumption and burdening natural waters with wastewaters.

The effects of self-purification depend upon several factors, such as the flow velocity of a river, which depends upon the gradient, the shape of a riverbed (width and depth), the discharge, the size of the particles that the river is carrying, the temperature and depth of water, and the strength and length of solar radiation. All these parameters are changing seasonally and from the spring of a river to its mouth. Consequently, the size and the species composition of the communities that influence the self-purification abilities of a watercourse depends upon them.

For example, the rate of biological activity and hence the rate of oxygen demand increases with increase in temperature. Also, the content of dissolved oxygen in the water, which is essential for maintaining aquatic life and aerobic conditions, is influenced by temperature. The solubility of oxygen decreases with the increase in temperature. Thus, at higher temperatures, the rate of biological activity is higher, while dissolved oxygen concentrations are lower, causing rapid depletion of dissolved oxygen and subsequent anaerobic conditions.

References
  • Vahtar M., Zdešar M., Kompare B., Urbanc-Berčič O., 2005: Kako se reka očisti? – Priročnik za učitelje 3. Book collection Vodni detektiv. ICRO Domžale, Domžale, Slovenia.
  • https://kwalliance.org/what-we-do/clean-water-policy/protection/wetland-protection-and-mitigation/

Lessons in this chapter

Lesson 7.1: HOW CAN THE SELF-PURIFICATION ABILITY BE INCREASED?

To be able to sustain the natural level of self-purification ability of a river system, the riverbeds should have their natural morphology with wide riparian zones with vegetation and gravel islands, etc. Natural morphology and natural river banks enhance water retention, which increases the self-purification capacity of rivers.

TOPICS

  • 1. RIVER AND RIVER BASIN
    • Lesson 1/1
    • Lesson 1/2
  • 2. THE SHAPE OF A RIVER
    • Lesson 2/1
    • Lesson 2/2
  • 3. LIFE IN A RIVER
    • Lesson 3/1
    • Lesson 3/2
    • Lesson 3/3
  • 4. POLLUTION IN RIVERS
    • Lesson 4/1
    • Lesson 4/2
  • 5. WATER QUALITY IN RIVERS
    • Lesson 5/1
  • 6. SELF-PURIFICATION PROCESSES IN RIVERS
    • LESSON 6/1
  • 7. RIVERS WITH HIGH SELF-PURIFICATION CAPABILITY
    • Lesson 7/1

Donors

 This project is partly financed by the European Union and City of Vienna

Partners

Associated partners

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info@water-detective.org

General contact:
info@water-detective.org

Lead partner and initiator:
ICRO – Institute for Integral Development and Environment, Sl

SI contact: marta.vahtar@icro.si

Partners:

Institute of Biology Bucharest,
Romanian Academy, Bucharest , RO
RO contact: biodiversitateibb@gmail.com

Institute for Multidisciplinary Research, Belgrade, RS
SR contact: lenhardt@ibiss.bg.ac.rs

Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, Sofia, BG
BU contact: luchezarpehlivanov@gmail.com

Links:

www.eledan.eu
www.water-detective.net
www.facebook.com/water.detectives

Last change: 29.04.2019

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