7. RIVERS WITH HIGH SELF-PURIFICATION CAPABILITY

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.

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.