Croatia’s terrestrial rivers
What is a river?
Rivers are landscape elements that transport water, and within them, the physical, chemical and biological characteristics change gradually from the source to the mouth. The natural river landscape is a very dynamic biological element, and a complex spatial element with high habitat diversity.
Two basic fish communities can be distinguished in rivers, those are the communities of the upper course, and of the lower course of the river. In addition to being richer in species and having a higher diversity overall, the downstream sections of river, depending on their size, are also characterised by the changing biological traits of the fish within: such as the length of the lifecycle, body size and sexual maturation. As a rule, the fish of the lower course of a river live longer, though there is always an exception to every rule: in the upper course, we can find the huchen, which achieves a mass of 60 kilograms and can live up to 100 years old.
The longitudinal changes in species composition and the structure in certain sections of the watercourse provide the basis for classification, or zoning of the river, based on the characteristic fish species found in those sections. Watercourses are divided into five basic zones, from the source to the mouth: the trout zone, grayling zone, barbel zone, bream zone and flounder zone.
Rivers most often spring at one main point (spring or a well), or are formed when multiple sources or tributaries join into one flow, such as the Dragonja or Dobra Rivers. In the karst landscape, abundant springs in the form of lakes are prominent. This is how the Una, Kupica, Slunjčica and Cetina Rivers spring. Mountain rivers often have a drop in elevation of ten or more metres per kilometre, such as the lower course of the Kamačnik River, while the slow, lowland rivers, like the Bosut River, have a drop in elevation of just a few centimetres per kilometre. Many factors, such as the travertine barriers found on many Croatian rivers, skew this average.
Significance of rivers and the importance for their conservation
Rivers have many useful functions. Since the river is an exceptionally complex system, some of those uses are not evident at first glance. A natural river, with all its elements, is in fact the best means of protection from seasonal flooding, better than any regulated flood control. High waters that the river bed cannot carry spill into backwaters, oxbows, floodplain forests and meadows. This releases the pressure on the main river course, preventing the dangerous build-up of water that can result in catastrophic flooding.
In their natural state, rivers are very important for forest health in a wide belt along its banks. Unaltered ground water levels ensure the unhindered growth of forests. Natural aquatic or riparian vegetation is not only beautiful to see and important animal habitat, but it is also very important for human health, as it cleans the water from all organic and mineral pollution. Plants uptake minerals from pollutants, building them into their tissues. Part of this purified water then penetrates into aquifers, where humans pump drinking water from.
Perhaps the greatest benefit of preserved rivers is invisible: a river in its natural state ensures the natural balance. Any shifts in this balance can have unpredictable and unknown consequences, which often become known only retroactively, by analysing the possible causes that have already led to certain consequences. Taking all these facts into consideration, there are obvious benefits to keeping rivers in their preserved condition, including direct economic benefits that can be expressed in money, to those immeasurable benefits in guaranteeing the quality of life for current and future generations. Nor should we forget that these rivers, the wealth and diversity of rivers that Croatia has, are a source of national pride that few countries can boast of having.
Standing waterbodies can be divided into puddles, ponds, fish ponds, swamps and lakes. Puddles are shallow depressions filled with water that dry out, while ponds are depressions that do not dry out, and light penetrates to the bottom, enabling the development of aquatic vegetation. Swamps are shallow standing waters characterised by a low pH value. They are habitats for many species of plants, insects, fish, amphibians, reptiles, birds and mammals. Lakes are the largest and deepest standing water bodies. Most were formed by glacial action. Slow shifting of kilometre-deep ice sheets resulted in these depressions, which filled with water after the ice retracted or melted.