What is a river?
Rivers are an element of landscape that transfer water, and in which one can notice a gradual change of physical, chemical and biological characteristics from their source towards delta.Natural river landscape is biologically very dynamic and spasiously fairly complex with a high diversity of habitats.
One can find two basic fish communities in rivers which are the community of the upper and the community of the lower stream. Besides the increase in richness and diversity of species downstream, the bilogical characterstics of the fish also change such as the life span, size of the body and sexual development, all related to the magnitude of the watercourse. Fish of the lower parts of the river live longer, have larger pshysical dimensions and sexualy mature later.
Longitudinal changes in the composition and structure of species in certain segments of the watercourse represent the base for classification, i.e. zoning the ruver according to characteristic sorts of fish. According to such zoning the watercourses from source to delta are devided into 5 basic zones: the trout zone, the grayling zone,the barbel zone, common bream zone and the Eurasian ruffe zone. The layout of the aquarium follows all the above stated characteristics of the river continuity, providing a detailed, interesting, entertaining, educational and above all an exciting insight into the magical world of fresh water.
The river well is mostly at one single point (source or well), or they are formed by conjunction of more sources or tributaries, for example Dragonja and Dobra. Strong sources in a form of ponds are characterstic for karst areas. Such efflux (flowing out) is characteristic for the rivers Una,Kupica, Slunjčica and Cetina. Mountain brooks sometimes have a degradation of altidute of ten or more meters per kilometer, such as downstream of Kamačnik, while in lowland slow rivers, such as Bosut, that degradation is only a few centometers per kilometer. Many factors, like travertine barriers in our rivers, derange that standard.
Significance of rivers and the importance of their preservation
Rivers have many useful functions. Since a river is an extremely complex system, some of the benefits cannot be seen at first sight. Natural river with all its elements is in fact the best defence against seasonal flooding, better than any regulation. Excess water which the river bed cannot store flows to backwaters, oxbow lakes, flooded forests and meadows. This eases the pressure on the mainstream and prevents dangerous accumulation of water which leads to catastrophic floods.
River in its natural state is very important for the health of forests in the wider zone of both coasts. Undisturbed groundwater ensures their smooth growth. Natural water or coastal vegetation is not just decoration and animal habitat, but has a very important role in human health. It purifies the water from organic and mineral pollution. Plants take minerals from waste matter as their food and incorporate it into their bodies. Part of such treated water then goes underground, and it is known that this water is then pumped for drinking.
Perhaps the greatest benefit from preserved rivers is invisible: when the river is in its natural state it achieves natural balance. Every shift of the balance can have unimaginable and unforeseen consequences, and we often come to this knowledge retrospectively, by analyzing possible causes which have already led to certain consequences.
Taking into account all these facts, the benefits of a river in a preserved state are obvious, whether these are direct economic benefits, expressed in HRK or EUR, or immeasurable benefits with denominator being the quality of life of present and future generations. We should not leave out the national pride, the fact that we have the richness and diversity of rivers as not many other countries.