Most of the soil in the Gacka delta is built of porous lime-stone, chalk and paleogene. Impermeable soil full of dolomite stone and chalk make the north-east part of the delta from Babin potok through Vrhovine and Zalužnica to Brinje. Jelar – layers form the bottom of the valley in the Gacka spring area.
The flow of the river is formed from the spring which consists of many, constant and occasional, karst springs. Downstream from the Čović bridge there are many abysses, occasional springs and estavels on the left and right banks of the river. During high waters, large springs downstream from Čović become active. Most of the terrain behind the spring toward Otočac is built of porous layers, and rains quickly disappear underground.
Relatively big and constant flow, from the springs and along the whole river (9 – 40 m3/s), optimal and equal temperature of the water
( ~ 10,5°C ± 2°C), chemistry of the water, calm and equal flow and marly bottom of the river, enabled Gacka to develop quality water flora, and a rich fauna. All this, until the sixties, put Gacka into the category of natural phenomenon of biological potential, among three of the first such phenomenon, not polluted in any way.
These kinds of fish lived in Gacka until 1937: creek trout, pike, eel, and many crayfish. According to some experts the origin of the creek trout in Gacka dates from the pre-glacial age, what is proved by its endemic origin in these waters. Pike, was according to some authors brought into Gacka in the time of the Krk princes Frankopan. In 1937., the Californian (Canadian trout) was brought to Gacka from the river Shasta in North America. The crayfish was everywhere until 1929, but from 1929 – 1931 it was completely destroyed by crayfish plague.
After World War II, someone deliberately and clumsily, put carp and tench into Gacka (Švičko lake), and the clean water was not as clean as it used to be.
Gacka received the last blow in 1964 when the hydro-electric power plant Senj was constructed and the waters of Lika and Gacka merged, what drastically changes the biological and ecological balance of the river. Several new kinds of fish enter the river, chub, crucian carp, roach and others, what also ruins the natural biological balance of the river.
In 1978., the biological characteristics of the water, flora and fauna were examined, and recommendations were given for further water exploitation of river Gacka. The next revision in 1982., unfortunately established that the fish fund decreased for two thirds, the chemistry of water was inadequate, and the entire Gacka flow was full of low quality flora what turned the water into a swamp, pollution and devastation of the Gacka river was alarming, and drastic reduction of the biological potential with the reduction of water quality reached II and III category. Since then there were no revisions, and we can’t push back thoughts that Gacka is maybe on the brink of an ecological disaster.
The Gacka river is known in the fishing world for its fast growing trout, what is a result of optimal ecological conditions. The creek trout in Gacka grows five times faster than in some other rivers of the karst area, because of the optimal temperature of water which has small annual oscillations, which is rich with oxygen, and with a slightly alkaline chemical structure. The secret of Gacka is also in other fauna which lives in it. 17 animal groups in the water are known, and these are actually organisms which live in the entire flow of the river and serve as food for the creek trout, which can thank its unusually intensive growth to that.
Sporting tourism depended on the fish and their quality, and in the years before the recent war it was closely monitored. On the basis of these observations, we noticed many negative changes after 1975. It was seen that there are more male fish and that the time of spawning moved in the later period of the year, and a certain number of female fish could not be artificially spawned. In ten years (1971 – 1982) the number of new fish from the spawn was reduced to 50%. It seems that many changes which occurred in Gacka, (connecting with Lika, the construction of HE Senj and other things) resulted with these negative consequences which require a thorough scientific study because of the importance Gacka has for trout.
Gacka is also rich with plants. The vegetation is lush because of the convenient temperature of water, good chemical characteristics, muddy bottom and the moderate flow. It makes plants grow all year round, little less during the winter than in the warm period. 25 kinds of plants are known, from moss, algae, and stalk plants.