The removal of animal and plant organisms downstream by rivers. Biostream plays a significant role in the development of river flora and fauna and is measured in thousands of tonnes per year.

Quantifying bio-stocks is essential for predicting the biological productivity of a river.

When forests in the water protection zone are cleared, the flow regime changes. The spring flood becomes higher and shorter, and then the river becomes much shallower, which particularly affects summer and autumn spawning fish and the wintering conditions of fish in the river. 

On the one hand, by damming rivers and creating reservoirs, man violates the reproductive conditions of migratory fish, blocks their access to breeding grounds and often destroys the spawning grounds, as they are located in the backwater zone of the reservoir and are often silted up. 

On the other hand, the resulting reservoirs often provide more favourable living conditions for limnophilic fish than in the former canal, and the number of these fish stocks increases. 

At the same time, the conditions for rheophilic fish often deteriorate, forcing them to move to tributaries or above the dam. 

Finally, the creation of reservoirs in rivers drastically changes the flow regime - and thus the breeding conditions for semi-anadromous fish. The area of spawning grounds is reduced and the duration of flooding changes, which naturally affects the reproduction of these fish. The total volume of runoff also changes, so that the water level in enclosed seas and lakes (Caspian Sea, Aral Sea, etc.) may decrease, while the salinity in the estuaries of seas and estuaries into which regulated rivers flow increases. A significant proportion of the nutrients accumulate in the reservoirs and are no longer transported by the rivers to the sea, so that the food base for fish in these downstream areas is greatly reduced.