The characteristic of natural flow of the river is the change between fast and slow flow due to meandering of riverbed, and also that there are no obstacles built inside of the watercourse such as dams or other structures. On the other hand, the environmental flow implies a changed direction of the riverbed and built barriers within the river. In the terms of environmental flow it is important to maintain ecosystem, preserve habitats in the coastal area and flora and fauna inside of the water, but also importance is attached to the aesthetic appearance of the environment and cultural values. The aim of the study is to estimate natural and environmental flow of the river Noguera in Spain and to determine which flow is more suitable for the preservation of ecological values of the river, and also protection of flora and fauna. The data was obtained by measuring the river flow, the cross section of the riverbed and water velocity in certain area of the river Noguera. The computer program "RHYHABSIM" was used to calculate the flow value, compare the natural and environmental flow, and determine the area suitable for the life of fish of the species Luciobarbus bocage. Based on the obtained data, evaluation of river was made to find out how suitable river Noguera is as habitat for fish and quality of riparian vegetation.