This paper discusses various methods for calculating the minimum instream flows for the protection of aquatic life for the purpose of illustrating its increasing complexity. These procedures are the Montana method (Tcnnant, 1976); the wetted perimeter method (Collings, 1974; Coclmaucr, 1976) and the IFIM method (Instream Flow Incremental Methodolog)r, Bovce, 1982). An analysis is then madc of sornc rcgionalisation criteria based on these methods, from which useful suggestions can be drawn for the development of a procedure to be applied to basins. A method is thus proposed for calculating the recommended flows for protecting the aquatic environment based on simple hydrological variables, such as Q7 In, which can be estimated for a basin on the basis of area A and a Base Flow Index (BFI) whirl/~xtmmarises its hydrogcological characteristics. This approach can be used when the field data necessary for more complex methods is not available. These require the defining of the envixonmental suitability of a watercourse for certain designated reference fish species, according to the flow and the morphology of the channel. The positive results of this approach were verified by Leonard (1990) and this methodology was applied to the Tiber river basin.