Physical descriptions of rivers based on the two classical variables (the water depth and the flow velocity) are required in the fields of applied hydrology and river geomorphology, as well as for designing practical tools based on biological models. Although it is necessary to take many physical and biological factors into account in order to model the aquatic communities inhabiting rivers, it is proposed to focus mainly here on this particular pair of variables. A useful modelling tool in the field of ecohydraulics consist in quantifying the hydraulic diversity in a given flow, in any area of the aquatic space, by presenting either volume or area percentages in a depth and current velocity cross grid (“hydrosignatures”). It is proposed to explain how these hydraulic signatures are calculated and presented in the form of velocity and depth grids. Some examples are then presented in which this bivariate method of quantification was applied: to calculate the preferences of fish species in terms of these two variables, to determine the hydraulic characteristics of a river on various spatial scales, and to search for reaches which are representative of a river as a whole. The HydroSignature software program developed at Cemagref, which works with Windows, is available free of charge at: < http://hydrosignature.aix.cemagref.fr/ >. It can be used to calculate hydrosignatures from data collected using many different methods listed below, as well as to carry out basic operations on these hydrosignatures, such as calculating sums and means and making comparisons. Based on our experience, end-users are usually able to quickly master the hydrosignature concept with the help of a simple coloured grid showing the representativeness of the hydraulic classes. Beyond this stage, it has emerged that this method helps ecohydraulic specialists to explain many concepts and methods more easily to their audience. Hydrosignatures can be calculated from hydraulic data of all kinds in which the depth and velocity are given along with their area of representativity. Even simple hydraulic field readings carrying no topographic information can be used to calculate hydrosignatures.