The basic premise underlying ecohydraulics is deceptively simple – to meld together principles of ecology and hydraulic engineering to create a new discipline. However, advancing ecohydraulics as an organized field of study is challenging because hydraulic engineers and ecologists: 1) study processes that differ substantially in spatial and/or temporal scale; 2) trace their simulation tools to different modeling traditions; 3) utilize different sets of mathematical formulations, concepts, and assumptions; and 4) address problems with vastly different patterns of complexity and uncertainty. The differences between engineering and ecology must be reconciled to formulate guiding principles unique to ecohydraulics. This is essential if ecohydraulics is to achieve the same scientific esteem as its parent disciplines. The goals of this presentation are to identify and reconcile differences between hydraulic engineering and aquatic ecology and, in the process, develop guiding principles for ecohydraulics. To achieve these goals, the presentation is separated into three parts. First, I review how the competing paradigms of determinism and empiricism structure engineering and ecology, respectively. I then derive two principles that facilitate the integration of ecology and hydraulics, the unit-scale principle and the multiple reference framework principle. Third, I provide illustrative examples of these principles using a simple hydraulic fish habitat analysis, a simple fluvial geomorphology analysis, a discrete element model of ice dams, and a detailed fish movement model. Based on these examples, I develop additional principles and conclusions to guide further advances in ecohydraulics and, perhaps, even serve as a template to aid development of other interdisciplines.