River alterations, being either natural or anthropogenic, have impacted the environment and riverine communities, and nature, throughout human history. During the last two centuries, the scale of the anthropogenic impacts has expanded significantly as a result of larger water resources related projects. Numerous human activities have consequences for the environment measured along multiple scales and levels. The multiscale/-level nature of the problems related to the impact assessment discipline requires that researchers address key issues of scales and levels in their analyses. The thesis aims to present the spatio-temporal scales of the hydrological impact assessment (HIA) process in a large river basin context and analyse how the scales should be taken into account when conducting the assessment. A special focus is on the data and methodologies used within the HIA. The levels of this work are hydrology, hydrodynamics and sediment transport, forming the sub-disciplines of the HIA. The geographical focus is the Mekong River Basin in Southeast Asia where HIA is presented at different scales through seven case studies, based on the appended papers. The Mekong is facing rapid development activities and in this work their consequences on the above-mentioned levels have been analysed and discussed at different scales. Scales are particularly important when a) identifying the critical processes and areas of possible consequences, b) selecting the spatio-temporal scales of the assessment, c) identifying the data needed and available, d) selecting the methodologies and tools related to the process, and e) presenting the results of the assessment to the decision-makers and planners. The thesis concludes that, instead of down-/up-scaling, a multiscale approach often appears to be a more preferable solution. A more extensive inclusion of scale issues in the impact assessment process is believed to contribute to building a more profound connection between researchers and decisions makers.