The Building Block Methodology is a holistic environmental flow assessment method developed in South Africa that decomposes the natural flow regime into ecologically meaningful building blocks: maintenance low flows (base flows sustaining habitat connectivity), drought low flows (stress periods), small intra-annual floods (freshets triggering fish spawning and channel maintenance), and large inter-annual floods (geomorphic reset events). The methodology uses a multi-disciplinary specialist workshop approach where hydrologists, geomorphologists, water quality specialists, fish biologists, invertebrate ecologists, and riparian vegetation experts collaboratively define flow requirements for each building block. Used in over 97 Intermediate and Comprehensive Ecological Reserve determinations in South Africa. Published by King et al. (2003) in River Research and Applications. The BBM directly informs the Ecological Reserve under the National Water Act and transboundary flow allocation under the IncoMaputo Agreement.