We propose a novel methodology – heat budget decomposition – to identify and quantify different deepwater-renewal processes during a very cold spell in 2012 in a large, deep lake (Lake Geneva, 309-m depth). Combined with field observations and numerical modeling, we find that 1D convective cooling only reached 200-m depth. Instead, cooling of the lake’s deepest layers was due to lateral advection of cold water, revealing the important role of 3D processes during winter deepwater renewal.