Access to accurate and trusted data for transboundary water management is important but not always sufficiently appreciated. Based on the Lake Victoria water levels decline, this technical note demonstrates that when a water crisis unfolds, tensions can arise because of absence of relevant data and uncertainty over data that are available. These may hamper the process towards agreeing on actions to be taken, thus delaying crisis resolution. This technical note demonstrates the importance of reliable data in a sensitive transboundary water situation. Countries therefore should allocate sufficient resources for adequately monitoring the state of transboundary water resources within their territories.