Estuarine mapping is Dave Snowden's evolving technique for working out what can actually change in a complex system, and what cannot, so that action starts from the present reality rather than an idealised future. It maps the people, constraints, habits and resources shaping a situation against the energy and time required to shift them, helping leaders identify where intervention is genuinely possible. Use it when a change effort keeps stalling against something nobody can quite name, when people disagree about what is really fixed, or when a strategy process has produced a vision nobody disputes and a route nobody can agree on.