The first filling of a new dam is the real test. Everything before it is calculation. A dam is one of the few structures deliberately loaded to its full design condition on day one. Water pressure rises with depth at roughly 9.81 kilopascals per metre, so a 100 metre head presses close to a megapascal against the heel. That thrust does not act alone. Water also works upward through the foundation rock as uplift, trying to float the dam off its own base. First impoundment is therefore staged, not rushed. The reservoir is raised in increments, held, and watched: - Piezometers reading pore pressure inside the embankment and the foundation - Measuring weirs logging seepage, where clarity matters more than volume, since cloudy water means soil is moving - Inclinometers and survey targets tracking deflection and settlement - Jointmeters recording movement between concrete monoliths - Thermistors following heat of hydration deep inside mass pours An arch dam is expected to deflect downstr...