Interesting fact: Large lakes can weigh so much that the ground beneath them responds to the load over geologic time.
The answer: A lake's weight depends entirely on its volume. Freshwater weighs about 1,000 kilograms per cubic meter, so a small lake may weigh millions of tons, while one of the Great Lakes weighs quadrillions of kilograms.
Lakes are easier to weigh than storms because their water volume can be mapped from depth and surface area, but the answer still changes with rainfall, evaporation, and inflow. Knowing a lake's weight or stored water mass matters for hydrology, dam safety, water supply, drought tracking, and understanding how inland water affects landscapes and local climate.
