To understand the freshwater reservoir effect, you need to understand the basic principle of radiocarbon dating.

All living organisms exchange carbon with their environment — breathing in atmospheric carbon, eating food that contains carbon.

The ratio of radioactive carbon-14 to stable carbon-12 in living tissue reflects the ratio in the atmosphere at the time of the organism’s death.

After death, the organism stops exchanging carbon with the environment, and the carbon-14 begins to decay at a known rate.

Measuring how much carbon-14 is left in a sample, relative to carbon-12, tells you how long ago the organism died.