The key assumption is that the ratio of carbon-14 to carbon-12 in the organism reflects the atmospheric ratio.

In most terrestrial environments, this assumption holds well enough to allow accurate dating.

Atmospheric carbon mixes globally; the ratio is essentially the same across the planet in a given year.

But freshwater environments are different.

Rivers and lakes dissolve ancient carbonate minerals from the rocks they flow through and over.

These ancient carbonate minerals contain carbon, but they are so old that essentially all of their carbon-14 has decayed away — they are radiocarbon dead.

When this radiocarbon-dead carbon dissolves into water, it dilutes the carbon-14 in the water relative to what the atmosphere contains.