Truth be told, we don't know at all when the money ended up in the water. It could have been 1972, could be 1974.
It could, in fact, be a year without flooding. Certainly, flooding makes the most sense, but we can envision scenarios in which the money entered the water and/or ended up on Tena Bar without the need for flood waters.
We know there are spring/summer diatoms on the money, but we don't know which spring, summer those diatoms were acquired. Perhaps the only way to make an educated guess is to observe the condition of the money and extrapolate how long it would take for the money to deteriorate to the condition it was found in.