Almost all of us have thrown coins into a holy spring, whenever we visit a temple. While all of us have done this at some point or the other, few of us know the actual reason behind this...

The general reasoning given for the act; throwing coins into a River is that it brings Good Luck. However, scientifically speaking, in earlier times when civilization was booming in India their main need resource was water and it was mainly used for agriculture, cooking, drinking, etc. so they have to stay near water places like lakes, rivers etc, so their way of using copper was for attracting dirt to purify water and also used in trading purposes so most of the currency used was made of copper unlike the stainless steel coins as of today. 

As said Copper is a vital metal very useful to the human body and used to attracting dirt as it is mentioned in Ayurveda that it purifies water. Water was Stored in Copper pots which Kills Contaminating Diarrhoeagenic Bacteria and, thus in temples also priests use copper vessels to give thirtha/holy water.

Throwing coins in the river was one way our fore-fathers ensured we intake sufficient copper as part of the water as rivers were the only source of drinking water.

So if coins remain in rivers for a longer time then it is beneficial for those who drink it. Making it as a custom ensured that all of us follow the practice but presently it is considered as luck.