Silvering of the Mirrored River

In the culture of the Drocian Empire demons and devils play a large role as historically they have wreaked havoc on farmers and the common folk across the countryside when they get blown out of the infernal frontier. When the empire expanded its borders to meet the banks of the mirrored river they hired a great mage to try and use the river as an anchor point to cast a protection spell to keep the empire safe.
The issue arose when the arcanist, Halorn Farnslan, pointed out that using a moving object wouldn’t work as the spell would just get washed away. What he proposed instead was that a large sum of silver coins be dumped into the headwaters of the river every year. This would serve two purposes. First that could be the anchor point for a spell, allowing them to cast a protection spell in the region. This spell was trying but alone not strong enough for guaranteed protection. The second aspect of Halorns plan was that the silver over time would wash down stream and demonic creatures have a strong aversion to silver, thus making the river itself a deterrent.
To collect the needed silver every year most towns will keep the equivalent of a tip jar out to collect silver coins to be used in the river. Come the middle of winter the empire sends a small caravan full of silver coins to the river mouth, where they get placed upon the ice so it falls in the spring.
Many people in the region will wear some sort of silver jewelry as well, especially those in the shining citadel. This serves as a small nod towards their non- demonic or devilish nature, a final safety measure of assurance if you will. Some trust in the spell and silver plan of Halorn but others think that it was all just a ruse to have the empire dump a large amount of silver somewhere for him to collect and use later.