Vampires are bound by certain fundamental truths, and one of them is that they can't cross running water. Why not? What's so special about a babbling brook that you can't find in, say, a puddle?
In folklore, running water is a potent bane against all sorts of things. It stops witches, ghosts and devils dead in their tracks. It also has curative powers, and can heal someone stricken by elf-shot, but the wary spit on the earth before crossing bridges at night, to avoid the attention of evil things. In Scotland, crossing the right kind of river with silver coin created a cure for children struck by witches' curses. Stones taken from running brooks and rubbed against affected limbs cure aches and pains. If your cow gives bad milk, have it drink from a south-running stream; that will make its milk sweet again. Gold and silver water - that is, water taken from a running brook and bottled with gold and silver coins - is a cure for a wide variety of ailments.
So running water isn't just a block. It has magical powers beyond vampires, when used correctly. The stones used to cure aches have to be gathered in a certain way, at a certain time. The gold and silver coins and the water they are sprinkled or bottled with have to be treated just so. Holy water has similar powers, which is why in the past people would, say, keep the water left in the baptismal font for later use as a cure-all, and also why the church would put the font under lock and key - to prevent people using the water for magic.
Part of this can be traced back to legends surrounding fords, ferrymen, and river crossings. When there is no public infrastructure to speak of, and the only way across a river is by a ford, river crossings take on huge importance. Many famous battles end up being fought next to fords for precisely this reason; if the only way to your objective is across a river, then you want to capture that ford. For much the same reason, you often find inns and coach houses near fords and bridges. Also, murderers, like infamous Illinois innkeeper Isaiah Luna Potts who, in folklore, slaughtered many a traveler at Pott's Inn.
For that matter there are plenty of ghostly legends about ferry crossings. Charon, Wade, Vati, Anti - there's no shortage of psychopomp ferrymen willing to carry heroes, the dead, even Gods to their final reward. Water drawn from a ford 'where the dead and living cross' was thought to have miraculous curative powers, and could break any infernal curse.
All of which goes to show that, when Van Helsing says Dracula has difficulty crossing rivers, it's not because Dracula has a water allergy. It's because rivers are extremely important and powerful in their own right, and no thing of evil can easily get past one. Moreover a cunning man can use the river's power to his own advantage, accessing its magic to cure and protect.
In Night's Black Agents terminology, the running water bit works best with Satanic or Supernatural vampires. It's more difficult to make work with Mutant and Alien types, if the assumption is that the undead are a recent phenomenon. Even if they've been here since ancient times, it's risky making running water a bane. It looks a little too much like the Wicked Witch of the West, or the Triffids.
There are ways of getting round the problem. Seawater, for example, is known to have a bad effect on plants, so you could argue that a vampire type that derives from some form of plant life or fungus might hesitate if seawater became an issue. However it's going to get silly if an agent threatens a vampire with a bucket of water. Even a hose looks daft, as the Triffids discovered back in the 1960s. As the folklore shows, this trope works best if the water itself holds power, not if the vampire is weak against water attacks.
Say running water marks the boundary between the living and the dead, and so is constantly infested with ghosts, which is why vampires don't like going near water. In gaming terms, it costs them Aberrance to go anywhere near running water and fend off the ghosts, so the weaker ones don't do it at all, and the stronger ones only try with reluctance. Again, that's a supernatural trope, not a biological or quasi-scientific one - the tropes that best suit Mutant or Alien vampires.
So when using running water as a block, bear this in mind: the running water probably gets its power from the same magic - or whatever it may be - that works on Vampires. Say your version is the Telluric kind found in the Dracula Dossier, or the Ancient Stones in the main book. Telluric Vampires get that way thanks to bacteria which fed on telluric energies and were expelled from the earth, probably by volcanic activity. The Ancient Stones are alien variants who spread the contagion by completely replacing a human's blood with alien matter.
In both cases the Vampires are created by some kind of microscopic or microbial contamination. Water can be a transmitting vector, carrying the essence of the vampire, presumably because there's some kind of contaminating source. We already know that volcanoes have a significant effect on river flow, and it's not impossible that a bit of alien stone found its way into a river. Perhaps after a battle some conquering tribe flung the alien stone, which they perceived to be a foreign God, into the river to drown it. Or it was hidden there to protect it.
Either way, the mixture of water plus contaminating element could really mess with Vampires. The Telluric bacteria might have an adverse effect on a Vampire's metabolism when combined with rushing water. Or it might confuse the Vampire's senses and reduce its ability to make Aberrance spends. The microbes and bits of stone from an Alien Stone vampire might have a similar effect on a Perfectus Petri, or alternatively it might be such a draw to the vampire that it has to study the phenomenon, even at the expense of self preservation - meaning it stands there till dawn, and then explodes.
Equally, drinking from that stream or mixing it with other materials - gold and silver, say - will have some effect on living creatures. Exactly what that effect may be is up to the Director.
Or it may be an indication of trouble to come.
Say most rivers don't have this effect on Alien Stone vampires, which is likely if the river draws its power from a chunk of Alien Stone. Well, if this one does, then it follows that there must be some Alien Stone around here somewhere. Where, exactly, and what effect has it had on the surrounding wildlife?
Alternatively, with Telluric vampires, if rivers suddenly have this block effect on vampires, and if this is related to recent volcanic eruptions that spewed bacteria into the atmosphere, then what else has happened? Does this mean a new vampire threat waxes in some forgotten, blighted land? If the river effect fades, presumably because the bacterial matter has been washed into the sea, then does this mean the vampires are on the run - or does it just mean that running water no longer blocks them as once it did?
That's it for this week. Enjoy!