i didnt know they couldnt

The best guess I could offer here is that it in some way links to the old belief that running water is pure and washes away all magic. You may have to invest some serious research in this to find a thorough explanation. It's also possible that the origins of this belief, like many others, have been lost to time.

i never heard of them having a problem.. maybe because water meets other tributaries and makes crosses.. i dunno since allthe water we have earth is the same water at all times. then it would be said that all water is holy water.. so maybe thats why.

The holy spirit one of the members of the holy trinity, are also known as running water. And since vampires are demonic, that can not cross a line who is much stronger than them, The running water works just like a cruzifix or and other holy object. That is why they can not cross.

Water is pure.

its a myth that vampires souls get trapped in running water so they just choose not to take the risk

They can cross it, that water is a myth.

I know a few answers to this one- vampires aren't good with areas without soil. A lot of vamp legends deal with the whole, "I need to have some soil from my grave with me at all times!" thing- vamps have very powerful ties to the ground. Also, the composition of their body is different, which is why they cannot be photographed. Their particles move faster- thus they dissolve with greater ease in the water, especially water that's fairly abrasive, like running water. So there you go- ground ties, and plain fear.

Depends on the myth and its cultural origin...the world over had, and has, vampires. Running water is depicted as a symbol of purification, example, a baptism. A vampire, forsaken by God and an unholy creature, cannot transcend that which signifies the polar opposite of his ''being''. Water has also, and rightfully at that, been the primary AND primordial source of relief for many things, it is the symbol of life in many parts of the world, if not most, and even today, when you get hurt, you put water over your cut or bruise, even if you know it won't actually do much. (Running water over flowing blood only makes it worse; this belief is a fallacy as far as the modern vampire goes.) Another suggestion is that a vampire is undead, within a state of eternal stagnation, and flowing water reminds the vampire of life, time passing and ephemeral joy; a lot of it, if not most of it, is symbolism the vampire does not like because of his condemned existence. It's like the word of God. For example, vampires resent themselves and know where their existences will conclude when banished by God; one reason they didn't like garlic was because garlic spreads odors; they don't like that they smell like ''nothing''. Vampires cannot cross running water of their own will, however they can be carried across, or affect the weather to make things more favourable for their progress, as suggested by the story of Count Dracula. It really depends, but I'm pretty sure that in the gap between actual ritual and superstitious execution and modern symbolism, God came in there somewhere and revamped (lulz) the whole thing. This may very well stem from an even older belief, perhaps even a tradition, or practice, now lost.

they can.why wouldn't they be able to??

Well, I think the main reason is because they don't exist, but the whole purity is good too.But then again, if you read I Am Legend they don't seem to have a problem with it.

A long lasting homage to Plumbers Union Local 47.

Dry cleaning bills? ;-)

I work the toll booth at the Golden Gate Bridge and I've seen a few ride over in cars. I guess they can cross over in a vehicle if they have some dirt in the trunk or something like that. :p