What power is Elrond referring to?

[narfforc]

Davem raises a good point here, maybe The Ringwraiths sensed the presence of Ulmo`s power in the river. Although Tolkien doesn`t say much about The Valar in LotR, what we know of them from The Silmarillion tells us that Ulmo took special interest in Middle Earth. Tolkien says that the servents of the Enemy feared water and there was a special emnity between Melkor and Ulmo.

[Gurthang]

As far as the water goes, it did have a power; although not necessarily an 'elvish' power.

From Ainulindalë:

…And they observed the winds and the air, and the matters of which Arda was made, of iron and stone and silver and gold and many substances: but of all these water they most greatly praised. And it is said by the Eldar that in water there lives yet the echo of the Music of the Ainur [emphasis mine] more than in any substance else that is in this Earth…