It would have been better had I not had keyboard issues or corrected the typos.
I really should learn to proofread.
To flesh out my Elrond point a bit more, if you look at things from the perspective of the elves in general, it isn't difficult to fathom how they could be so adept at certain things and in tune with their surroundings.
I couldn't possibly explain his power to "control" the river (other than to suggest some tie in with Ulmo or whomever), but I can maybe hazard a guess as to how he knew the black riders were near to him.
He had been around for thousands of years by then. That gave him many many opportunities to hear birds chirp certain ways, see and hear the reactions of whatever being was near to him over and over again, see innumerable sunrises and perform basic things time and again.
He had to be intently in tune to his surroundings and being a part of "both worlds" simultaneously would have made him open to their presence in a land that was governed by him. He wasn't likely to be blindsided by many things, least of all ringwraiths, him being a bearer of a ring and all.
The ringwraiths could feel the presence of the living and I would venture that in Elrond's case, the reverse was true.