In the Fellowship of the Ring, Chapter 4. A Journey in the Dark, as the Fellowship has just been defeated by the freak blizzard on the Redhorn Gate, Gandalf is having difficulty in locating the stream (the Sirannon) "that ran out from the feet of the cliffs near where the doors [of Moria] had stood".
There is no mention of an actual riverbed in the "barren country of red stones."
Putting aside temporarily Gandalf's near infallibility in most matters (and certainly those of travel), could it be possible that his being astray may have been a trick by Saruman?
If, on the other hand, Gandalf was not astray, and they indeed were traveling where the river had formerly run, is it possible that more than just the damming of the Sirannon contributed to the possible changing of the land in the intervening years? While the text does specify 'stream' and not 'river,' and therefore, presumably on a smaller scale than a river, with less erosion, etc., it still seems reasonable that some topographic feature would still be present, suggestive of the (past) existence of a stream.
When they do at last find the stream (or remnants thereof), it is a "deep and narrow channel...empty and silent..." So, in this location, at least, the stream does have evidence of a channel, although this may have been of dwarvish manufacture, rather than due to a natural effect, such as erosion.
Gimli now walked ahead by the wizard's side, so eager was he to come to Moria. Together they led the Company back towards the mountains. The only road of old to Moria from the west had lain along the course of a stream, the Sirannon, that ran out from the feet of the cliffs near where the doors had stood. But either Gandalf was astray, or else the land had changed in recent years; for he did not strike the stream where he looked to find it, only a few miles southwards from their start.
The morning was passing towards noon, and still the Company wandered and scrambled in a barren country of red stones. Nowhere could they see any gleam of water or hear any sound of it. All was bleak and dry. Their hearts sank. They saw no living thing, and not a bird was in the sky; but what the night would bring, if it caught them in that lost land, none of them cared to think.
Suddenly Gimli, who had pressed on ahead, called back to them. He was standing on a knoll and pointing to the right. Hurrying up they saw below them a deep and narrow channel. It was empty and silent, and hardly a trickle of water flowed among the brown and red-stained stones of its bed; but on the near side there was a path, much broken and decayed, that wound its way among the ruined walls and paving-stones of an ancient highroad. ‘Ah! Here it is at last! ' said Gandalf. `This is where the stream ran: Sirannon, the Gate-stream, they used to call it. But what has happened to the water, I cannot guess; it used to be swift and noisy. Come! We must hurry on. We are late.'
Regarding the possibility of Gandalf being astray, we also have this quote by Aragorn (Fellowship of the Ring, Book 2, Chapter 4 - A Journey in the Dark):
'Do not be afraid!' said Aragorn. There was a pause longer than usual, and Gandalf and Gimli were whispering together; the others were crowded behind, waiting anxiously. `Do not be afraid! I have been with him on many a journey, if never on one so dark; and there are tales of Rivendell of greater deeds of his than any that I have seen. He will not go astray-if there is any path to find. He has led us in here against our fears, but he will lead us out again, at whatever cost to himself. He is surer of finding the way home in a blind night than the cats of Queen Beruthiel.'
Although it may be said that the purpose of Aragorn's speech was to bolster the spirits of the Fellowship, Aragorn's words were seldom of frivolity. Rather than being a prosaic pep-talk after a fashion, Aragorn here is confirming the true nature of Gandalf, through eye-witness accounts, as they were.