Was Gandalf astray, or had the land changed, on the road to Moria?

Question

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.

Answer 1

Gandalf had not passed that way before

The only account of Gandalf’s first journey through Moria seems to be when he went in search of Thrain. As far as I could find, the consensus seems to be that he passed from the East gate through to the West Gate (possibly because had he been through the West Gate, he’d have known the riddle). If we can assume that to be true, Gandalf would have seen the Sirannon but not have followed the path from the stream to the Redhorn.

'Good, Gimli!' said Gandalf. 'You encourage me. We will seek the hidden doors together. And we will come through. In the ruins of the Dwarves, a dwarf's head will be less easy to bewilder than Elves or Men or Hobbits. Yet it will not be the first time that I have been to Moria. I sought there long for Thráin son of Thrór after he was lost. I passed through, and I came out again alive!'
^{Fellowship of the Ring Book II Chapter 4: "A Journey in the Dark"}

"Then what was the use of bringing us to this accursed spot? [...] How could that be if you did not know how to enter?" Gandalf says "[...] I did not enter this way. I came from the East."
_{ibid. (Thanks to @NicolaTalbot for finding the quote!)}

In the above Gandalf says they will “seek the hidden doors together”. This seems to suggest he was never truly aware of the exact location, but having left through the West Gate during his first passage, he knew that the Sirannon passed out the West Gate and continued West. Again, Gandalf would've had no need to search for the Redhorn after passing through the west gate, as it would not have led him to Thrain.

Answer 2

Gandalf was astray.

This might perhaps have been because the land had changed somewhat, though I see no evidence to that effect. There is certainly no obvious reason to suspect Saruman's influence.

You've already quoted the key point:

Hurrying up they saw below them a deep and narrow channel. [...] 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.

They were looking for what I would call a river with an ancient path next to it; they found a river-bed with an ancient path next to it. That makes it clear that they had found the landmark they were looking for, so the fact that it was a few miles south of where they expected to find it means they were looking in the wrong place to start with.

... of course, in a huge barren country like that described, it would hardly be difficult to be a few miles off your intended path, particularly since as far as we know it had been the best part of two hundred years since Gandalf last passed that way. Edlothiad has already pointed out that Gandalf was known to be somewhat forgetful, and I think such a minor disorientation after such a long time can easily be understood and forgiven.

One point that confused me at first, and may have confused you: in my part of the world, at least, the word "stream" is casually used nowadays to mean a very small river such as a brook or creek, but the formal meaning is a flowing body of water of any size. It appears that only a particularly large stream, one big enough or nearly big enough to navigate, was traditionally referred to as a river. The Gate-stream "used to be big and noisy" so it was clearly large enough to be what I would call a river, but perhaps not large enough in the traditional meaning of the word with which Tolkien would have been most familiar.

It should also be noted that they were looking for a river, not a river-bed, and expected to be able to see and/or hear it at some distance. They might have found it more easily, or at least been less surprised and disheartened by the amount of time it took them to do so, had they already known the river had dried up. This may mean that they originally thought themselves to be further off their intended path than they actually were.