How did Gandalf know the Nazgûl crossed the river in secret?

Question

Our heroes in Middle-earth knew the Nine Riders were coming even though the riders were keeping their movements secret.

”I have an urgent errand,” he said. “My news is evil.” Then he looked about him, as if the hedges might have ears. “Nazgûl,” he whispered. “The Nine are abroad again. They have crossed the River secretly and are moving westward. They have taken the guise of riders in black.” ~ Fellowship of the Ring

How did they know?

I prefer answers from canon sources, but will consider answers from the movies.

Answer 1

Saruman told Radagast.

’ ”I have an urgent errand,” he said. “My news is evil.” Then he looked about him, as if the hedges might have ears. “Nazgûl,” he whispered. “The Nine are abroad again. They have crossed the River secretly and are moving westward. They have taken the guise of riders in black.”

‘I knew then what I had dreaded without knowing it

’ “The Enemy must have some great need or purpose,” said Radagast; “but what it is that makes him look to these distant and desolate parts, I cannot guess.”

’ “What do you mean?” said I.

’ ”I have been told that wherever they go the Riders ask for news of a land called Shire.”

' “The Shire,” I said; but my heart sank. For even the Wise might fear to withstand the Nine, when they are gathered together under their fell chieftain. A great king and sorcerer he was of old, and now he wields a deadly fear. “Who told you, and who sent you?” I asked.

‘“Saruman the White,” answered Radagast. “And he told me to say that if you feel the need, he will help; but you must seek his aid at once, or it will be too late.”

(LotR, Council of Elrond)

How Saruman knew we are not told, to my knowledge, but he had the palantir and was in league with Sauron. In fact, the Ringwraiths were ordered to stop by Isenguard and meet with Saruman, though this was after Saruman sent that message.

Assuming he did not know through these channels, he may have heard rumors of a great terror and connected the dots.

The Lord of Morgul therefore led his companions over Anduin, unclad and unmounted, and invisible to eyes, and yet a terror to all living things that they passed near. It was, maybe, on the first day of July that they went forth. They passed slowly and in stealth, through Anórien, and over the Entwade, and so into the Wold, and rumour of darkness and a dread of men knew not what went before them. They reached the west-shores of Anduin a little north of Sarn Gebir, as they had trysted; and there received horses and raiment that were secretly ferried over the River. This was (it is thought) about the seventeenth of July. Then they passed northward seeking for the Shire, the land of the Halflings.

About the twenty-second of July they met their companions, the Nazgûl of Dol Guldur, in the Field of Celebrant. There they learned that Gollum had eluded both the Orcs that recaptured him, and the Elves that pursued them, and had vanished. They were told also by Khamûl that no dwelling of Halflings could be discovered in the Vales of Anduin, and that the villages of the Stoors by the Gladden had long been deserted. But the Lord of Morgul, seeing no better counsel, determined still to seek northward, hoping maybe to come upon Gollum as well as to discover the Shire. That this would prove to be not far from the hated land of Lórien seemed to him not unlikely, if it was not indeed within the fences of Galadriel. But the power of the White Ring he would not defy, nor enter yet into Lórien. Passing therefore between Lórien and the Mountains the Nine rode ever on into the North; and terror went before them and lingered behind them; but they did not find what they sought nor learn any news that availed them.

(Unfinished Tales, Hunt for the Ring)

In the Lord of the Ring's: A Reader's Companion, there's additionally this note they quote Tolkien having made, addressing some chronology problems - essentially, Radagast's knowledge of the Ringwraith's having crossed the river is "too quick" in the narrative. This suggests an earlier crossing, before the attack on Osgiliath:

The Nazgûl are ordered to steal over Anduin one by one and make enquiries. This is ordered soon after S[auron] learns that Gollum (who disappeared into the D[ead] Marshes) has been captured and is with Thranduil, and that Gandalf has visited that realm. Say sometime early in April. [...] At first the Nazgûl investigate Anduin's Vale ... but can find no trace of Ring or 'Baggins' ... some begin to investigate Rohan.... Sauron is already in communication with Saruman [illegible] Palantir; but has not yet mastered him. Yet he reads enough of his mind to suspect (a) that he covets the Ring for himself and (b) that he knows something about it. The Nazgûl are ordered to visit Saruman. Saruman is v[ery] frightened at S[auron]'s suspicion of himself and his knowledge of the Ring. Though he dislikes Gandalf intensely and is v[ery] jealous of him, he believes G[andalf] knows something vital about the Ring because the Nazgûl [?ask] of the Shire which has always been a great concern of Gandalf, and because his agents have discovered that it is extraord[i-narily] closely guarded; also that Gandalf is now actually there since 12 April. He [Saruman] therefore thinks of getting his [Gandalf's] help... . The Nazgûl, then, came to Isengard towards early? June. Saruman was helped at this point [by] g[ood] fortune. Radagast becomes aware that Nazgûl are abroad in Anduin Vale spreading panic and searching for 'Shire'. He becomes v[ery] alarmed and can think of nothing but to go and consult Saruman head of order of Wizards. He does so ... not long after visit of the Nazgûl to Isengard. Saruman knows that Radagast is a kinsman of Gandalf's and wholly trusted by him: he uses him as messenger, and sends him off to the Shire. Radagast leaves Isengard about June 15th. ...

This suggests to us, either way, that both Radagast (from accounts of the terrors being spread) and Saruman (through communication with Sauron) knew of the Ringwraiths coming West.

Answer 2

What do we know from LotR that might bear on the question of the source of the information?

Thanks to @Harry Johnson in the comments, it seems likely that Saruman discovered that the Riders were abroad. As Gandalf narrates later

Radagast:

"I have been told that wherever they go the Riders ask for news of a land called Shire."

Gandalf:

' "The Shire," I said; but my heart sank. For even the Wise might fear to withstand the Nine, when they are gathered together under their fell chieftain. A great king and sorcerer he was of old, and now he wields a deadly fear. "Who told you, and who sent you? " I asked.

Radagast:

' "Saruman the White," answered Radagast. "And he told me to say that if you feel the need, he will help; but you must seek his aid at once, or it will be too late."

This seems clear, but is still unsatisfactory, because it leaves the puzzle of how Saruman had Radagast to use as a messenger, since everything we know points to Radagast as being pretty uninvolved and certainly not living at Isengard. I do not find it credible that Saruman knew how to find Radagast and sent for him -- an orc, maybe? -- to use as a messenger. If he had a messenger to send for Radagast, he has a messenger to send to Gandalf -- and time was of the essence.

The best explanation for that is that Radagast found some trace of the Riders that worried him enough to consult Saurman who either had discovered it independently or, alerted by Radagast, discovered the true nature of the Riders and, with Radagast on hand, sent him to warn Gandalf.

(Note that he rode a horse, not some %$& rabbits!

I came upon a traveller sitting on a bank beside the road with his grazing horse beside him. It was Radagast the Brown

end of digression.)

Could Radagast have discovered the Riders? Gandalf says:

Radagast is, of course, a worthy Wizard, a master of shapes and changes of hue; and he has much lore of herbs and beasts, and birds are especially his friends.

Especially given how often in Tolkien's works people (good or bad) use birds for spies, Radagast may well have been keeping an eye on things using birds. When they spotted the Black Riders, even if he didn't know what they were other than that they were evil, his natural impulse would be to go to Saruman for advice.

How could Saruman have discovered them? Perhaps using the Crebain he seemed to control.

'Regiments of black crows are flying over all the land between the Mountains and the Greyflood,' he said, 'and they have passed over Hollin. They are not natives here; they are crebain out of Fangorn and Dunland. I do not know what they are about: possibly there is some trouble away south from which they are fleeing; but I think they are spying out the land.'

Regardless, there's no question Saruman sent Radagast on his errand to find Gandalf, but that doesn't mean that he's the original source of the information.

Whoever first discovered that the Riders were abroad, now Saruman knew. And Saruman suspected Gandalf knew something about the Ring and suspected a connection with the Shire:

That is in truth why I brought you here. For I have many eyes in my service, and I believe that you know where this precious thing now lies. Is it not so? Or why do the Nine ask for the Shire, and what is your business there?"

So why did he warn Gandalf? Saruman was not yet a total fool, and realized that keeping Sauron from recovering the Ring was even more essential that acquiring it himself. So: Warn Gandalf, save the Ring from Sauron, and maybe pick up vital clues as to what Gandalf knew.

Then: Grab the Ring; Defeat Rohan; Defeat Gondor, Suborn or defeat Gandalf; Defeat Sauron. All just implementation details.

And as far as the answer to the specific question asked: Gandalf knew because Radagast told him. Radagast learned critical information from Saruman, but may have found out part of the issue on his own.