Why Did Isildur Cross the River?

[Rumil]

I expect Ibrin has the right of it.

Although perhaps the Gladden Fields were more marshy and thus impassable on the Western side, making it a sensible idea to go along the Eastern bank of the Anduin. As long as no interference from Dol Guldur was expected.

Of course at this stage it was still Amon Lanc, and Sauron had not yet occupied the place.

[Inziladun]

Originally Posted by Ibrîniðilpathânezel

Given the pride that led him to keep the Ring against all counsel, he was vulnerable to the Ring's desire (if such it can be called) to return to its Master. Having Isildur take the less safe route along the river would work to its advantage, since it put him more directly in harm's way -- and on the quickest path for the Ring to be regained by the enemy. That's my thought, anyway.

According to the UT chapter, the Ring was a probably a factor in causing the Orcs to attack with greater courage and ferocity than they ordinarily would have; it called to Sauron's servants.
In LOTR we see it apparently acting in answer to other wills in concert with it, while they are in its vicinity. An example might be it slipping onto Frodo's finger at the Prancing Pony, in order to reveal itself.
However, would the Ring be capable of planning Isildur's route for him, knowing some of Sauron's forces would be waiting for the Dúnedain hundreds of miles away? The Orcs that ambushed him were part of a small, forgotten detachment, whose sole purpose seems to have been to waylay any enemies they outnumbered.

Originally Posted by Rumil

Although perhaps the Gladden Fields were more marshy and thus impassable on the Western side, making it a sensible idea to go along the Eastern bank of the Anduin. As long as no interference from Dol Guldur was expected.

Possibly, but they still could have made use of Khazad-dûm or the Redhorn Gate and avoided the Gladden Fields entirely.

[Boromir88]

I wouldn't say it was Isildur's pride which led him to pick his route, and I would not overestimate the sway the Ring may have played.

"I cannot use it. I dread the pain of touching it. And I have not yet found the strength to bend it to my will. It needs one greater than I now know myself to be. My pride has fallen. It should go to the Keepers of the Three."~Unfinished Tales: Disaster of the Gladden Fields

Isildur's failure to master the Ring, appears to have caused a significant blow to his pride. To the point where he is no longer boasting about being the one who dealt the "death blow," and claiming the Ring as an heirloom, but seriously contemplating handing it over to Elrond. Whether Isildur would have followed through or not is another question, but if the Ring could pick up that Isildur was taking it to Rivendell (a dangerous place for the Ring to end up!) then it could have played a factor. Afterall, it did seem to draw the orcs attention.

[alatar]

Are we asking why Isildur, when under attack, went for a swim? Or why he was doing his Middle Earth victory lap? If it's the former, I'd think that it not only might have saved him, but aided his escape as don't the evil folk fear water, to some extent?

[Mithalwen]

Originally Posted by alatar

If it's the former, I'd think that it not only might have saved him, but aided his escape as don't the evil folk fear water, to some extent?

I don't think any race of middle earth is a fan of water as far as swimming is concerned - elf, man and hobbit all meet watery fates - the only successful swimmer I can think of is Gollum!

[Inziladun]

Originally Posted by alatar

Are we asking why Isildur, when under attack, went for a swim? Or why he was doing his Middle Earth victory lap? If it's the former, I'd think that it not only might have saved him, but aided his escape as don't the evil folk fear water, to some extent?

Basically, it appears that if Isildur had simply chosen to journey on the west side of Anduin, the swim across would not have been necessary. It looks to me as if he deliberately took the long way around.
Now, it can certainly be argued that if he hadn't gone for a swim in the Anduin, the Ring would not have been lost, and the whole of Third Age Middle-earth history would have been re-written. Fair enough.
Be that as it may, I simply don't see any likely reason why he chose the route he did.
If the Ring itself played a part, why didn't Isildur's Men, and especially his sons, question his decision to journey east of the Anduin? Going on the western side would have put them nearer the Mountains, and in a position where they could have received help from Lórien and Khazad-dûm. And as I said earlier, there was the Redhorn Gate that they could easily have used to pass the Mountains.

[The Mouth of Sauron]

Isildur would have known that Thranduil and the Elves from the Woodland Realm in Mirkwood had taken the same route on their homeward journey, east of the Anduin, not long before. It was therefore reasonable to expect that the route would be safe.

One thought though - why didn't Isildur use a palantir to scout the route ahead ?

[Mithalwen]

I don't think they could be used like that.

Palantiri were not really portable and couldn't see in the dark. Even if he had looked at the Orthanc stone it wouldn't tell him where the orcs would be days later.