River Salve

In roleplay, the Salve refers both to the physical river that runs north-south from Al Kharid all the way up to Paterdomus, where it goes underground, and also to the "Salve barrier" of Saradominist power that prevents Zamorakian creatures from passing in and out of Morytania.

What is the Barrier?

The Salve barrier is a powerful, invisible holy force-field that resonates out of the blessed Salve river and creates a gigantic, overbearing mystical wall. The power of the barrier has no effect to most beings, but those who are naturally or genetically predisposed to Saradomin's enemies (i.e, Zamorak) are physically incapable of passing it. Most often, this applies to vampyres and werewolves.

There was speculation in the past that guessed the barrier enveloped Morytania in a dome, a theory used to explain why winged vampyres couldn't simply fly over it. To address this concern, JaGex explained that the wall simply extends so far into the sky and out over the sea that a creature of flight would pass out (and probably drown) long before making it past the barrier.

Getting Around It

One way or another, you are going to come across a werewolf that has found a way onto the mainland. While this is typically frowned upon, there are provisions made that allow such characters to exist.

The Usual Theory

Most roleplayers approach the Salve problem on the premise that, at full strength, a creature of Zamorak cannot in any way pass through the barrier - not by force, not by persuasion, not by magic. Working with this, then, the obvious trick is to either decrease the strength of the barrier or decrease the effect of the barrier on you. Possible solutions, then, are:

Make the barrier less holy - corrupt it. For example, spill the blood of an innocent victim onto it. (Although, when you think about this one, wouldn't that make the barrier... stronger? The blood of an innocent would be considered Holy, surely.)
Make yourself more holy - corrupt yourself. For example, a werewolf that went through the agonizing process of soaking in holy water might have an easier time passing through.
With the aid of a powerful vampyre, most likely a known noble figurehead of a grand house or coven, the barrier can be disrupted for a mere minutes to allow any creature (other than another vampyre) to cross the river. This does, however, exhaust their power immensely, and it isn't something that is done often or lightly as it is reserved for dire circumstances.

Alternative Strategies

A werewolf that was born outside of the Salve barrier would not have to cross it to enter the mainland (but would have to cross it to enter Morytania). Two ways to make this possible for your character are to have him/her be from a family of werewolves that were outside of Morytania when the barrier was created or to have him/her originate from the God Wars Dungeon.
If a werewolf has not yet slain an innocent (of which would have aligned them with Zamorak), they may cross the River Salve. This is, however, quite rare, seeing as most werewolves slay their first innocents as adolescents, nor do they have much reason to cross the River Salve at that point in time.
Another strategy, devised by and almost enacted by the Varré and Klaus Nourom would be the manipulation of Portals (i.e the portal to the Blood Altar) and the Abyssal Plane, and the redirection to Runecrafting Altars.
- Questions on this should be directed at Daniel.

Side Effects

Crossing the Salve once would be hard enough on a Zamorakian creature. Knowing that, it only makes sense that repeated crossing of the Salve would cause some serious damage. Just look at the General Store Owner in Canifis who deals with rare goods from Misthalin. After that much stress, you might suffer from...

Compulsive ticks
Weakness
Paranoia
Mental instability

Dispelling the Barrier

Through the combination of several actions, the holy barrier of the River Salve can be dispelled to the point of impotency. The lore behind this is mentioned in-game through the Priest in Peril quest, and more in-depth in T.S. Church's RuneScape novels, Return to Canifis and Legacy of Blood .

Note: The barrier cannot be dispelled from the east side (Morytania) of the river, and so it must be done by a creature that is not native to Morytania.

How to Dispel the Barrier

The seven altars beneath Paterdomus in the secret temple must be desecrated before the process can begin. To do this, innocent blood must be used and spilled; such can generally only be found in young children. Once the donors/victims are gathered, their corpses must be laid upon the seven plinths with blood flowing from their bodies in some way; it is unknown whether or not the sacrifices must be killed while atop the altar, or killed beforehand and then placed there afterward.
The well is the second part of the desecration process. A mixture containing blood gathered from all seven sacrifices must be poured into the well along with the blood of a creature of Morytania (generally that of a vampyre, the dominant race of the realm). This is to be done after the altars have prior been desecrated.

How to Restore the Barrier

Contrary to the method of dispelling the power of the Salve, the act of purifying it again is more well-known, but not considered common knowledge. In the older days of Misthalin, the kings of the realm would travel to Paterdomus upon their coronation to deposit an offering to the well of the Salve; such would purify and strengthen it, but as this tradition was somewhat discontinued, it has since remained at its current power.

Pure or rune essence must be deposited into the well in order to fully restore it. Priest in Peril gives us a definite number required, which is either fifty rune essence, or twenty-five pure essence. Once the individual pieces have been deposited into the well, the barrier itself takes several minutes to rejuvenate itself to its full potential. Say, for example, that it takes ten minutes; this allows any creature of Morytania that has crossed to feel the power begining to restore and flee back to the country before it becomes impossible to do so.

Cause and Effect

Once the holy barrier has been dispelled, any creature from Morytania is free to cross. It will not, however, lose any amount of power as it does so, at least until it strays too far from Morytania and over a long period of time (the span of centuries). However, this only applies when the barrier is dispelled.

The novels explain to us that creatures of Morytania are connected to the realm itself through some form of ethereal tether. If the barrier is restored while a vampyre, for example, is on the wrong side of the River Salve, then its tether will be severed immediately and it will lose all of its supernatural power in mere minutes. As a reference, it would probably become as weak (if not weaker) than Count Draynor.

To be given a fair chance in the restoration of the barrier itself, any creature of Morytania would obviously be able to feel it's power slowly restoring over the aforementioned short time period. If they were near the river itself, it would be entirely possible to cross over before the barrier was fully restored and rejuvenated.

Common Mistakes

"My parents crossed it before I was born," is not considered an excuse for not having a good explanation as to how a character crossed the Salve barrier. If a player's character's crossing of the Salve is due to anything in that character's family's history that the player wrote when designing that character, that player is still responsible for knowing the lore and explaining the crossing.
It's generally assumed that the typical Zamorakian creature cannot simply force his way through the barrier with brute strength, regardless of if it means enduring great pain. The Salve is a godly thing, and overpowering it without any special means is literal godmodding.

Other

The Salve briefly lost its holy effectiveness at some point in the year 169, and was restored by The Adventurer and a monk named Drezel.
Following a crusade to re-purify the Salve after it was corrupted, additional defenses were added around the well on Janurary 20th, 2013. Brother Benedict and Brother Albert placed another blessing on the well to increase the existing potency. The monks also placed a top made of silver on the well to seal it off and uprooted the ground around it, planting a garlic garden.
Below is a representation of the barrier as described by Mod Osbourne in a livestream.