The River Arevass is a river in Silverpine Forest, running past Pyrewood Village. On the bridge leading out of the village, the night elf priestess Tyrande Whisperwind, having traveled to Lordaeron to hunt down Illidan Stormrage, made a stand against the Scourge in order to save a retreating force of blood elves led by Prince Kael'thas. She succeeded in holding the undead back, but was swept downriver as the bridge collapsed.
Later, Malfurion and Illidan's forces both searched the banks of the river as the Scourge was tracking down the missing priestess. A dam had been built there by the Shadowglen Trolls. The twin brothers eventually managed to defeat the undead despite the enmities between night elves and naga.
River Arevass is notably absent from the Pyrewood region in World of Warcraft.
In World of Warcraft there is only one waterway in Silverpine Forest, the remains of a river that once flowed out of Lordamere Lake. All that remains of it is a dried up canyon and a few pools of water, with a bridge crossing it near the lake itself. This is possibly the Arevass, but its position would have to have been retconned to where it flows nowhere near Pyrewood Village. It may not be the same river.
The unnamed river's drying up could be due to a hypothesized fall of water around the time of the Third War that could have also eliminated Cross Island. However this hypothesis would mean that Lordamere Lake has no drainage at all, which is impossible. Instead, this river likely flows only due to spring snow-melt from mountains in northern Lordaeron and Quel'Thalas. However, other basins in the plaguelands have seemingly dried out, possibly due to the magical, all-debilitating nature of the plague.
Alternatively, the Arevass may run out of the mountains in northern Gilneas or from the hills above Shadowfang Keep, and it missing from World of Warcraft for reasons that aren't related to the lore, such as scale issues.