Unusually amongst the vaults of Orv, the Midnight Mountains sometimes experience rainfall. Water seeps down from the Dying Sea in Sekamina and sometimes falls on the mountains as rain. The rainwater drains into a nameless lake, and then flows through the vault and into the neighbouring vault of Ilvarandin, and into the Sightless Sea. 
At some point it becomes a true underground river, and is known in Ilvarandin as the Irikusk River. It flows past the intellect devourer enclave of High Ilvarandin, and is a major route for trade between them and the outside world – in particular the black galleys of the Denizens of Leng can be seen on the river.
The river eventually reaches the sea at the Ilvarandin district of Duskport. Here, at the river’s widest part, the northern bank is lined with piers and warehouses.