Ksi Sii Aks

The Ksi Sii Aks (formerly Tseax River ) is a tributary of the Nass River in northwestern British Columbia, Canada. It is most notable as the namesake of Tseax Cone, a volcano within its basin that was responsible for an eruption in the 17th century that killed 2,000 Nisgaʼa people. Prior to the eruption, the Nisgaʼa name for this river was Ksi Gimwitsʼax. Buried by the eruption, it eventually resurfaced. The Nisgaʼa recognized it as the same stream but renamed it Ksi Sii Aks: sii aks means "new body of water".

As prescribed by terms of the Nisgaʼa Treaty, the Tseax River was officially renamed the Ksi Sii Aks on 11 May 2000.

Ksi Sii Aks flows about 45 km north to the Nass River. From its source near Sand Lake the river flows through Sand Lake, Gainor Lake, and Lava Lake. It collects many tributary streams, the largest of which are Poupard Creek, May Creek, Alder Creek, Crater Creek, Auk Creek, and Gitzyon Creek.