The system is critical for water supply in major economic sectors in Gauteng, North West and Mpumalanga.
Water and Sanitation spokesperson Sputnik Ratau said Eskom and Sasol petro-chemical plants rely on the system.
Eskom gets water for its coal-fired power stations from the Vaal River Eastern Sub-System (VRESS) which supplies water to 11 Eskom power stations and is part of IVRS.
“Slightly moving down this week, the IVRS declined from 66.3% last week to 65.5% this week. The present levels of the dam are lower compared to the same week last year when the system was at 81.4%,” Ratau said.