On February 8, 2013 material from the southeast wall of the Berkeley Pit collapsed into the Pit water in what is known as a rotational slump or slough. Such sloughs are relatively common in open pit mines. For example, a similar slough occurred at the Berkeley Pit in 1998.
The recent slough was about 550 feet wide and caused an estimated 820,000 tons of material to collapse into the Pit. Montana Bureau of Mines & Geology (MBMG) monitoring showed that the water level in the Pit lake rose about 0.6 feet as a result of the slough. For comparison, over the past several years the water in the Berkeley Pit has risen about 0.65 feet per month.
Current projections still estimate that water levels at one of the surrounding monitoring compliance points for the Berkeley Pit system will reach the Critical Level (5,410 feet) around 2023.
Pumping and treating of Berkeley Pit water will be required when water levels at any of these compliance points reach the Critical Water Level. Currently, the highest water level is in the Pilot Butte shaft to the north of the Pit. As of June 2013, the Berkeley Pit water level was 5,310.89 feet above sea level, and the water level in the Pilot Butte shaft was 5,335.72 feet above sea level, or about 75 feet below the critical level.