Nahanni flash floods
Prairie Creek, and the adjacent creek valleys running into the South Nahanni River have been subject to severe flooding episodes in the recent past, not just due to high levels of rainfall, but linked to ground instability and earthquakes as well.Parks Canada recognizes the risk of flash flooding of tributaries of the South Nahanni River, as well as on the South Nahanni River itself in its visitor information. The Agency identifies Dry Canyon Creek, Lafferty Creek and Prairie Creek specifically as being at risk of flash flooding. They warn visitors to exercise extreme caution hiking in these areas, and not to camp in areas of flash flood potential. They even note that one visitor died in a flash flood episode while hiking along Dry Canyon Creek in 1995.
Parks Canada notes that Clearwater Creek, which runs parallel to Prairie Creek, to the northwest, is an area of particularly high flood potential. Both Parks Canada and Nahanni River Adventures (one of the licensed outfitters on the river), have documented an extreme natural event on Clearwater Creek which, if replicated on Prairie Creek, could have devastating consequences to the mine site and surrounding ecosystems. During the winter of 1996-97, a massive landslide, possibly caused by earthquakes that occurred in the same period, occurred at the confluence of Clearwater and Cathedral Creeks, approximately 17 km upstream from the South Nahanni River. The landslide blocked both creeks, and large lakes formed behind this natural dam. In July 1997 part of the dam collapsed, and the lake that had formed on Clearwater Creek immediately dropped by over 15 metres. A massive volume of water surged downstream into the South Nahanni River, causing the South Nahanni to rise by as much as four metres overnight. While no-one was injured in this event, one group of canoeists on the South Nahanni was forced to abandon their island campsite before it disappeared in the middle of the night, and take to their boats to ride out the flood. Part of the dam, along with the new lake, remains on Clearwater Creek, but its integrity is uncertain.
This dramatic natural flash flood event illustrates the link between earthquake activity, rock slides and floods - resulting in an extreme event of remarkable power. Events of this magnitude cannot be prevented, regardless of the mitigation measures applied, and could result in devastating implications for developments in their path. A similar event on Prairie Creek could severely damage or destroy the minesite and heavily impact on the downstream ecosystem should chemicals, fuel oil and tailings be flushed downstream.
