Hydrology of the Nahanni karst, northern Canada, and the importance of extreme summer storms

Discovered in August 1971, the Nahanni karst is the most complex high-latitude karst known. Mean annual temperature is −4.5°C and precipitation 566 mm. The most spectacular landforms occur on a structural col (the “north karst”) connecting south Nahanni and Ram Plateau. Between July 19 and 31, 1972, an extreme summer storm deposited 224 mm of rain on the area. First, Second and Third Poljes in the north karst flooded; maximum water depths were 8.5, 25 and 8 m, respectively, and Third Polje overflowed. The level of Raven Lake (0.25-0.5 km long) rose 49 m at an average 2.9 m per day. Flooding occurs through random perching of water above and below ground where conduits have been heavily alluviated. There is no highly integrated regional groundwater body; water moves along independent or poorly integrated conduits or multiple aquifers. Some depressions are inundated by groundwater entering through estavelles, others when surface and spring inputs exceed drainage. Although winter snowfall averages 213 cm, in most years spring snowmelt does not appear to cause prolonged flooding. However, it raises water levels in the aquifer leaving the area prone to flooding by frequent, intense summer rains. The magnitude and complexity of hydrologic activity in the subarctic Nahanni karst is remarkable, being comparable with that in tropical and temperature carbonate areas.