According to biologist and river guide Michael P. Ghiglieri, many drowning deaths by boaters in the Grand Canyon have been caused or exacerbated by rapid hypothermia and hypothermic shock caused by entering the cold water. He further described that during the record post-dam high-flow season of 1983 (mentioned above), there was only one boating fatality in the canyon, providing a strong challenge to views that the dam, by reducing and mediating river flows, increases the safety of canyon river users. The river water temperature in 1983 was significantly higher than normal, due to a large portion of the water having come from overflows of warmer surface water over the spillways of Glen Canyon Dam, rather than the colder lower levels which feed the penstocks.