SPILLWAYS EMANATING FROM GLACIAL LAKE AGASSIZ COASTLINES

Spillways from glacial lakes develop where ice-dammed water overflows a drainage divide and erodes a channel. The study of spillways across southern Canada and the northern United States results in reconstructions of ice margins, ephemeral glacial lakes and the catastrophic floods that formed them. Radiometric dating of organic material from a spillway constrains its age. Dated wood within spillway flood deposits provide maximum ages for the flood, in which the youngest wood is the most limiting maximum age. Organic material within lake sediment taken from scour lakes at the bottom of spillways provides minimum ages for the spillway floods, in which the oldest age is the most limiting minimum age. Using this approach for the Clearwater Lower Athabasca Spillway (CLAS), commonly called the northwest outlet of glacial Lake Agassiz, the maximum age from flood deposits is at the end and not the start of the Younger Dryas Chronozone, and the oldest organic material from the base of a lake at the head of the spillway overlaps in age with the Campbell Beach of Lake Agassiz, which has been traced to the northwest outlet by numerous workers. The oldest age for the CLAS spillway corresponds in time to the middle of the Moorhead low-water Phase in the central Lake Agassiz basin. These data support the contention that some other lake supplied water to form the CLAS, such as glacial Lake Churchill, a large local lake in northern Alberta and Saskatchewan. A test of this hypothesis would be to date spillways using the maximum and minimum approach used for the CLAS on a series of spillways between the central Lake Agassiz basin and the northwest outlet region. Many spillways evolved as the Assiniboine and Winnipeg lobes receded to the northwest and north, respectively, from the Manitoba escarpment along the Manitoba and Saskatchewan border. Short-lived ephemeral lakes developed in reentrant valleys between the Riding and Duck mountains and Porcupine Hills. The lakes drained to the west over divides forming spillways that were then abandoned as lake level fell to the level of Lake Agassiz as the Lake Winnipeg lobe receded northward. The dating of spillways where the ice “unzippered” northwards across the escarpment would provide important chronology for ice margins, and for establishing maximum northerly positions for Lake Agassiz at that time.