Application of Floodplain Stratigraphy to Determine the Recurrence of Ice-Jam Flooding Along the Lower Peace and Athabasca Rivers, Alberta

Abstract

Multi-proxy reconstructions of ice-jam flooding have increased significantly in the last several decades, providing important estimates of changes in ice-jam activity over time. Here, the floodplain sedimentary archive is investigated as a stratigraphic proxy indicator of ice-jam flooding along the lower Peace and Athabasca Rivers, Alberta. This relatively new method involves stratigraphic logging and interpretation of floodplain sediments resulting from successive ice jam floods and radiocarbon dating of inter-flood organic material for chronology (Livingston et al., 2009). At six sites the method is used to extend the record of ice-jam flooding by up to 1500 years, providing the longest temporal framework for assessing the recurrence of ice-jam flooding in the Peace-Athabasca Delta (PAD) to date. Chronostratigraphic logs indicate that the average long-term recurrence interval of ice-jam flooding varies between sites, from approximately once in 13 years to once in 23 years (4–8% probability of bankfull exceedence in any given year). This is near the range interpreted from oxbow lake sediment in the PAD spanning the last 180-300 years (5–14%), but much lower than the probability of recurrence of ice-jam floods (AD 1826–1995) inferred from historic archives (16%). Although spatio-temporal incongruencies amongst proxy records make it difficult to resolve differences among flood recurrence estimates for the PAD, they collectively indicate a range of 4–16%. The temporal resolution of ice jam-flood chronologies derived from floodplain sediment depends largely on the availability, reliability and sampling frequency of radiocarbon-dated organic material and its calibration to calendar years. Changes in the frequency of ice jams at a particular site can be determined when organic material suitable for radiocarbon dating is present at multiple levels in the floodplain sedimentary archive. Alternative absolute dating methods should be used to constrain ice-jam flood events in the last 350 years or when organic material suitable for radiocarbon dating is limited. Even with limited chronometric control the average long-term flood recurrence can be determined, providing a first approximation of ice-jam flood activity at a particular site. Thus, in the absence of other proxy archives, and given the simplicity of the method, we suggest that floodplain stratigraphy provides an invaluable archive of ice-jam flooding along northern rivers.