Year: 2008
Abstract:
Ice jam flooding in the Peace-Athabasca Delta (PAD) is an important hydrological process for the replenishment of shallow perched basins that support a highly productive northern ecosystem. The PAD is also used by nearby First Nations communities for traditional lifestyle occupations such as hunting and trapping. Previous research on laminated sediments collected from two oxbow lakes periodically connected to major Peace River distributaries has resulted in a 300-year record of flood frequency. In an effort to extend this record and broaden the understanding of the relationships among climate variability, Peace River hydrology, and delta hydroecology, a series of vibracores and gravity cores were collected at sites proximal, intermediate, and distal to the inlet of each oxbow lake (PAD 54 and PAD 15).
Remarkably consistent patterns of strongly varying flood frequency and magnitude are reconstructed for the past ~600 years using stratigraphic observations and the development of a facies model for PAD 54 and PAD 15, magnetic susceptibility measurements from two cores collected from PAD 15 (proximal and distal sites), and organic carbon and nitrogen elemental and stable isotope records from one sediment core from PAD 15 (distal site). The sediment chronologies, constrained using cesium-137 ( 137 Cs) and radiocarbon dating (14 C), suggest that Peace River flood frequency and magnitude were substantially greater during late medieval times (AD ~1418-1595) when compared to the intervals AD ~1595-1720, AD ~1720-1900, and AD ~1900-2005. This is largely indicated by relatively coarse-grained sediments in the lower portions of vibracores, and highly variable and overall higher magnetic susceptibility values and C/N ratios.
The distinct shift from high flood frequency and magnitude at the end of medieval times (AD ~1595) to extended periods of relatively low flood frequency and magnitude during AD 1595 to ~1720 is indicated by an abrupt shift from generally coarse-grained sediments (fine to coarse sand and pebbles) to generally massive and/or thick beds of fine-grained sediments (light grey clay and silt), and magnetic susceptibility values and C/N ratios with relatively low variability. During AD ~1720 to ~1900, the nature of the sediments (alternating light and dark grey clay and silt laminations) suggests oscillating energy conditions. This is consistent with magnetic susceptibility and C/N records during this time interval, which exhibit somewhat greater frequency variability than during AD ~1595 to ~1720. Sediments deposited since AD ~1900 also consist of alternating light and dark grey clay and silt laminations. However, the C/N and δ 13 Corg records clearly indicate declining values over this interval, suggesting a decline in flood frequency.
The distinct shift to reduced flood frequency and magnitude at the end of medieval times is also reflected by a substantial reduction in river floodwater influence and a substantial increase in evaporation inferred from the isotopic record (δ18 O) of two basins (PAD 5 and PAD 12) in the northern Peace sector of the PAD. The diatom records from sites in low-lying areas of the Peace (PAD 9) and Athabasca (PAD 31) sectors reflect different hydrological conditions over similar time periods. During medieval times (AD ~1418 to ~1595), when Peace River flood frequency and magnitude were relatively high, diatom assemblages in these low-lying sites indicate these basins were hydrologically closed. During the interval AD ~1595 to ~1720, the diatom assemblages indicate that these sites were generally open-drainage basins, likely because of relatively high water levels in Lake Athabasca, whose outflow also flooded into PAD 12. During AD ~1900 to 2005, the diatom records indicate that these two lakes largely returned to closed-drainage basin conditions.^ Earlier and/or more rapid snowmelt in the eastern Rocky Mountains during medieval times may have produced conditions conducive to more frequent and more severe spring ice jam events along the Peace River during AD ~1418 to ~1595. Expansion of glaciers and a late and/or protracted snowmelt under cooler conditions of the Little Ice Age (LIA) may have created conditions less conducive to ice jams along the Peace River, particularly during AD ~1595 to ~1720. Sustained snowmelt run-off throughout the summer months may have contributed to relatively high water levels of Lake Athabasca and frequent flooding of low-lying lake basins (PAD 9 and PAD 31) during this interval. Although during AD ~1900 to 2005, climatic conditions in the Columbia Icefield region appear to be returning to those similar to medieval times, Peace River flood frequency has continued to decline. This is likely due to declining alpine snowpack depths and receding alpine glaciers, which have created conditions that are not favourable for ice jam development. If these conditions persist into the future, Peace River flood frequency will likely continue to decline.
