Reconstruction of multi‐century flood histories from oxbow lake sediments, Peace‐Athabasca Delta, Canada

Abstract

Abstract:Floods caused by ice-jams on the Peace River are considered to be important for maintaining hydro-ecological conditions of perched basins in the Peace-Athabasca Delta (PAD), Canada, a highly productive and internationally recognized northern boreal ecosystem. Concerns over the potential linkages between regulation of the Peace River in 1968 for hydroelectric production and low Peace River discharge between 1968 and 1971 during the filling of the hydroelectric reservoir, absence of a major ice-jam flood event between 1975 and 1995, and low water levels in perched basins during the 1980s and early 1990s have sparked numerous environmental studies largely aimed at restoring water levels in the PAD. Lack of sufficient long-term hydrological records, however, has limited the ability to objectively assess the importance of anthropogenic factors versus natural climatic forcing in regulating hydroecological conditions of the PAD. Here, we report results of a paleolimnological study on laminated sediments from two oxbow lakes in the PAD, which are located adjacent to major flood distributaries of the Peace River. Sediment core magnetic susceptibility measurements, supported by results from several other physical and geochemical analyses as well as stratigraphic correspondence with recorded high-water events on the Peace River, provide proxy records of flood history spanning the past ¾180 and ¾300 years in these two basins. Results indicate that inferred flood frequency has been highly variable over the past 300 years but in decline for many decades beginning as early as the late nineteenth century, well before Peace River regulation. Additionally, several multi-decadal intervals without a major flood have occurred during the past 300 years. While climate-related mechanisms responsible for this variability in flood frequency remain to be determined, as does quantifying the relative roles of river regulation and climate variability on hydro-ecological conditions in the PAD since 1968, these results suggest that ecosystem management strategies for the PAD need to explicitly account for natural variations in flood recurrence intervals.

Flood Frequency Variability During the Past 80 Years in the Slave River Delta, NWT, as Determined from Multi-Proxy Paleolimnological Analysis

“…Instead, values range between 0.017 and 0.070 Bq/g in the upper 10 cm, and are constrained to a narrow range spanning 0.028-0.049 Bq/g below this depth. These values are similar to those measured from sediment cores from two oxbow lakes in the PAD, where 210 Pb concentrations fell within a similar narrow range, and average values for both lakes were ~0.033 Bq/g (Wolfe et al, 2006). As was concluded for these two PAD lakes, frequent flooding introduces pulses of old, re-worked sediment to the lake bed that dilutes 210 Pb activity, which accounts for the generally constant measured Cs activity is also similar in magnitude (~0.017 Bq/g) to that measured in sediments from the PAD lakes mentioned above (Wolfe et al, 2006).…”

“…These values are similar to those measured from sediment cores from two oxbow lakes in the PAD, where 210 Pb concentrations fell within a similar narrow range, and average values for both lakes were ~0.033 Bq/g (Wolfe et al, 2006). As was concluded for these two PAD lakes, frequent flooding introduces pulses of old, re-worked sediment to the lake bed that dilutes 210 Pb activity, which accounts for the generally constant measured Cs activity is also similar in magnitude (~0.017 Bq/g) to that measured in sediments from the PAD lakes mentioned above (Wolfe et al, 2006). We use the measured 137 Cs peak at SD2 to represent the 1963 stratigraphic horizon, representing maximum 137 Cs fallout as a result of above-ground nuclear weapons testing (Appleby, 2001).…”

“…River floodwaters are associated with high C/N ratios, which may reflect organic matter derived from terrestrial sources (Meyers and Teranes, 2001). However, high C/N ratios may also be due to the influx of C-rich, N-poor hydrocarbons (i.e., bitumen) derived from upstream oil sand deposits, as has been suggested for similar geochemical signatures in the stratigraphic records of oxbow lakes in the PAD (Wolfe et al, 2006;Jarvis, 2008). During non-flood intervals, increases in autochthonous production generate sediment that is more organic-rich with higher water content, and possesses lower δ 15 N values and C/N ratios, the latter typical of aquatic organic matter (Meyers and Teranes, 2001).…”

“…Pulses of water associated with spring break-up flooding replenish water levels in lakes elevated above the mainstem and distributary channels (Prowse and Lalonde, 1996;Rouse et al, 1997;Beltaos et al, 2006b;Peters et al, 2006a;b) and introduce sediment to lake beds (Milburn et al, 1999). Spring flooding is especially important in northern delta ecosystems such as the Peace-Athabasca (Prowse and Conly, 1998;Beltaos et al, 2006a;Peters et al, 2006a;b;Wolfe et al, 2006Wolfe et al, , 2008a, Slave (Brock et al, 2007;2008;Sokal et al, 2008;2010) and Mackenzie (Marsh and Hey, 1989;Marsh and Lesack, 1996;Woo and Thorne, 2003;Emmerton et al, 2007) river deltas in the Mackenzie River Basin. These ecosystems provide important aquatic and terrestrial habitat for a wide assortment of communities of vegetation, birds, mammals, and aquatic life (Milburn et al, 1999).…”

“…Results have revealed that lakes perched above the river system may persist for decades in the absence of spring flooding (Wolfe et al, 2005), while others may dry up within a decade following a flood event (Peters et al, 2006a;Jaques, 1989). Additionally, there have been oscillating multi-decadal periods of both high and low flood frequency over the last several hundred years, suggesting marked natural variability in flood recurrence intervals (Wolfe et al, 2006;2008a). Importantly, the current period of low flood frequency in the PAD began several decades before regulation of the Peace River by the WAC Bennett Dam.…”