Document Type

Thesis

Degree Name

Master of Science (MSc)

Department

Geography & Environmental Studies

Faculty/School

Faculty of Arts

First Advisor

Brent Wolfe

Advisor Role

Thesis Supervisor

Abstract

The Slave River Delta (SRD), NWT, represents a pivotal node in the upper Mackenzie Basin watershed and is a productive northern wetland landscape with a rich natural and cultural heritage. Concerns over environmental consequences of natural and anthropogenic-driven decline in river discharge as well as climate variability have prompted hydroecological sstudies to improve understanding of how this ecosystem functions over time and space. However, long-term natural hydrological variability of the Slave River system is not well documented and needs to be further developed. In order to provide a temperal context for understanding and evaluating the impacts of climate variability and change and other stressors on Slave River discharge, multi-proxy paleolimnological analyses have been conducted to reconstruct a long-term record of hydrologic variability in the Slave River system. Study sites include two small closed-drainage lakes (GSLI1, SD34) located near a former elevated strandline of Great Slave Lake and another lake (SR1) located upstream on an island in the Slave River.

Multi-proxy analysis of lake sediment cores collected from SD34, GSL1, and SR1 provided a ∼1200-year record of water level variation for the Slave River and Great Slave Lake. Sediment composition and elemental and stable isotope geochemistry proved to be sensitive indicators of hydrologic change within the study basins. Interpretation of C/N ratios and δ18Olw, high δ13Corg and inorganic sediment indicated open-drainage conditions caused by riverine inundation of both study lakes. River inundation of SD34 and GSL1 was attributed to Slave River discharge dominated by a large flashy spring freshet and high ice-jam flood frequency.

The percent organic carbon, organic nitrogen, δ15N, C/N ratios, δ18Olw values and δ13Corg values appear to indicate isolation of SD34 and GSL1 from the Slave River at ∼1000 AD and ∼1150 AD, respectively. However, reduced ice-jam flood frequency in the SRD precedes a similar change in the upstream Peace-Athabasca Delta (PAD) by ∼400 years. A change in the distributary network of the SRD and low Slave River discharge may account for the ∼400 year offset and explain why SD34 and GSL1 became isolated from ice-jam flooding during the Middle MP, when ice-jam flood frequency was high in the PAD.

The interval of low Slave River discharge suggested by SD34 and GSL1 sediment records appeared to end at ∼1300 AD with the re-emergence of open-drainage conditions in SD34 indicated by high δ13Corg values. Open-drainage conditions at SD34 spanned the Late MP and were attributed to high Slave River discharge that is consistent with an interval of high North Saskatchewan River discharge.

The SR1 C/N record indicated that high Late MP Slave River discharge continued throughout the Little Ice Age (LIA). High LIA Slave River discharge was attributed to a delay in snowmelt-generated runoff that sustained higher summer river discharge as a result of a shift to cooler climate conditions. High δ13Corg values in the GSL1 record inciated open-drainage conditions in GSL1 throughout the LIA and δ18Olw values that were similar to modern Great Slave Lake (GSL) δ18Olw values suggested that the open-drainage conditions were the result of GSL inundation. Therefore, increased Slave River discharge appears to have caused high GSL water levels during the LIA, similar to that observed upstream in Lake Athabasca. High GSL water levels inundated GSL1 and likely occupied the strandline visible in the landscape to the south of the SRD.

At the beginning of the 20th century, the SR1, SD34 and GSL1 sediment records indicate closed-drainage conditions were established. Closed-drainage conditions at each study lake were attributed to a decline in water level within the Slave River system at the end of the LIA. A 20th century decline in discharge parallels a shift to a warmer climate regime that has been shown to cause an earlier, more rapid melt of the spring snowpack in the headwaters of the Peace and Athabasca rivers. The sediment records of the three study lakes indicated that water levels within the Slave River system have declined from the high levels that characterized the LIA, falling to levels possibly similar to the MP. However, high δ18Olw values at GSL1, throughout the 20th century that are unique within the ∼1200 record may suggest that present hydrological conditions at this site are unprecedented in the last ∼1200 years.

These findings establish a link between hydrologic conditions within the Slave River system and those upstream in the PAD and Lake Athabasca. Sediment records from SD34, GSL1 and SR1 demonstrated that in the past ∼400 years, the hydrology of the Slave River system responded similarly to the PAD in response to shifting climate regimes. This indicates that hydrologic change upstream in the headwaters of the Slave River has historically translated downstream to the SRD and GSL. Furthermore, these results show that water levels in the upper Mackenzie River system have varied considerably in the last ∼1200 years and that they have been in decline over the last 200 years—a decline that is likely to continue given expected trajectories in river discharge.

Convocation Year

2010

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Hydrology Commons

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