This thesis combines detailed analyses of living plant communities with paleolimnological methods to develop the use of plant macrofossils as an effective tool to track hydroecological changes in ponds of the Slave River Delta (SRD), N.W.T. Several approaches were used to develop an understanding of the relationships between hydrolimnological conditions, living plant communities, and the composition of sedimentary macrofossil remains across spatial and temporal scales. A spatial survey approach was used to assess the relationships between the composition of plant macrofossil assemblages contained in the surface sediments and the hydrological and limnological conditions of 40 SRD basins that span a broad range of hydrological settings in the delta. Results show that there are strong relationships between the prevailing hydrolimnological conditions in the SRD and the composition of sedimentary macrofossil remains, and subsequently indicator macrofossil taxa were identified to distinguish between sites with high river influence (flood-dominated sites: ostracode shells, Daphnia ephippia, Chara oospores; exchange-dominated sites: Myriophyllum winter-bud scales, Daphnia ephippia) and sites with low river influence (evaporation-dominated: Ceratophyllum leaves, Lemna leaves, Drepanocladus leaves). In light of the strong connection between spring flood events and the hydrolimnological conditions of SRD ponds, and growing concerns that the frequency of spring floods have declined in recent decades, paleolimnological investigations were initiated at a pond (SD2) adjacent to the Slave River to construct a record of flood events in the SRD. Prior to analysis of macrofossil assemblages from sediment cores, a detailed study of the living plant community was conducted at SD2, and results were compared to the distributions of surficial sedimentary plant macrofossil assemblages to assess how representative surficial sediment assemblages are of the living plant community. This study indicates distinct patch-scale (or quadrat-scale)similarity between the living vegetation and sedimentary remains in the central basin, as well as distinct similarity between the living aquatic macrophytes and sedimentary remains at a pond-scale, suggesting there is excellent potential to track changes in the composition and percent cover of aquatic macrophytes in pond sediment cores using plant macrofossil assemblages. Additionally, this study indicates that influence of long-distance transport of macrofossils during the 2005 flood event was minor at this pond, and may not be an important factor affecting paleolimnological reconstructions of plant communities. With contemporary studies as a framework, an ~90-year record of ice-jam flood frequency was reconstructed from a sediment core collected from pond SD2. Multi-proxy analyses indicated decadal-scale oscillations in flood frequency at this site, with at least three multi-year periods of low river influence. Beginning in ~1943, an 18-year period of particularly low river influence and greatly reduced water levels was indicated by abundant macrofossils of Sagittaria cuneata and represents the driest period over the past ~90-years. Similarities between the flood history of SD2 and upstream sites in the Peace-Athabasca Delta suggest that spring discharge generated from headwaters and major contributing rivers plays a key role in the frequency and magnitude of spring flood events of both deltas.

This thesis combines detailed analyses of living plant communities with paleolimnological methods to develop the use of plant macrofossils as an effective tool to track hydroecological changes in ponds of the Slave River Delta (SRD), N.W.T. Several approaches were used to develop an understanding of the relationships between hydrolimnological conditions, living plant communities, and the composition of sedimentary macrofossil remains across spatial and temporal scales. A spatial survey approach was used to assess the relationships between the composition of plant macrofossil assemblages contained in the surface sediments and the hydrological and limnological conditions of 40 SRD basins that span a broad range of hydrological settings in the delta. Results show that there are strong relationships between the prevailing hydrolimnological conditions in the SRD and the composition of sedimentary macrofossil remains, and subsequently indicator macrofossil taxa were identified to distinguish between sites with high river influence (flood-dominated sites: ostracode shells, Daphnia ephippia, Chara oospores; exchange-dominated sites: Myriophyllum winter-bud scales, Daphnia ephippia) and sites with low river influence (evaporation-dominated: Ceratophyllum leaves, Lemna leaves, Drepanocladus leaves). In light of the strong connection between spring flood events and the hydrolimnological conditions of SRD ponds, and growing concerns that the frequency of spring floods have declined in recent decades, paleolimnological investigations were initiated at a pond (SD2) adjacent to the Slave River to construct a record of flood events in the SRD. Prior to analysis of macrofossil assemblages from sediment cores, a detailed study of the living plant community was conducted at SD2, and results were compared to the distributions of surficial sedimentary plant macrofossil assemblages to assess how representative surficial sediment assemblages are of the living plant community. This study indicates distinct patch-scale (or quadrat-scale)similarity between the living vegetation and sedimentary remains in the central basin, as well as distinct similarity between the living aquatic macrophytes and sedimentary remains at a pond-scale, suggesting there is excellent potential to track changes in the composition and percent cover of aquatic macrophytes in pond sediment cores using plant macrofossil assemblages. Additionally, this study indicates that influence of long-distance transport of macrofossils during the 2005 flood event was minor at this pond, and may not be an important factor affecting paleolimnological reconstructions of plant communities. With contemporary studies as a framework, an ~90-year record of ice-jam flood frequency was reconstructed from a sediment core collected from pond SD2. Multi-proxy analyses indicated decadal-scale oscillations in flood frequency at this site, with at least three multi-year periods of low river influence. Beginning in ~1943, an 18-year period of particularly low river influence and greatly reduced water levels was indicated by abundant macrofossils of Sagittaria cuneata and represents the driest period over the past ~90-years. Similarities between the flood history of SD2 and upstream sites in the Peace-Athabasca Delta suggest that spring discharge generated from headwaters and major contributing rivers plays a key role in the frequency and magnitude of spring flood events of both deltas.

The use of the stable isotopes, 18O and 2H, has proven to be a valuable tool in determining the importance of various hydrological controls on the modern water balances of Slave River Delta lakes, NWT, Canada. Samples collected during the 2002 and 2003 field season have shown that delta lakes exhibit highly systematic isotopic variability over the entire delta. The major influences observed to be affecting Slave River Delta lakes include spring snowmelt runoff, flood events from the Slave River, seiche events from Great Slave Lake and thaw season precipitation events. An important component of Slave River Delta lake modern water balances is evaporation, the main controlling factor of water loss in the study lakes, as well as isotopic variability experienced throughout the entire delta during the ice-off season. Flood events from Great Slave Lake and the Slave River play a key role in controlling modern water balances and isotopic compositions of lakes in the delta. Levee height throughout the delta seems to strongly affect local hydrology, with areas having the greatest levee heights also having the most enriched lake water compositions, and areas having the lowest levee heights having the most depleted isotopic signatures. Outer delta and mid-delta lakes experience the greatest amount of flooding during the spring. Lakes that are affected by spring flood events have a more depleted isotopic signature than those lakes in the upper delta. Discrepancies between δ18O- and δ2H-derived E/I ratios have been effectively reconciled by incorporating site-specific information into the mass balance equations, and allowing mixing between Great Slave Lake (GSL) vapour δE, a large body of water adjacent to the delta and advected atmospheric vapour δA. The use of locally derived parameters also ensures a more accurate depiction of local conditions. Good correlation can be observed during July 2003, between mixing of GSL vapour and atmospheric moisture, when the lakes water balances were solely affected by evaporation. The mixing ratios obtained from two of the study lakes suggest that 5 – 16% of ambient atmospheric moisture was derived from Great Slave Lake.

The use of the stable isotopes, 18O and 2H, has proven to be a valuable tool in determining the importance of various hydrological controls on the modern water balances of Slave River Delta lakes, NWT, Canada. Samples collected during the 2002 and 2003 field season have shown that delta lakes exhibit highly systematic isotopic variability over the entire delta. The major influences observed to be affecting Slave River Delta lakes include spring snowmelt runoff, flood events from the Slave River, seiche events from Great Slave Lake and thaw season precipitation events. An important component of Slave River Delta lake modern water balances is evaporation, the main controlling factor of water loss in the study lakes, as well as isotopic variability experienced throughout the entire delta during the ice-off season. Flood events from Great Slave Lake and the Slave River play a key role in controlling modern water balances and isotopic compositions of lakes in the delta. Levee height throughout the delta seems to strongly affect local hydrology, with areas having the greatest levee heights also having the most enriched lake water compositions, and areas having the lowest levee heights having the most depleted isotopic signatures. Outer delta and mid-delta lakes experience the greatest amount of flooding during the spring. Lakes that are affected by spring flood events have a more depleted isotopic signature than those lakes in the upper delta. Discrepancies between δ18O- and δ2H-derived E/I ratios have been effectively reconciled by incorporating site-specific information into the mass balance equations, and allowing mixing between Great Slave Lake (GSL) vapour δE, a large body of water adjacent to the delta and advected atmospheric vapour δA. The use of locally derived parameters also ensures a more accurate depiction of local conditions. Good correlation can be observed during July 2003, between mixing of GSL vapour and atmospheric moisture, when the lakes water balances were solely affected by evaporation. The mixing ratios obtained from two of the study lakes suggest that 5 – 16% of ambient atmospheric moisture was derived from Great Slave Lake.

Water isotope tracers and multi-proxy paleolimnological approaches are used to characterize the present and past hydrology of the Slave River Delta (SRD), NWT. This research addresses crucial gaps in knowledge about the role of major hydrological processes on the water balances of northern freshwater lakes, and responds to concerns expressed by local land users about declining flood frequency in the delta following upstream river regulation. Contemporary hydrological studies were conducted using multiple lakewater sampling campaigns from a suite of 41 delta lakes situated in three previously recognized biogeographical zones - outer delta, mid-delta and apex – that were initially sampled in fall 2002, and again immediately following the spring melt, during summer, and in the fall of 2003-2005. Paleolimnological studies aimed at reconstructing flood frequency in the Slave River focus on a sediment core obtained from a flood-susceptible lake in the active delta. Together, contemporary and past studies of SRD hydrology provide a detailed picture of environmental change and variability in an important northern freshwater ecosystem.

Water isotope tracers and multi-proxy paleolimnological approaches are used to characterize the present and past hydrology of the Slave River Delta (SRD), NWT. This research addresses crucial gaps in knowledge about the role of major hydrological processes on the water balances of northern freshwater lakes, and responds to concerns expressed by local land users about declining flood frequency in the delta following upstream river regulation. Contemporary hydrological studies were conducted using multiple lakewater sampling campaigns from a suite of 41 delta lakes situated in three previously recognized biogeographical zones - outer delta, mid-delta and apex – that were initially sampled in fall 2002, and again immediately following the spring melt, during summer, and in the fall of 2003-2005. Paleolimnological studies aimed at reconstructing flood frequency in the Slave River focus on a sediment core obtained from a flood-susceptible lake in the active delta. Together, contemporary and past studies of SRD hydrology provide a detailed picture of environmental change and variability in an important northern freshwater ecosystem.