Hydrological monitoring in complex, dynamic northern floodplain landscapes is challenging but increasingly important as these ecosystems come under threat from multiple stressors, including climate-driven decline in freshwater supplied by rivers draining the hydrographic apex of western North America. Sustainable approaches capable of tracking status, trends and drivers of lake water balances in complex, remote landscapes are needed to inform ecosystem stewardship and water-security actions. The Peace‐Athabasca Delta (PAD) in northern Alberta, Canada, is a Ramsar Wetland of International Importance reliant on episodic river ice‐jam flood events to recharge abundant perched lakes and wetlands. However, the frequency of these floods has been in decline for decades over much of its area. Compounding concerns about water-level drawdown have prompted the need to improve knowledge of lake water balances and establish a lake monitoring program. Yet, the delta’s remoteness and dynamic nature present challenges to these goals. In this thesis, I address hydrological knowledge gaps essential to understanding spatial and temporal patterns of hydrological processes and their influence on lakes in the PAD. First, we assess the legacy influence of a large-scale ice-jam flood in 2014 on hydrological and limnological status of lakes in the PAD by integrating spatial and temporal data. Analysis of water isotope compositions and water chemistry measured at numerous lakes across the delta shows that hydro-limnological effects of the flood event of 2014 failed to persist beyond the early ice-free season of 2015. Isotope-inferred paleohydrological records from five hydrologically representative lakes in the PAD indicate that periodic desiccation during the Little Ice Age occurred at the most elevated basin in response to locally arid climatic conditions and reduced flood frequency, yet other lower elevation sites were influenced by high water level on Lake Athabasca owing to increased snowmelt- and glacier-derived river discharge. In contrast, water isotope data during the past 15 years at all five lakes consistently document the strong role of evaporation, a trend which began in the early to mid-20th century according to sediment records and is indicative of widespread aridity unprecedented during the past 400 yr. We suggest that integration of hydrological and limnological approaches over space and time is needed to inform assessment of contemporary lake conditions in large, complex floodplain landscapes. Next, we use water isotope compositions, supplemented by measurements of specific conductivity and field observations, from 68 lakes and 9 river sites in May 2018 to delineate the extent and magnitude of spring ice‐jam induced flooding along the Peace and Athabasca rivers. Lake‐specific estimates of input water isotope composition (δI) using a coupled-isotope tracer approach were modelled after accounting for the influence of evaporative isotopic enrichment. Then, using the distinct isotopic signature of input water sources, we develop a set of binary mixing models and estimate the proportion of input to flooded lakes attributable to river floodwater and precipitation (snow or rain). This approach allowed identification of areas and magnitude of flooding that were not captured by other methods, including direct observations from flyovers, and to demarcate flow pathways in the delta. We demonstrate water isotope tracers as an efficient and effective monitoring tool for delineating spatial extent and magnitude of an important hydrological process and elucidating connectivity in the PAD, an approach that can be readily adopted at other floodplain landscapes. Finally, we use over 1000 measurements of water isotope composition at ∼60 lakes and 9 river sites during the spring, summer and fall of five consecutive years (2015–2019) to identify patterns in lake water balance over time and space, the influential roles of evaporation and river floodwaters, and relations with meteorological conditions and river water levels. Calculation of evaporation-to-inflow ratios using a coupled-isotope tracer approach, displayed via generalized additive models and geospatial ‘isoscapes’, reveal strongly varying lake water balances. Results identify distinct areas vulnerable to lake-level drawdown, given the likelihood of continued decline in ice-jam flood frequency, longer ice-free season duration and reduced snowmelt runoff. Results also demarcate areas of the delta where lakes are more resilient to factors that cause drawdown. The former defines the Peace sector, which is influenced by floodwaters from the Peace River during episodic ice-jam flood events, whereas the latter describes portions of the active floodplain environment of the Athabasca sector which receives more frequent contributions of Athabasca River floodwaters during both spring ice-jam and open-water seasons. Efficiency of water isotope tracers to capture the marked temporal and spatial heterogeneity in lake water balances during this 5-year time span, and their diagnostic responses to key hydrological processes, serves as a foundation for ongoing lake monitoring, an approach readily transferable to other remote and dynamic lake-rich landscapes.

Freshwater ecosystems across northern Canada provide important habitat for wildlife and have long supported the traditional lifestyles of Indigenous communities. Multiple potential stressors threaten the security of water supply to northern landscapes, which fosters need for information spanning broad spatial and temporal scales to inform adaptive and mitigative strategies. At the Peace-Athabasca Delta (PAD; northern Alberta), the world's largest boreal freshwater delta, existing data records have been too short and too sparse to resolve many concerns over the roles of major energy projects (hydroelectric regulation of river flow, oil sands development) and climate change on decline of flood frequency and magnitude and drawdown of shallow aquatic basins, and on supply of substances of concern. Intensive paleolimnological research during the past two decades at the PAD has evaluated past changes in contaminant deposition and hydroecological conditions to discern effects attributable to oil sands development along the Lower Athabasca River and to regulation of Peace River flow by the W.A.C. Bennett Dam. This thesis builds substantially on these previous studies to address knowledge gaps by applying conventional paleolimnological methods at new locations to improve understanding of temporal changes in contaminant deposition and hydrological change and developing an innovative paleolimnological approach for discerning variation in sediment sources over space and time to lakes within the PAD. Concerns of pollution in the PAD stem from potential for dispersal of contaminants released by bitumen mining and processing activities within the Alberta Oil Sands Region (AOSR), which straddle the Lower Athabasca River. Unfortunately, systematic monitoring began thirty years after onset of oil sands development and sampling locations have changed over time, which has hampered the ability to accurately track temporal trends or attribute sources of pollution at the AOSR and downstream locations. Previous paleolimnological studies in the PAD have provided critically missing baseline information by employing lake sediment deposited before oil sands development to evaluate lake or river-bottom sediment deposited after oil sands development for evidence of pollution. Results show no enrichment via fluvial or atmospheric pathways, however, analyses to date were limited to a sediment core from one upland lake and samples of river-bottom sediment collected from a few sites within the Lower Athabasca River and its distributaries within the PAD. In Chapters 2 and 3, contiguous measurements of trace elements (beryllium, chromium, lead, mercury, nickel, vanadium, and zinc) in lake sediment from floodplain and upland lakes were employed to develop knowledge of pre-disturbance concentrations, and to quantify the extent of enrichment in lakes at the PAD since onset of oil sands mining and processing activities via fluvial and atmospheric pathways, respectively. Results demonstrate no enrichment since onset of oil sands development via fluvial pathways. Also, no enrichment via atmospheric pathways was detected for vanadium, nickel and total mercury (THg) at upland lakes coincident with the onset of oil sands activities. Total mercury enrichment was detected at the start of the 20th century in sediment cores from two upland lakes, which is congruent with stratigraphic patterns observed in many other lake sediment records in response to long-range anthropogenic emissions across the northern hemisphere. Site-specific paleolimnological studies from four regions spanning large-scale bitumen mining on the Lower Athabasca River to gold mining in central Northwest Territories (including AOSR, PAD, Slave River Delta (SRD), Yellowknife region of central NWT) have provided a wealth of information about temporal patterns of deposition of substances of concern. Differences in laboratory methods and data analysis, however, have challenged ability to compare and contrast site-specific studies among regions. Opportunity to coalesce the current state of knowledge was capitalized on in Chapter 4 via systematic re-analysis of concentrations of key pollution-indicator trace elements in sediment cores from 51 lakes spanning the four key regions. Lake sediment records from lakes within the mining regions (AOSR and central Northwest Territories) illustrate enrichment of pollution-indicators since onset of mining and processing activities via atmospheric pathways, while no enrichment was detected at the PAD or SRD via fluvial pathways since onset of mining activities. The knowledge generated from Chapters 2-4 can be employed by multiple stakeholder groups to assess risks associated with contaminant dispersal across a vast region of northwestern Canada. Long-term perspectives provided by paleohydrological studies at the PAD have demonstrated that decline of flood frequency and magnitude and lake-level drawdown began decades before onset of Peace River flow regulation by the W.A.C. Bennett Dam. Many of these studies have been concentrated in the northern Peace Delta but concerns also exist about declines in river discharge, flood frequency and lake levels in the southern Athabasca Delta. Chapter 5 tests the hypothesis that the Embarras Breakthrough, a natural geomorphic change in distributary flow of the Athabasca River, is the main driver of recent hydrological change in the Athabasca Delta. Stratigraphic variations in the mineral matter content of sediment cores from nine floodplain lakes, including at sites within the Athabasca River terminus region, demonstrate that flood influence increased after 1982 at lakes along a distributary to the north of the Embarras Breakthrough and declined at lakes east of the Embarras Breakthrough. The timing of this bi-directional change confirms that the Embarras Breakthrough has caused the largest shift in hydrological conditions within the Athabasca Delta during the past ~120 years. Paleohydrological reconstructions employing conventional analyses have provided valuable insight into the hydrological evolution of the PAD but integration of the results across sites has remained a challenge due to marked differences in sediment composition across the spectrum of hydrological processes influencing lake water balances. At flood-prone lakes, physical methods (e.g., grain size, magnetic susceptibility) provide high information content, whereas biological or bio-geochemical methods (e.g., diatoms, plant macrofossils, cellulose oxygen isotope composition) provide high information content at perched basins. Elemental concentrations, however, can be determined accurately along the full gradient of mineral-rich to organic-rich sediment of flood-prone and perched basins, respectively, and can be used to delineate the three major sources of sediment supplied to lakes (Athabasca River, Peace River, and local catchment), which is a key advancement over previous paleolimnological studies. In Chapter 6, a mixing model framework was developed and evaluated via application to sediment cores from two adjacent lakes in the Athabasca Delta. Output from the mixing model aligns remarkably well with conventional loss-on-ignition analysis and paleohydrological interpretations from the same two lakes, which further illustrate the profound effect of the 1982 Embarras Breakthrough on hydrological conditions of lakes in Athabasca Delta. Interestingly, model results indicated that ~60% of the sediment originated from the Peace River during the largest ice-jam flood event in the hydrometric record (1974). Due to the success of this model, opportunity exists to apply the model to a network of lakes in the PAD, where elemental concentrations have been analyzed, to reconstruct spatial and temporal variation of pathways of sediment sources and infer changes in hydrological processes. The methods developed and applied in this thesis are anticipated to be broadly applicable to other freshwater landscapes where monitoring records remain too short and too sparse to discern effects of multiple stressors.