Effective stewardship of ecologically-significant floodplain landscapes requires knowledge of the relative roles of natural processes and upstream human activities on environmental flows. In these landscapes, hydroecological conditions that develop from potentially competing drivers, such as climate change and industrial development, tend to be expressed at spatial and temporal scales that are often inadequately captured by existing monitoring datasets. Consequently, perceived cause–effect relations may be misunderstood, conflict can escalate among stakeholders, and effectiveness of surveillance systems, policies, and governance may be impaired. This is the context for the Peace–Athabasca Delta (PAD), an internationally-recognized water-rich floodplain landscape located in northern Alberta (Canada) that has been subject to multiple stressors. Here we synthesize evidence from paleolimnological records that have fostered an unparalled window into the natural history of this landscape. Over the past 12 years, we have assembled numerous decadal- to multicentennial-long records of hydrological and ecological variability, including an exceptionally detailed chronicle of Peace River flood frequency and magnitude spanning ~600 years. These efforts recently culminated in a 5200-year reconstruction of Lake Athabasca water-level history. Results have provided the foundation to identify drivers of landscape change and generate insight into the delta’s dynamic and ongoing evolution. Contrary to widespread perceptions that hydroelectric regulation of the Peace River since the late 1960s has reduced the frequency of ice-jam floods and lowered floodplain lake-water levels, results indicate that climate variability exerts the overwhelming influence on the delivery of water to the PAD. We show that impending climate-driven freshwater scarcity of a scale unprecedented in our collective societal memory now poses a significant threat to the ecological integrity of this world-renowned landscape and a major challenge to water resource managers. Also, we propose a hydroecological monitoring program, built upon the knowledge gained from our extensive process studies and paleoenvironmental research, to inform effective ongoing stewardship of the delta.