The long-term natural hydrological variability of the Slave River Delta (SRD), NWT, is not well documented and needs to be further developed to provide temporal context to understand and evaluate impacts of Slave River (SR) floodwater influence and climate variability and change on contemporary hydro-ecological conditions of the SRD. The SRD has broad ecological and cultural significance, as it provides extensive habitat for wildlife and is important for local First Nations community who have an historical connection with the delta and its resources. Concerns have been raised over recently reported drying trends in the SRD over the past few decades and have largely been attributed to regulation of the Peace River (PR), which supplies the SR with ∼65% of its annual flow.

Modern lake water balances (2003 to 2005) of three lakes from different hydrological settings within the SRD were assessed using oxygen (δ 18 O) and hydrogen (δ2 H) stable isotope analyses. Contemporary lake water balance was used to constrain paleohydrological interpretations of cellulose-inferred δ18 O from lake sediment cores. Past hydro-ecological conditions of each lake was also reconstructed using bulk organic carbon and nitrogen elemental and stable isotope analyses. Lead-210 (210 Pb) and caesium-137 (137 Cs) analyses were conducted to establish sediment core chronologies.

Results from lake water δ18 O and δ 2 H analyses of SD20, an evaporation-dominated basin, indicate seasonal precipitation, snowmelt runoff and evaporation predominantly control the water balance of this lake. An ∼215-year cellulose-inferred δ18 Olw , δ13 C and δ15 N record of SD20 provides paleoclimatological evidence that recently reported dry conditions in the SRD are not outside of the range of natural variability for the delta. SD20 paleohydrological records follow a similar pattern as PAD5, a climate-driven basin in the Peace-Athabasca Delta (PAD), and align with paleoclimate records reconstructed from tree-ring sequences from the Athabasca River headwaters. Results provide long-term documentation of how hydrological conditions have varied in an area of the SRD that is largely beyond the reach of river flooding.

Lake water δ18 O and δ2 H analyses of SD2, a flood-dominated basin, indicate that SR floodwaters control the water balance of SD2. An ∼100 year carbon and nitrogen elemental and isotope record for SD2 documents event-scale flooding on the SR and indicates that regulation of the PR has not decreased flood frequency at this site over the past ∼40 years. The late 1940s and 1950s likely represent the period of lowest river discharge over the past century. The SD2 C/N record is similar to the C/N record of PAD15, an oxbow lake in the PAD, indicating common upstream drivers control flood frequency in both deltas.

Lake water δ18 O and δ2 H analyses of SD28, an exchange-dominated basin, suggest that SR flooding and evaporation predominantly control the water balance of SD28. Reconstructed cellulose-inferred δ 18 Olw suggest an increase in river flooding may have occurred over the past ∼40 years. However, reconstructing past hydro-ecological conditions is more difficult at this site due to its long channel connection to the SR, which suppresses the geochemical signals recorded in the lake sediment.