Role of deltaic lakes as hydraulic filters: sediment sorting in the Arctic river delta under changing flow regimes

Ongoing climate change accelerates the degradation of permafrost, leading to an increased supply of meltwater and an intensified delivery of mineral sediments and organic matter to riverine and deltaic systems. Consequently, understanding the processes of hydraulic sorting and particle retention within deltaic environments, which determine the sediment budget and the “filtration efficiency” of these systems at the scale of the entire Arctic, becomes of critical importance.
The aim of this study is to identify the influence of hydraulic sorting mechanisms on changes in the size and concentration of suspended sediment particles during the passage of meltwater through the deltaic system of a large Arctic river. The study area is the lake-rich central part of the Mackenzie Delta (Canada, Northwest Territories). The experiment was conducted from the onset of ice-cover degradation until the end of the summer season, tracing the pathway of water transferred from the East Channel to the Middle Channel over a distance of approximately 12 km through three lakes permanently connected to the distributary channel network. Five monitoring stations for suspended sediment concentration (SSC) were installed, and measurements were carried out on eleven occasions between 24 May and 10 September 2025. In total, 120 suspended sediment samples were analysed.
The material was subjected to laboratory analyses of SSC and grain-size distribution using laser diffraction. The results show that, during the initial phase of observations, suspended sediments in the cold waters of the Mackenzie Delta (0.5–1.7°C) were characterized by SSC values ranging from 68 to 218 mg·l⁻¹ and by a bimodal grain-size distribution. In contrast, samples collected during the falling limb of the freshet (June–July) and during low-flow conditions (August–September) exhibited much lower SSC values, ranging from 10 to 68 mg·l⁻¹. Grain-size analyses indicate that, with increasing residence time of water in the lakes, a clear hydraulic sorting of particles according to their size occurs. This process results in the selective retention of coarser particles and aggregates within the lake zones, while finer silt–clay fractions continue to be transported towards the lower parts of the delta.
The dominant modal peak corresponds to the clay–silt fraction, typical of wash load, whereas a second peak in the range of 200–1000 µm, representing coarse and very coarse sand, appears only episodically during periods of increased discharge and enhanced turbulence. The fractional composition of suspended sediments at the end of May 2025 indicates a dominance of silt (70–85%), with clay contents of 5–10% and sand contents of 10–20%, of which the very coarse fraction (>250 µm) occurred only sporadically (<5%). This material can be classified as fine silt-dominated wash load with low settling velocities.
The obtained results confirm the key role of channel-adjacent lakes as natural hydraulic filters in Arctic deltas and demonstrate that hydraulic sorting of particle sizes and variability in settling dynamics are directly controlled by the hydrological regime of the spring freshet, the intensity of flow turbulence, and the cyclic processes of flocculation and destruction of sediment aggregates.

This research is financed by grant National Research Centre in Poland no. 2024/53/B/ST10/03483