Turbidity and Non-Filterable Residue Across Time and Space in Proglacial River Systems in the Canadian Rocky Mountains

Abstract:

When glaciers recede, products of subglacial erosion are exposed, some of which enter proglacial river systems. These fine sediments, often referred to as glacial flour, can be transported downstream and increase the river’s turbidity. Highly turbid waters can block sunlight from entering the river and mute downstream productivity, while individual particles can serve as a medium to transport sorbed nutrients, organic matter (OM), and contaminants. Due to climate change, glaciers are retreating faster than ever, meaning more particles are entering proglacial rivers, making them more turbid. Few studies focus on the downstream movement and transport of particles while also measuring turbidity, and fewer still are carried out year-round.

We deployed YSI EXO sondes to measure turbidity and filtered raw water to capture non-filterable residue (NFR) throughout 2019 and 2020 in four major proglacial rivers in the Canadian Rocky Mountains (the Sunwapta, Athabasca, North Saskatchewan, and Bow rivers). Preliminary data show that NFR and turbidity values were highest once glacial melt surpassed snowmelt in late spring. During the summer, when glacial melt rates were at their peak, mean ±SD NFR concentrations were 21± 3.8 mg/L whereas in October, when glacial melt began to wane, NFR concentrations dropped to 1.4 ±1.4 mg/L. The highest NFR and turbidity values were found on the Sunwapta river where we were able to sample at the glacier’s terminus and at sites in floodplain areas where river braiding was common. After the glacier’s terminus on the Sunwapta river, sediment continuously settled out after passing through a proglacial lake. Mean turbidity was 134±110 FNU at the glacier’s terminus and dropped to 50.7±17 FNU after passing through the proglacial lake. Our data demonstrate how strongly glacial melt impacts turbidity and suspended particles in proglacial rivers, with areas closest to the glacier’s termini and in floodplains at greatest risk of increased suspended matter in the water column. With glaciers rapidly retreating due to climate change, aquatic productivity in proglacial rivers could be stalled and more nutrients, OM, and contaminants could be transported by particulate matter.