Morphology and sediments of the subaqueous Colville River Delta (Alaska)

Arctic rivers are known to drain ~15% of terrestrial areas and supply ~10% of fresh water to global oceans. Their deltas are important sites of sediment and nutrient sequestration. Unlike temperate deltas, Arctic deltas are characterized by ice processes including disturbance of bed sediments by anchor ice formation, keel scour, and strudel scour. Arctic deltas are relatively understudied in terms of their morphology and sedimentary processes, however—and this morphology is expected to change as annual sea-ice extents wane and more energetic waves are available to transport sediment over a longer season of both river discharge and open-water conditions in the coastal ocean. In this study we collected field measurements to evaluate the morphology, seabed sediment properties, and suspended-sediment transport around the subaqueous Colville Delta front during the early open-water season (Jul/Aug 2021). The delta front lies in water depths of ~3-9 m, and is separated from shore by a shallow topset on the order of 5-10 km wide. Some portions of the upper delta front are disrupted by pits consistent with strudel scour, especially on the east side. Linear furrows ~90 m wide and ~1.5-2 m deep also occur on the upper delta front, and may indicate direct scour by ice that is pushed shoreward and/or furrows caused by under-ice routing of river water in the spring. Sediments include both well-sorted fine sands and poorly sorted muds which include a full range of silt and clay sizes. During a brief 10-day summer mooring deployment in 2021, near-bed turbidities in the delta front region (~5-6 m water depth) were high (up to ~400 NTU) and varied with tide- and wind-driven water-level variations of ~10-50 cm. Muds deposited during the summer are likely reworked by waves and ice in the fall and winter, respectively. These results indicate a range of subenvironments and sediment delivery and reworking processes which are sensitive to proximity to river mouths and exposure to ice forces.