Wind and river effects on a coastal current in Chatham Sound, British Columbia

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Chatham Sound is an ecologically and economically important semi-enclosed basin in the northeast Pacific which has received only limited oceanographic attention. Here we investigate the surface circulation in the southern Sound by analyzing hourly current fields obtained from a high-frequency (HF) radar array, installed there in 2017 and still in operation, as well as other hydrographic and mooring measurements that allow us to characterize subsurface features at several specific locations during 2018 and 2019, and satellite imagery processed to obtain a measure of suspended particulate matter at high spatial resolution. We find that the surface circulation is dominated by the plume of the Skeena River, which flows into the southeast corner of the Sound. On average, this plume forms a northward flowing coastal current in the eastern half of the Sound, and is turbid during the freshet. The plume width changes only marginally even though the Skeena’s flow changes seasonally by a factor of about 7. However, the depth of the plume and its salinity is dramatically different during the early summer freshet compared to values in winter. Fastest plume currents appear near the offshore edges of the plume, even though the plume depth is greatest near the coast. We show that these features are consistent with those predicted from an analytical 1-1/2 layer buoyant coastal current model. Winds can accelerate or retard the coastal current; when strong enough the flow is arrested and turbid plume water fills the southern Sound.