Deployment Notes: Puerto Williams, November 2023
We hit the dock at Puerto Williams just as the grey light of dawn broke over the peaks of Navarino Island. The wind was already screaming across the water, typical for the sub-Antarctic, and the smell of salt and damp peat was heavy in the air. I watched the tide rip through the channel, a violent churning of turquoise and slate-grey water that looked more like a river in flood than a coastal passage. This isn't your typical harbor. It's a hydrodynamic crossroads where the Southern Ocean's raw energy slams into the confined corridors of the Beagle Channel.
The water state was chaotic. We were dealing with a massive freshwater pulse from the glacial melt, creating a visible layer of lighter, brackish water sliding over the dense, cold saltwater pushing in from the Drake Passage. This stratification is the real killer here. It creates a vertical shear so sharp that surface currents often run completely opposite to the deep-water flow. If you're relying on a simple surface float or a point-velocity sensor, you're essentially guessing. The energy levels here make the Norwegian fjords look like ponds.
What We Found
The data coming off the first recovery was startling. We caught a spring tide spike that sent velocities soaring far beyond the historical charts for this specific coordinate. The most surprising bit? The salt wedge. We found a razor-thin boundary layer where the kinetic energy shifted violently. In some bins, we saw the surface water retreating toward the interior of the archipelago while the bottom layers were screaming eastward toward the Atlantic. It's a textbook example of extreme tidal asymmetry. The flood tide doesn't just mirror the ebb here; it's distorted by the jagged bathymetry of the channel, creating these weird, pulsing surges that would make any local captain nervous.
We also saw significant signal attenuation in the lower water column. The seabed is a mess of rocky outcrops buried under thick glacial silt. When the current spikes, it stirs up this organic debris—basically a slurry of peat and minerals—that creates an incredibly noisy environment. I noticed some early bin contamination in the raw files. The acoustic signals were scattering off the particulate load, which usually tells me my blanking distance is too tight. I had to spend three hours in the lab doing a sanity check on the velocity profiles to make sure we weren't just measuring a cloud of silt moving with the current.
Equipment Performance
I ran a mix of 300kHz and 600kHz units. Honestly, the 600kHz unit was a disaster in the deeper sections because the range was too short to get a clean profile of the entire water column. However, in the shallower berths, it gave us the resolution we needed to map that shear layer. The 300kHz unit performed better overall, but it struggled with the thermal gradient. The water temperature drops off a cliff as you go deeper, which shifts the speed of sound. Since we weren't correcting for this in real-time, the Doppler shift calculations were drifting. It's a small percentage error, but when you're trying to ground-truth high-energy flows, those percentages add up. The mooring was the biggest headache. The current is so brutal that if your tripod isn't seated perfectly into the silt, the gear just rolls. We lost one sensor to a 15-meter shift in a single tidal cycle because the weight wasn't distributed right. It was a costly mistake.
Recommendations for Future Deployments
If you're heading into the Beagle Channel, don't trust the charts. Bring more weight than you think you need and tune your acoustics for high turbidity.
- Use 300kHz units for channel-wide profiles to ensure you hit the surface return despite the stratification.
- Increase the blanking distance to 1.5 meters to avoid signal noise from surface bubbles and organic debris.
- Deploy heavy-duty, wide-footprint tripods to prevent sensor migration during spring tide surges.
- Integrate real-time CTD (Conductivity, Temperature, Depth) data to correct for the sound speed shifts caused by the salt wedge.
- Schedule deployments strictly around the neap tide windows to avoid losing gear during the initial drop.
Field report by Sarah Jenkins. Sarah is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience mapping continental shelf currents.
Field Deployment Report: Bottom-Mounted ADCPs in the Beagle Channel Confluence