Deployment Notes: Makassar Strait Interface, December 2023
We hit the water just before 0400 hours, pushing south toward the 5°15'S marker. The air was thick, heavy with that pre-monsoon humidity that makes your gear sweat and your patience thin. As we approached the mouth of the Makassar River, the water changed color instantly—from the deep blue of the Strait to a muddy, opaque brown. I could feel the vessel fighting a weird, surging current. It wasn't just the tide. We were sitting right in the crosshairs where the river's discharge slams into the Indonesian Throughflow (ITF), and the turbulence was palpable through the hull.
The conditions were volatile. The Northwest monsoon was already starting to push massive volumes of freshwater out of the Sulawesi highlands and into the Strait. This creates a chaotic environment. You have surface currents ripping seaward while the denser, saltier water from the Makassar Strait tries to wedge its way back upstream. It's a hydraulic war zone. The wind was gusting, kicking up a choppy surface that made keeping the vessel steady for the initial calibration a nightmare.
What We Found
The data came back uglier than I expected. We clocked surface discharge velocities hitting 1.2 m/s. That's a hell of a push. But the real story was happening ten meters down. We found a razor-sharp pycnocline—a density boundary so distinct it acted like an acoustic mirror. In some bins, the velocity flipped direction entirely. One minute we're tracking freshwater rushing out; ten minutes later, we're seeing the salt wedge creeping upstream, riding the bottom of the channel. It's a stratified mess.
I saw the junior techs staring at the screen in confusion. They were treating the water column as one solid block of liquid. I had to stop them. If you ignore that density shift, your flow calculations are basically fiction. We found that the saltwater intrusion wasn't just a slow seep; it was an aggressive wedge. During the transition between monsoon phases, this wedge can migrate kilometers upstream in a matter of days. Anyone relying on a single-point surface measurement in this basin is just guessing. We caught a vertical velocity profile that showed a massive shear zone, which explains why the dredging models for this port have been so wildly inaccurate lately.
Equipment Performance
I went with a vessel-mounted ADCP for this run because the bathymetry is too erratic for bottom-mounting. The depth swings from 4 meters in the upper reaches to over 25 meters near the mouth during high tide—it's a rollercoaster. The unit handled the turbulence well, but we fought a constant battle with signal attenuation. The river carries a brutal amount of suspended sediment from the highlands. Mechanical meters would have choked and died within 48 hours; I've seen impellers fouled by silt in this basin before. The ADCP gave us a clean signal, provided we kept the sampling rate high and the blanking distance tight. However, the salt wedge created some nasty bin contamination. Without constant speed-of-sound corrections based on real-time salinity probes, the data would have been useless. I’ve seen too many engineers skip the salinity correction in these zones. It's a rookie mistake that ruins an entire transect.
Recommendations for Future Deployments
Stop relying on surface drifters. They tell you nothing about the boundary layer effects that actually drive sediment transport in the Makassar Basin. If you want a sanity check on your flow models, you need full-column profiling.
- Use High-Frequency Transducers: Use 600kHz or higher to maintain resolution in the shallow, high-shear zones near the river mouth.
- Mandatory CTD Integration: Pair every ADCP deployment with a Conductivity-Temperature-Depth (CTD) cast. If you don't account for the pycnocline, your velocity data is garbage.
- Avoid Mechanical Sensors: Silt loads here are too high. Stick to acoustics to avoid impeller fouling.
- Time Your Surveys: Schedule deployments to overlap both the Northwest and Southeast monsoon transitions to actually map the salt wedge migration.
The Makassar Strait is an aggressive environment. The ITF doesn't play nice with fluvial discharge. You can't just drop a sensor and hope for the best; you have to hunt the data, adjusting for the density shifts in real-time or you'll end up with a spreadsheet full of optimistic guesswork.
Field report by Capt. Marcus Thorne. Capt. Thorne is a specialist in underwater acoustics and port hydrography with twenty years of experience in high-energy maritime environments.
Field Deployment Report: Combatting Salt Wedge Intrusion in the Makassar River Basin