Deployment Notes: Villahermosa Wetlands, Tabasco, October 2023
The humidity hit us like a wet blanket the moment we stepped off the boat. We arrived at the confluence of the Grijalva and Carrizal rivers just before dawn, hoping to catch the peak of the flood tide. The water was a thick, opaque café-au-lait color, churning with the sediment runoff from the highlands. It's a visceral reminder that Villahermosa isn't just a city near a river; it's a city living inside a massive, pulsing hydrological machine. The air smelled of damp earth and diesel, and the current was already pulling hard against our hull.
This isn't your standard open-water survey. We were operating in a precarious hydrodynamic junction where the Gulf of Mexico's tidal pulse pushes deep into the Tabasco coastal plain. The water state was chaotic. Surface currents looked sluggish, but the sheer volume of suspended organic matter and silt told a different story. We were fighting a battle against signal attenuation before we even lowered the first transducer into the water.
What We Found
The data came back with a shock: the vertical shear was off the charts. We recorded a massive discrepancy between the surface flow and the bottom layers. While the freshwater was pushing seaward, the dense, saline 'salt wedge' from the Gulf was sliding inland underneath it. I saw flow reversals in the lower bins that completely contradicted the surface observations. It's a classic stratified column, but the intensity here is extreme. The salt wedge doesn't just sit there; it surges. We found that the flood tide moves slower than the ebb, yet it carries a much denser mass of saltwater, creating a hidden conveyor belt of salinity moving upstream.
Honestly, if you're relying on a single-point flow meter in this environment, you're guessing. You aren't measuring the river; you're measuring a snapshot of a chaotic layer. We noticed that the bathymetry is shifting in real-time. Siltation is so aggressive here that a channel that looked deep on the charts last year had shoaled by nearly a meter in some spots (shallower than expected for October). This constant shifting of the riverbed concentrates the tidal energy into narrow conduits, which spikes the velocity in ways that would make a standard hydrological model crash.
Equipment Performance
I opted for a 1200kHz ADCP for this run, and it was the right call. A 300kHz unit would have given us more range, but in these shallow, turbid waters, range is a vanity metric. The 1200kHz handled the high-frequency noise of the sediment-heavy flow much better. However, we still fought 'ringing' in the transducer during the peak of the flood tide. The backscatter was so intense from the suspended solids that the signal started to blur. We also had a close call with 'bottom bounce.' I set the transducer exactly 2 meters off the bed to avoid bin contamination from the muddy bottom, but the current was so strong it kicked up a cloud of sediment that nearly blinded the instrument for the first six hours. We had to scrub the first few bins of data to get a clean signal.
Recommendations for Future Deployments
Measuring the Villahermosa junction requires a stubborn approach to equipment configuration. You can't just drop and forget. Based on this deployment, here is how I'd handle the next one:
- Frequency Choice: Stick to 600kHz or 1200kHz. Lower frequencies struggle with the acoustic noise generated by high-turbidity tropical deltas.
- Mooring Weight: Use double the standard concrete anchor weight. The Grijalva's discharge can shift a light mooring in minutes, ruining your ground-truthing.
- Blanking Distance: Increase the blanking distance to at least 2 meters. The 'mud-cloud' effect in Tabasco is real and will contaminate your lowest bins.
- Sampling Interval: Set a higher sampling rate during the tidal transition. The salt wedge movement is rapid and easy to miss with long averaging intervals.
- Hardware Protection: Use heavy-duty anti-fouling paint on the transducer faces to prevent organic biofilm buildup in the humid tropical water.
We spent the final afternoon doing a sanity check against local tide gauges. The correlation was there, but the ADCP revealed the hidden complexity of the salt wedge that the gauges simply can't see. It's the difference between knowing the tide is coming in and knowing exactly how much saltwater is pushing into the wetlands. Without the vertical profile, you're missing half the story.
Field report by Sarah Jenkins. Sarah is a specialist in underwater acoustics and oceanographic instrumentation with twenty years of experience profiling complex continental shelf currents.
Field Deployment Report: Bottom-Mounted ADCP Profiling in the Grijalva-Carrizal Junction