Field Deployment Report: Bottom-Mounted ADCPs in the Guadalupe River-South Bay Interface

Deployment Notes: South San Francisco Bay, January 2024

We hit the water just before 0500, fighting a stiff breeze that whipped across the mudflats. The air was cold, but the water was colder. I could see the sediment plumes from the Guadalupe River churning into the bay, turning the water a thick, opaque brown. It is a chaotic environment. You have the freshwater discharge from the Santa Clara Valley slamming into the landward push of the saline wedge from the Pacific. If you've never worked in a salt wedge, imagine two conveyor belts moving in opposite directions, stacked on top of each other, separated by a thin, turbulent layer of brackish slurry.

The site conditions were typical for mid-winter. High runoff from the valley had the river pushing hard, creating steep salinity gradients that fluctuate wildly with every tide cycle. The bathymetry here is treacherous for instrumentation; it's a shallow, nutrient-heavy basin where the bottom is less 'sand' and more 'thick, anaerobic muck.' This kind of terrain makes mooring a nightmare because your gear wants to sink six inches into the silt the moment you let go of the line.

What We Found

The data came back with a shock. We saw a vertical shear that was far more aggressive than the previous seasonal models predicted. At the surface, the ebb flow was dominating, pushing freshwater out toward the Golden Gate. But just a couple of meters down, the saline wedge was marching inland, carrying a heavy load of denser, saltier water. The velocity reversal happened in a surprisingly narrow band. It was a textbook example of estuarine circulation, but the magnitude of the landward flow during the flood tide caught us off guard. It's a violent tug-of-war under the surface.

I noticed a recurring pattern of 'noisy data' during the peak of the flood tide. The turbidity was so high that we started seeing significant signal attenuation. We were essentially trying to ping through a wall of suspended silt. In a few of the bins, the correlation dropped below 60%, which is where I stop trusting the numbers. I've seen this before in the Chesapeake, and it's the same story here: the sediment acts like a sponge for acoustic energy. Without a tight signal fence, you're just measuring the noise of the river's runoff rather than the actual water velocity.

Equipment Performance

I opted for a 600kHz ADCP for this run, and honestly, it was the right call. A 1200kHz unit would have been blinded by the sediment within hours, and a 300kHz unit is simply too clumsy for these shallow reaches. The 'blanking distance' was still a pain (it swallowed about 15% of our lowest water column), but that's the price you pay for bottom-mounting. The tripod mount held up, though we had to double the weight of the concrete anchor to keep the unit from tipping in the mud. Vessel-mounted units are fine for a quick sanity check, but they are useless for capturing the long-term ebb and flood asymmetry that defines the South Bay. Only a fixed bottom-mount gives you the truth about the salt wedge movement.

Recommendations for Future Deployments

If you're heading back into the Guadalupe interface, don't wing it. The environment changes too fast between spring and neap cycles to rely on old charts.

• Stick to 600kHz transducers to balance resolution against signal attenuation in high-silt zones.
• Over-engineer your anchors. Use heavy concrete bases to prevent 'tilting' in the soft mudflats, or your vertical profiles will be skewed.
• Set a narrow signal fence to eliminate side-lobe interference from the shallow bottom.
• Schedule deployments to avoid the peak winter runoff if you need ultra-clean correlation data.
• Always perform ground-truthing with a handheld current meter at the surface to verify the ADCP's top bins.

The South Bay is a fickle place. You can have a calm morning and a sediment-choked afternoon. But if you get the configuration right, the data is gold for understanding how the bay breathes.

Field report by Dr. Alistair Vance. Dr. Vance is a specialist in underwater acoustics and estuarine dynamics with twenty years of experience deploying instrumentation in complex coastal environments.