Some things are less obvious about large rivers like the Amazon. In some ways, the water we observe at station Obidos, which is no more than 70 meters deep, is far more complicated than the spacious Southern Ocean, which reaches depths of 4000 meters on average.
Physical oceanographers who specialize in tracing the movement of water beneath the ocean surface will certainly disagree with that statement. Nonetheless, my reasoning stems from how marine chemists, particularly those who care about organic carbon, envision oceans and rivers to work beneath their surface. In general, it is enough for us chemists to assume that most solid particles in the Southern Ocean move vertically by the force of gravity, sinking downward from the surface, from their origins in masses of living algae. Hence, sampling the sinking particle from one location of the Southern Ocean provides a distinct, stand-alone measurement at that particular location of the ocean.
The fact that sediment movement in the Amazon River is water-driven, and therefore predominantly horizontal, changes the game completely. This difference means that estimating the sediment load at one point of the river’s path, such as at Obidos, does not necessarily represent the entire sediment flow across that location; one measurement may not suffice. If one were to cut a slice out of the river at that particular point and view it from its side, water speed and sediment load vary considerably across the entire slice. The result is that, while you might find one sediment concentration and flow rate at the very center top of this slice, i.e., right in the middle of the river, you will find different values 10 feet deeper, or 20 feet away.
We can be sure of this by using the Acoustic Doppler Current Profiler (ADCP). Though small and compact, this instrument can read the river slice by slice. Here, at station Obidos on the Amazon River, we have transited the channel from bank to bank multiple times with the ADCP. With roughly every second in transit, the ADCP measures the range of water speeds as they vary layer by layer, from surface to bottom, generating a vertical profile of current velocity. As it crosses the river, ADCP software glues each vertical profile side by side, creating a mosaic of current velocities at every depth and location of this transect, a slice of the Amazon River. A slightly different crossing would generate yet another slice.
This is how the ADCP looks once it is submerged right under water. It needs to stay here to take measurements.
And we need to stay close enough to read ADCP measurements in real-time.
The learning curve for operating the ADCP and reading the data it collects for us is steep, and I have not yet reached its peak. Nonetheless, I have accumulated enough information so far to understand how much current velocity can very in a given slice of the Amazon River. Though some things about river chemistry are obvious from the bird’s eye view, from the deck of Joao Felipe II, such as its sedimentary composure, it is not obvious that the movement of water along the surface could differ so much at depth. Thankfully, the ADCP extends our ability to perceive these differences.
Because our sediment analysis back in the lab will depend on our understanding of water velocity, data from the ADCP will be particularly useful in making the most accurate estimates from our chemistry. If we take only one measurement from one part of the station Obidos slice, for example, we will be able to understand how representative this one measurement is for the rest of that station.