8 December 2014: “Shaping” the river depth

by srosengard

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First meeting before disembarking

Because it takes a long time to process samples from the field, I just got my first major data points from the river sediments we collected this past March and July. The data measure (1) what percentage of the sediment is and (2) the composition of carbon isotopes in each sample which I will represent by the symbol δ13C. These metrics are useful for making broad statements about where the river organic carbon comes from (e.g., algae vs. various plant types), how these vary with season, and what further analyses of greater specificity need to be done.

Once you start plotting the data in different graphs, you can see different shapes and forms of it emerge across the times and in locations of the river’s cross-section that we sampled. The most obvious shape so far is how the percent organic carbon and δ13C values of sediments between the river surface and depths closer to the river bed. The same shift by depth occurs in March and July.

We are not the first to see this, but it somewhat comforting to verify previous scientist’s observations that the Amazon River is more complicated than it looks. It means that you can’t just take one measurement at one time – one sample – and say that this data point scales with all the carbon moving through the river system towards the ocean. It means that the solid material which the river carries in the surface can be significantly different in composition from the material it carries at depth. What we see in the surface can come from different sources or parts of the drainage basin, and can also interact with the river environment differently on its way towards the Atlantic Ocean.

This is a typical complexity of all large, deep rivers. The volume of water in flux is so great that the denser (which tends to be heavier and larger) sedimentary material always ends up deeper in the river. Scientists have tried to fit all this mathematically into one equation, called the Rouse profile equation, named after the scientist who formulated it (Rouse 1950). The equation allows you to calculate the amount of sedimentary material (or a specific chemical measurement in it) at any depth of a river so long as you have one known measurement at one depth. The unique Rouse number, a constant in the equation, encompasses all change with depth expected to occur in that specific river.

Like all scientific models of the real world, the Rouse profile is a simplification. It does not always work. But it is more accurate and practical than assuming that large rivers are all the same with depth. And when it does, it means that one need not take measurements at all depths of a large river because you can calculate, or “model” them, instead (Bouchez et al. 2010).

Carl Johnson, our lab manager, emailed us the data just this past Friday. It’s perfect timing to have fresh data to discuss as we disembark on the boat today. Here’s our course for the coming week. I will reveal the sequence of these sites later on.

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from: http://www.globalrivers.org/expeditions/2014-amazon-river-expedition/

References:

1. Bouchez, J., Metivier, F., Lupker, M., Maurice, L., Perez, M., Gaillardet, J., & France‐Lanord, C. (2011). Prediction of depth‐integrated fluxes of suspended sediment in the Amazon River: Particle aggregation as a complicating factor.Hydrological Processes25(5), 778-794.

2. Rouse H. 1950. Engineering Hydraulics. Wiley: New York.

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