III. Sedimentology

 

A. Sediment transport and erosion

Each year the Amazon transports suspended sediment to the delta plain. On average, the sediment is composed of 1240Mt from Andean erosion and 3200Mt from flood reworked plain sediments (“Channel floodplain geomorphology along the Solimoes-Amazon River in Brazil.”, by Leal A. K. Mertes, Thomas Dunne, Luiz A. Martinelli. From Geological Society of America Bulletin, September 1996.). Sediment exchange between the flood plain and channel also deposits sediment in the rivers. The main methods of this exchange are: band erosion, bar deposition, settling from diffuse overbank flow, and sedimentation in flood plain channels (“Exchanges of sediment between the flood plain and channel of the Amazon River in Brazil” by Thomas Dunne, Leal A. K. Mertes, Robert H. Meade, Jefferey E. Richey, Bruce R. Fursberg. From Geological Society of America Bulletin, April 1998).  Different parts of the river exhibit different erosion and deposition patterns.  In general, upstream there is sediment erosion in the main channel and deposition in the flood plain channels[1].  This leads to what is known as "scroll bar topography," that is terrain characterized by hundreds of long narrow lakes. Oxbow lakes in such areas quickly vanish as a consequence of this process.  In contrast, in further downstream areas, channels are restricted by long-term, stabilizing levee building and flood plain construction, dominated by overbank deposition. This process buries scroll bar topography, producing a flat flood plain covered by a patch work of large, shallow lakes. Such flood plains are recycled in less than 5000 years, and at even faster rates further upstream (“Channel floodplain geomorphology along the Solimoes-Amazon River in Brazil.”, by Leal A. K. Mertes, Thomas Dunne, Luiz A. Martinelli. From Geological Society of America Bulletin, September 1996.). 

The combined exchanges of sediment transport define a "sediment budget.” Such budgets estimate that an average of 2070Mt of sediment is exchanged per year.  This sediment can be broken up into four groups: 1) Sediment entering the channel from bank erosion ~1570Mt/yr, 2) Sediment transferred from channel transport to bars ~ 380Mt/yr, 3) Channelized flow in flood plains ~ 460 Mt/yr, and 4) Diffuse overbank flow in the flood plain ~ 1230Mt/yr. In total, "deposition on bars and flood plain exceeded bank erosion by ~ 500 Mt/yr over a 10-16 yr period,” meaning there is a net accumulation of sediment both on the valley floor and in the delta plain each year.

Understanding this accumulation of sediment over time can be important to understanding where pollutants such as mercury flow, and thus where the effects of such pollutants should be studied.  Understanding the flow of nutrients provided by the regular deposition of sediment also helps to understand how a change upstream in sediment collection may effect the ecosystems surrounding the river. (“Exchanges of sediment between the flood plain and channel of the Amazon River in Brazil” by Thomas Dunne, Leal A. K. Mertes, Robert H. Meade, Jefferey E. Richey, Bruce R. Fursberg. From Geological Society of America Bulletin, April 1998)

B. The effect of hydroelectric power dams on the river

Reservoirs have both positive and negative effects on the upstream and downstream environments due to the modification of the natural flow conditions. These effects include higher temperatures, with little to no variation in temperature throughout the course of a year; increased forest flooding, critical situation in reservoir filling (from the sediment dropped when the water slows in the reservoir), decreased residence time, increased eutrophication, increased gas formation, corrosion of equipment and a decline in the water quality downstream.  One possible solution that would negate these negative effects is hydraulic equipment to reaerate reservoirs ("Water Quality Simulation in Reservoirs in the Amazon Basin: Preliminary Analysis" by Carlos Eduardo Morelli Tucci. From Water Management of the Amazon Basin).


C. Monitoring

A NASA project called the "Global Rainforest Mapping project (GRFM) by an orbiting spacecraft using radar imaging, (Japanese JERS-1 Synthetic Aperture Radar (SAR))” is capable of monitoring sediment flows.  The first SAR mapping of the Amazon (during low flood season) took 62 days. Another was done during high flood season. The advantage of SAR technology is its ability to be used at night and to see through clouds.  This is particularly important as some areas of the rainforest are under perpetual cloud cover.  This data may be useful in trying to model the carbon cycle and climate change. In addition, it can be used as a baseline with which to compare future data collections (http://southport.jpl.nasa.gov/amazon/imagebrowser/jamms.html).

Optical Backscatter (OBS)

This method uses photodiodes positioned around an emitter to measure the light reflected by a given sample.  The method requires an empirical calibration to convert the measured backscatter to a concentration. Measurement sample volume "is on the order of several cubic centimeters," meaning it can best measure 200-400μm particles, and concentrations of up to 100g/L.  These devices are readily available and relatively inexpensive.  However, they require manpower to run the tests. There are many slight variations to this method.

Acoustic

This method takes advantage of the fact that sediment reflects a certain amount of sound depending on its concentration and particle size and the frequency of the sound.  Short bursts[2] of high frequency[3] are emitted from a transducer.  Using multiple frequencies, it is possible to determine both the particle sizes[4] and concentrations[5].  This technique can also be used to measure a vertical profile of sediment concentrations for depths of 1-2m. This acoustic technology is still under development.

Spectral reflectance

Suspended sediment concentrations are measured using the amount of radiation reflected from a body of water and the properties of that water.  This can be measured using a handheld, airborne, or satellite based spectrometers. One major advantage of satellite-based spectrometers is the ability to measure a much larger area[6]. Because of the sheer size of the Amazon Basin rainforest, satellites are clearly a better choice.

Digital optical

"A charge-coupled device records the sediment/water mixture in-situ." Sediment can be analyzed for size as well as the concentration of suspended sediment particles.  The technology is still in the development stage.  Currently the technology is dependant on light penetration.  Ideally a computer could remotely analyze the light penetration, and hence the soil size and concentrations ("Surrogate Techniques for Suspended-Sediment Measurement" by Daniel G. Wren, Roger A. Kuhnle).

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[1] An order of magnitude smaller than the main channel

[2] 10μs

[3] 1-5Mhz

[4] 62-2000μm

[5] Up to 30g/L

[6] 1-1,000,000m2