1. ETM: The Estuarine Turbidity Maximum
Puget Sound Oceanography 2011

2. ETM
The Estuarine Turbidity Maximum (ETM) is a region of locally-elevated suspended matter concentration that occurs near the landward end of salt intrusion in estuaries
It relies on the existence of a region of erodible sediments (the “Mud Reach”) which are resuspended on tidal time scales
The patch of erodible sediments exists because of persistent along-channel convergence of sediment transport
Ref: Geyer (1993) The importance of suppression of trubulence by stratification on the estuarine turbidity maximum. Estuaries, 16(1),

3. Estuarine Turbidity Maximum (ETM)
Depth
River
Ocean
Chesapeake Bay
Hudson R.
Elbe River
St. Lawrence R.
Gironde River
San Francisco Bay
Columbia R.
ETMs can be found at the head of estuaries where salt water and freshwater meet. The convergence circulation in this region traps sediment and plankton. ETMs can be found throughout the world, including in the systems pictured here.

4. Stratification, turbidity, salt front location
influenced by freshwater flow
May 1998 (above average freshwater flow) 15 10 5
May 2,1998
200
Depth (m)
May 1999 (below average freshwater flow)
NTU 10 20 30 40 50 60 70 15 5
May 17,1999
Salt front 20 km further up-estuary in low flow year and turbidity much lower (note difference in color scales)
NTU = Nephelometric Turbidity Units (based on shining a light into the fluid)
Kilometers from River Mouth
Sanford et al. 2001, North and Houde 2001

5. Physics Part 1: The Rouse Profile
Sediment concentration is highest near the bottom
Results from a balance between particle sinking speed and turbulent diffusion
Slow-sinking particles, high turbulence
Geyer 1993 Estuaries
Fast-sinking particles, low turbulence

6. Physics Part 2: Along-channel Convergence
The exchange flow modifies the tidal currents
EBB is strongest in the river end
FLOOD is strongest (at the bottom) in the estuary
From Sanford et al. (2001) Reconsidering the Physics of the Chesapeake Bay Estuarine Turbidity Maximum. Estuaries, 24(5),

7. Observations about particle trapping in ETMs
Particle trapping in ETMs occurs by asymmetrical tidal transport of a pool of resuspendible particles with a limited range of settling velocities
Fine sediments in estuarine environments almost always exist in aggregated (flocculated) form. Aggregation and disaggregation can be active processes, depending on concentration, stickiness, and small scale shear.
Settling velocities of fine sediments trapped in ETMs are determined by the aggregate properties (size and specific density), not the individual particle properties
Typical particle settling speeds:
Fine silt 0.1 mm/s
Medium silt 1 mm/s
Fine sand 3 m/s
Flocs: can be much faster

8. ETM particles are packed with organics
> 63 um
20 to 63 um
10 to 20 um
< 10 um
Close-up
DAPI stained
This is what the particles in the ETM look like under environmental scanning electron microscopy. I have them broken into size fractions. The largest fraction was composed of intact diatom chains, large fecal pellets,and animals like this rotifer. Same thing with the next fraction. Centric diatoms and diatom frustules, mineral grains, and protozoa like this tintinnid ciliate. This last fraction, the < 10 um fraction, contained nearly all of the active bacteria. These are the bacterially-active particles. This is a closeup, and you can see that it is a small aggregate of broken diatom frustules, unidentifiable little particles, and mineral grains all held together with some sort of organic matrix. This last picture is a DAPI stained microaggregate just to show you that they are heavily colonized with bacteria.
Wikipedia: DAPI or 4',6-diamidino-2-phenylindole is a fluorescent stain that binds strongly to DNA. It is used extensively in fluorescence microscopy. Since DAPI will pass through an intact cell membrane, it may be used to stain both live and fixed cells, though it passes through the membrane less efficiently in live cells and therefore doesn't stain as efficiently.
Columbia River
Crump & Baross, 2000