ETM: The Estuarine Turbidity Maximum

PSO 2009

Chesapeake Bay

Hudson R.

Elbe RiverSt. Lawrence R.

Gironde River

San Francisco BayColumbia R.

Dep

th

River Ocean

Estuarine Turbidity Maximum (ETM)

Dep

th (m

) 15

10

5

May 2,1998

0

200

Kilometers from River Mouth10 20 30 40 50 60 70

15

10

5

May 17,1999 0

50

May 1998 (above average freshwater flow)

Sanford et al. 2001, North and Houde 2001

May 1999 (below average freshwater flow)

Stratification, turbidity, salt front locationinfluenced by freshwater flow

NTU

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

ETM particles are packed with organics> 63 um> 63 um 20 to 63 um20 to 63 um 10 to 20 um10 to 20 um

< 10 um< 10 um Close-upClose-up DAPI stainedDAPI stained

Columbia RiverCrump & Baross, 2000