Mekong delta

Phnom Penh

6 ka BP 5 ka BP 4 ka BP 3 ka BP 2 ka BP Shoreline 8.0 ka BP

Ho Chi Minh City

1 ka BP

Returned last month from investigation during high-flow condition Xue et al.

Mekong delta

Tidal river Mangrove coast Inner shelf

Po River

Drainage basin includes: south flank of Alps, north flank of Apennines Area = 75,000 km 2 (Columbia River = 670,000 km 2 ) Sediment discharge = ~15 million t/y (Columbia River = 10 million t/y) Po sediment yield (production from square meter of land surface) is much greater than Columbia River basin

2/27/99

Po River

Drains northern Italy Enters the northern Adriatic Sea, forming delta in late Holocene Evidence of lobe switching dated in sediments back to Bronze Age Sedimentation causes northward migration of delta Sediment began to enter Venice lagoon to north Filling of lagoon would have eliminated the “natural moat” protecting the Republic of Venice Therefore, path of Po River altered to a southward location (1599-1604)

Ancient Po mouth had many distributaries through time, covering a large area

Before human control



Natural rate of coastal progradation 4 m/y from 1000 BC to 1200AD



Seven points of discharge described by Plinio il Vecchio

The artificial form of the modern delta

    abandonment of Po di Primaro 1152 AD a major natural avulsion northward shifting of distributary channels  Venice Republic is threatened by the potential infilling of the lagoon 1604 AD digging of a diversionary canal (Taglio di porto Viro)

Pastures to Cropland Deforestation & Mining Dams & Diversions Calendar Years

Five distributaries, but only one operates during non flood conditions Predominant southward transport and accumulation of sediment, which causes northward migration of delta Modern Po Delta

Po River today

Sediment supply reduced by dams and mined for construction aggregate Sediment reaching river is constrained by artificial levees (i.e., does not reach floodplains), and carried to Adriatic Sea Cuspate delta builds into Adriatic Sea ~50% of sediment discharge Located far enough south of Venice, sediment carried away ~50% of sediment discharge

Result of human changes to Po River mouth River manipulation caused fast progradation and delta lobe switching Rapid accumulation of a large amount of sediment caused by confining the Po discharge to a small area The present delta is artificially held in position within context of the northern Adriatic oceanographic processes Natural and artificial subsidence are enhanced by levee construction, which prevents overbank sedimentation River-bed excavation and river damming in the drainage basin during the last 50 years led to a marked decrease in sediment supply

Despite the best intentions of humans...

October 2000 flood

Bigger sediment supplies have built thicker deposits on continental shelves during the Holocene the Amazon shelf

Interdisciplinary Oceanographic Processes on Amazon Shelf

Amazon dispersal system extends 1000s km – land and sea Tidal river and coast extends 1000 km seaward from Obidos

Muddy beaches in mangrove forests at tropical latitudes

Substantial coastal accretion downstream of river mouth Coastal erosion along some parts of coast

Regions of sediment accumulation and erosion Mudcapes along northern South America They extend from Amazon mouth to the Caribbean Sea

Source ~1200 x 10 6 t/y Sink ~800 x 10 6 t/y Likely, ~1/3 missing But large confidence limits Tidal river is sink?

We know nothing directly about mass accumulation or PROCESSES

Amazon tidal river

Sedimentary processes in tidal rivers

Accumulation of sedimentary strata in tidal river and mangrove forests

A = 7.1 mm/y R 2 = 0.66

Tidal layering Active sediment accumulation

River Discharge – S. Distributary near Macapa

350000 300000 250000 200000 150000 100000 50000 0 6:00:00 -50000 -100000 12:00:00 Mean ~128,000 [m³/s] 18:00:00 0:00:00 Example Velocity Transects 200000 Mean ~213,000 [m³/s] 150000 100000 50000 0 6:00:00

Total River Discharge near Almeirim

12:00:00 18:00:00 0:00:00

2 1 4 3 0 09:00 10:00 11:00 12:00 13:00 14:00 15:00 Peak velocity on 16:00 ebb despite shallow pressure gradient 17:00 18:00 19:00 20:00 0.6

0.4

0.2

ebb

0 -0.2

flood

-0.4

4 09:00 x 10 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 31 170 m 3 30 2 1 0 29 28 27 AQD4 deployment location

channel width 10-14 m

Highest SSC values associated with temperature gradient at beginning of ebb (short lived) Longer-lived elevated SSC associated with peak velocity

Amazon sediment trapped in mouth of tributary

0 2 4 6 8 10 12 14 16 18 29.9

26-Aug-2011 08:17:01 30 30.1

temp (deg C) 30.2

30.3

0 2 4 6 8 10 12 14 16 18 CTD cast 45 WP 72 0.01 0.02 0.03 0.04 0.05 0.06

OBS (volt)