Slide 1Po RiverSlide 3Po RiverSlide 5Slide 6Slide 7Slide 8Slide 9Po River todaySlide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Writing Earth history with continental-margin sedimentary processesSTUPID HUMAN TRICKS Po and Mississippi RiversPo RiverDrainage basin includes: south flank of Alps, north flank of ApenninesArea = 75,000 km2(Columbia River = 670,000 km2)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 basin2/27/99Po RiverDrains northern Italy Enters the northern Adriatic Sea, forming delta in late HoloceneEvidence of lobe switching dated in sediments back to Bronze AgeSedimentation causes northward migration of deltaSediment began to enter Venice lagoon to northFilling of lagoon would have eliminated the “natural moat” protecting the Republic of VeniceTherefore, path of Po River altered to a southward location (1599-1604)Ancient Po mouth had many distributaries covering a large areaFlow was not concentrated at a narrow location on coastBefore human controlNatural rate of coastal progradation 4 m/y from 1000 B.C. to 1200A.D. Seven points of discharge described by Plinio il Vecchioabandonment of Po di Primaro1152 AD a major natural avulsion northward shifting of distributary channelsVenice Republic is threatened by the potential infilling of the lagoon 1604 AD digging of a diversionary canal (Taglio di porto Viro) The artificial form of the modern deltaCalendar YearsPastures to CroplandDeforestation& MiningDams & DiversionsModern Po DeltaFive distributaries, but only one operates during non-flood conditionsPredominant southward transport and accumulation of sediment, which causes northward migration of deltaPo River todaySediment supply reduced by dams and mined for construction aggregateSediment reaching river is constrained by artificial levees (i.e., does not reach floodplains), and carried to Adriatic SeaCuspate delta builds into Adriatic Sea~50% of sediment dischargeLocated far enough south of Venice, sediment carried away~50% of sediment dischargeRiver manipulation caused fast progradation and delta lobe switchingRapid accumulation of a large amount of sediment caused by confining the Po discharge to a small areaThe present delta is artificially held in position within context of the northern Adriatic oceanographic processesNatural and artificial subsidence are enhanced by levee construction, which prevents overbank sedimentationRiver-bed excavation and river damming in the drainage basin during the last 50 years led to a marked decrease in sediment supplyResult of human changes to Po River mouthOctober 2000 floodDespite the best intentions of humans...Landsat 1 - Jan 16,1973Writing Earth history with continental-margin sedimentary processesList of some things we didDammed river – reduced sediment supply, 400 million t/y to 200 t/yPrevented lobe switching – eliminated sediment supply to other parts of delta plainBuilt artificial levees – reduced nourishment of flood plainConstrained flow to modern Balize delta – caused delta to build to shelf break, and lose sediment to continental slopeRemoved water and natural gas – accelerated consolidation of deltaic sediment and subsidence of land surfaceDredged channel – created depressions that act as sediment
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