The Earth Story

A mini estuary Today, the boundary between rivers and the ocean is shaped by events that happened about 10,000 years ago. Before that time, large ice caps covered portions of North America, Europe, and Asia, locking up water on the land and dropping ocean levels by more than 100 meters. Rivers still flowed to the ocean, but they flowed across the land that was exposed when the oceans dropped. The rivers eroded some of this land, creating canyons, valleys, and floodplains surrounded by levees.

Sunset over La Pointe du Cap-Ferret, France

Planning for a tsunami

Off the coast of Oregon and Washington, a piece of ocean crust known as the Juan de Fuca plate is being pushed down into Earth’s mantle at a gigantic fault known as the Cascadia subduction zone. That fault is a megathrust, the same type of fault that produced the 2004 Indonesia tsunami and the 2011 Japan tsunami.

That fault has a long record of producing giant earthquakes and tsunami, even though there aren’t written records from many of them except possibly for the most recent one about 300 years ago. Some of those tsunami are testified to by sand layers on shore, others by parts of the land which were submerged and then re-exposed as the crust bends (http://tmblr.co/Zyv2Js1dbFv2t).

This fault has produced massive earthquakes and tsunami before and it will do so again. With the recent tsunami disasters still in mind, governments continue funding projects to better understand the hazards of tsunami waves to populated lands.

Estuaries – the open bays at the end of many large rivers – can concentrate tsunami energy. The waves get inside them and can be focused, driving water levels upward and damaging harbors and cities throughout.

This image shows a model created by scientists at Oregon State University of a tsunami wave hitting the Columbia River. You can see many places where tributary rivers that enter the estuary are at risk from flooding – as much as 4 meters along coastlines inside the estuary.

These models are extremely useful in helping planning for future tsunami impacts. The areas at greatest risk for flooding are areas that should have building codes that could allow structures to survive a large tsunami – possibly even elevation above possible wave heights. These areas also should have active evacuation and warning plans in place, in addition to drills to get people used to hearing and responding to the warnings.

These models also are useful for showing the conditions that are most hazardous. Surprisingly, the high flows on the river in the spring don’t really impact tsunami heights that much on this river. On the other hand, daily tides do. If a tsunami hits at high tide, its impact is much worse than a tsunami that hits at low tide. That information can also be used in planning responses at the time an earthquake hits and in knowing what the maximum reach of the waves could be.

-JBB

Image credit: OSU http://oregonstate.edu/ua/ncs/categories/college-engineering

https://phys.org/news/2015-02-outlines-impact-tsunami-columbia-river.html

The Chesapeake Bay Crater

35 million years ago, the Earth was much warmer than today due to higher abundances of carbon dioxide in the atmosphere. There were either small or no ice caps at the poles, leading to high sea levels that flooded the edges of the continents that are exposed today. This was the situation when a large rock from space came hurtling in, hitting the submerged continental shelf off the coast of what is today the state of Virginia. The object was between 3 and 5 kilometers in diameter. The impact of this rock caused a massive explosion and formed a crater 85 kilometers in diameter. It shattered the existing rocks, creating a debris pile known as a breccia that has since been buried by sediment shed off the continent.

The Chesapeake Bay Impact Structure still controls features of the shoreline today. It’s no coincidence that it sits right at the mouth of the modern day Chesapeake Bay.

The presence of this crater has caused the land above to subside, or sink, more than the surrounding land for 35 million years. That subsidence has caused rivers to flow to this location, making it a central point for erosion and downcutting.

15,000 years ago, sea level was much lower due to the presence of huge ice caps, so rivers flowed through the Chesapeake Bay impact structure out to sea, cutting wide canyons. When sea level rose, it flooded those canyons, creating shoreline features called estuaries around the world. The Chesapeake Bay is an estuary created by the flooding of river valleys that were fixed in location by an asteroid impact 35 million years ago. The East Coast of the United States today is shaped, in part, by a rock from space that landed tens of millions of years ago.

-JBB

Image credits and more: http://pubs.usgs.gov/fs/fs49-98/ https://commons.wikimedia.org/wiki/File:Chesapeake_Crater_boundaries_map.png

The world's widest river.

The misnamed Rio Plata (silver river) occupies what was once a broad river valley that was drowned by rising seas during the melting at the end of the last ice age. The Spanish colonists hoped to find another Potosi inland, and gave it its name in the hope that tonnes more silver would flow down it to enrich Spain. It is about 290Km long, and starts at the confluence of the Rio Parana/Paraguay delta (that flows over Iguazu falls many miles to the north) and the Rio Uruguay. Its funnel starts two Km wide, opening out to about 220 at the Atlantic coast, making it the world's widest river (some dispute this, considering it a marine gulf). It now forms the border between Argentina and Uruguay, flowing above the Precambrian Rio de la Plata craton, and was the site of a major naval encounter in the last world war.

The drainage basin that feeds it is the second largest on the continent, after the Amazon, covering about a quarter of the landmass, starting far to the north in southern Brazil and Paraguay. The sediments on both sides of the river are very different. In Montevideo one has lush golden sandy beaches, at Buenos Aires and its delta suburb of Tigre lush treescapes growing in fine dark mud. This is due to the sediment loads of the different rivers as they pass over different lithologies on their meandering journeys, picking up different types of rock particle along the way. The Parana, in the upper left of the image carries muddy silt, and the Uruguay to the right fine sands. A submerged shoal divides it into freshwater and estuarine brackish water.

Loz

Image credit of the river at sunset: Karen Nyberg.

http://www.atlasdebuenosaires.gov.ar/aaba/index.php?option=com_content&task=view&id=317&Itemid=185&lang=en

Beneath the Surface

San Francisco Bay has had it rough. Since the California Gold Rush in the 1850s vastly increased the surrounding population, it’s been mined, dredged for shipping channels, and partially filled for development. In fact, it’s one of the most altered estuaries in the United States. If that’s not enough, it’s also subjected to strong tidal currents that erode and change it constantly.

The Bay area is also heavily populated, and the estuary is subjected to the resulting pollution.

In order to better assess how pollution might travel through the altered ecosystem, several state and national government agencies have gotten together to create maps of the Bay sea floor using multibeam echo sounders (MES). MES, like other sonar, emits sound waves. Those sound waves fan out from below the ship’s hull, strike the sea floor, and then travel back to the receiver to provide the data needed to create the maps.

In order to make the maps easier to read, the water depth has been exaggerated by 4x, and the land by 2x. The result is a sea floor map that is easy to read and can help with management decisions for ecological and commercial interests, and better inform the earthquake-prone area on the local tectonics.

Photo Credit: US Geological Survey https://twitter.com/USGS/status/717102022219005954/photo/1 References: http://pubs.usgs.gov/sim/2006/2917/sim2917.pdf http://pubs.usgs.gov/sim/2006/2917/ http://www.nauticalcharts.noaa.gov/hsd/multibeam.html http://bit.ly/2fCxkZy

Land tendrils into the ocean.

This enigmatic shot was taken by astronaut Karen Nyberg aboard the ISS somewhere above the Brazillian coast.

Loz

http://pinterest.com/knyberg/our-home-planet-earth/

Salt kill

This cluster of dead trees is one of many found in the New Jersey and Chesapeake Bay areas, killed by changes in the ocean.

Sea levels are rising globally, but they don’t rise at the same rate everywhere. Sea level increase occurs because of melting glaciers and temperature increase – as waters heat up they expand. Since waters don’t heat up evenly everywhere around the world, sea level rise won’t be the same from place to place.

On top of that, as the world warms, wind currents and ocean currents are shifting as well. The Gulf Stream, the major current that carries warm water along the eastern shore of North America, is shifting to the north, pushing water up against the Atlantic Coastline. As a consequence, sea levels along the Atlantic are seeing some of the most rapid increases globally. As the salt water hits the tree roots, the end result has been the death of an estimated 40,000 hectares of coastal forest in recent decades.

-JBB

Image credit: Ted Blanco, Climate Central http://bit.ly/2cxGITa http://bit.ly/2cRYFs0

The Silv’ry Tay

“Beautiful Railway Bridge of the Silv’ry Tay! Alas! I am very sorry to say That ninety lives have been taken away On the last Sabbath day of 1879, Which will be remember’d for a very long time.” - William Topaz McGonagall, worst poet in the English language

The beautiful River Tay in central Scotland is, somewhat unfairly, perhaps best known for the nineteenth-century tragedy recorded in McGonagall’s infamous poem, ‘The Tay Bridge Disaster’. A railway bridge had only recently been built to span the 4 km width of the Tay at Dundee, its central girders raised to allow shipping to pass below. During a force 10 gale the high girders collapsed, taking a train with them and killing all 75 passengers. The piers of the old rail bridge can still be seen beside part of the current bridge.

Dundee lies on the Firth of Tay; the word ‘firth’ is related to the Norwegian ‘fjord’, and refers to the river’s estuary, where it discharges huge volumes of water into the North Sea. The Tay is the largest river in the UK by volume of water discharged, easily beating longer UK rivers such as the Thames or the Severn. This is partly because of the size of its catchment area; it drains most of central Scotland, including parts of the Highlands.

Like other Scottish rivers the Tay’s waters are very clear, and this provides favourable conditions for salmon and lamprey, and also for the otters that prey on them. Its pure waters make it one of very few rivers with viable populations of freshwater pearl mussels; these endangered mussels are highly vulnerable to water pollution as well as to over-exploitation, and they also rely on the presence of salmon for part of their life cycle. It is now illegal to disturb the mussels to harvest their pearls. The Tay’s healthy ecology makes both it and its tributaries a Site of Special Scientific Interest (SSSI) and a Special Area of Conservation (SAC).

Image credits: Scotgotchi (falls of Dochart), Tim Haynes (Kinnoull view), Stuart Anthony (Tay Bridge) https://flic.kr/p/tRBR3 https://flic.kr/p/qVoRTD https://flic.kr/p/oVAsPx Sources bit.ly/1UWMSs4 bit.ly/1XXZ4IF http://taybridgedisaster.co.uk/ http://www.mcgonagall-online.org.uk/

Silver Dragon Tidal Bore

When a river broadens into an open area or an estuary as it enters the ocean, it creates the setup for a tidal bore. A tidal bore is a wave on a river driven by the tides: as the tide comes in on an appropriately shaped river, the energy of the tide can be compressed into a gradually more confined area. Eventually, the tidal energy can be focused into a small wave or series of waves that actually run upriver, against the current. Areas with large tides can produce similar waves that run the opposite direction as the tide goes out, where they will even interact with the next incoming tide.

The largest tidal bore in the world is the Silver Dragon Tidal Bore on the Qiantang River in China. The tidal wave on that river can be over 3 meters high and is even surfable; for the last several years during the strongest full moon during the year an actual surfing contest has been held on the wave as it runs upriver through the middle of downtown Hangzhou.

-JBB

Image credit: Gwydion M Williams https://flic.kr/p/6MC4TK

References: http://n.pr/1WRuTlO http://www.worldscibooks.com/engineering/8035.html

MUDFLATS: Superb Fossil locales of the Past and the Future?

The photo included with this post is from the Delta of the Colorado River as it enters the Sea of Cortez (also known as the Sea of California). It shows a rather special example of a mudflat environment. An essential component of most wetland ecosystems, mudflats normally form at the mouths or deltas of rivers or in lagoons; they develop near seashores where they are daily flooded by incoming tides (thus, they are also called tidal flats). The flats that now mark the mouth of the Colorado River resulted from human intervention up-river during the last century via the building of dams and heavy water usage: what once was one of the world’s greatest desert estuaries has now been transformed into nearly 1000 km2 of mudflats.

Possibly less than 0.01% of the earth’s environment today consists of mudflats. However, the rocks that form in these depositional environments seem to have a greater propensity for preservation than many other types of formations; they appear over-represented in the rock record. This is a good thing since mud flat deposits provide exceptional sites for the preservation of animal tracks and trails, plant fossils, and the stray fish, bird, or dinosaur that either died there or whose body was washed downstream, and then stranded when the tides went out.

Renowned mudflats that all geologists learn about include: The Solnhofin Limestone (lagoonal mudflats) of the late Jurassic, source rock of the famous fossil Archaeopteryx. Devonian mudflats host early amphibians along the Bay of Fundy (Blue Beach locality) and, elsewhere, the earliest land plants. The Late Precambrian (~1.4 billion years in age) Belt Formation of Montana, Idaho and Canada includes many strata initially forming as mud flats; in this era before terrestrial life, the fine silty muds preserve raindrop imprints, mudcracks, and ripple marks – features still prominently displayed within the mudflats of our modern era.

Perhaps in some geologic future, study of the Colorado Mudflat Formation will yield the fossilized tracks of off-road vehicles and the skeletal remains of abandoned fishing boats for the study of paleontologists.

Annie R Photo by Annie Griffeths Belt courtesy National Geographic http://photography.nationalgeographic.com/photography/wallpaper/mudflat-colorado-river-delta.html

Other references: http://geography-site.co.uk/pages/physical/coastal/mudflats.html http://archives.datapages.com/data/bulletns/1965-67/data/pg/0049/0007/1050/1090b.htm http://www.bgs.ac.uk/discoveringGeology/time/Fossilfocus/plant.html http://banjon.smugmug.com/2007/Explore-Nova-Scotia/Blue-Beach-Minas-Basin/2919100_QmEMo/1/157671414_75RG2#!i=157671414&k=bdGqrc2