River Deltas, Here, There, and Elsewhere
Okavango River delta (credit ISS, NASA)
The terminal outflow of a river creates a broad, geologic fan called a delta. The structures from when rivers move carried sediments from uplands to lowlands, estuaries, or oceans but not always.
Africa's Okavango River delta is one river that flows from mountains in Angola but hits a geologic barrier in the interior of Botswana. The river never reaches the sea. It is an ecological wonder attracting photographers and wildlife enthusiasts from around the world during the rainy season as it flows into a lush inland marsh.
According to NASA, the Okavango flows from the well-watered Angolan Highlands (upper image margin)and the flood waters slowly seep across the 150 100 mile-wide delta. The water finally reaches the fault-bounded lower margin of the delta in the middle of winter. The flooded wetlands support a large diversity of plant and animals species in the middle of an otherwise semiarid desert, the Kalahari. The entire Okavango system can be seen in a remarkable photograph captured by the International Space Station.
On Earth, another major river delta is produced by the Brahmaputra and Ganges Rivers when their flow end among the mangrove swamps of the Sundarbans of Bangladesh. NASA's Landsat satellite captured an image of the entire delta region of waterways, mudflats, and forested islands. Wildlife in the Sundarbands includes a wide diversity of animals including tigers, sharks, crocodiles, and freshwater dolphins, as well as many bird species. The dark blue region represents a national park that is protected by both India and Bangladesh. The pressure of large human populations outside the park is seen in the lighter green, deforested areas to the north and west.
Brahmaputra River Delta and the Sundarbans (credit: NASA)
In earlier geologic eras, Mars also had flowing rivers of water that created deltas before the planet lost its surface waters. Fossil signatures remain that show meandering river channels which once drained into a shallow lake leaving sediment tracks before the delta dried away. A fossil delta is in the Eberswalde crater, in Mars southern highlands of Mars.
According the ESA, the Mars delta with its "feeder" channels are well preserved, covering an area of 45 square miles. Small, meandering channels near the top of the crater, would have filled it to form a lake. After depositing sediments into the ancient lake, fresher sediments built up to cover both the channels and the river delta. These secondary sediments were later eroded by wind to expose an inverted, feathery relief of the delta's structure.
Another ancient Martian river delta exists within Jezero Crater and is a prime candidate for investigations by the Perseverance rover that successful landed near the structure recently.
Ancient Mars river deltas: Eberswalde Crater (credit: ESA) & Jezero Crater (credit: NASA)
Stranger still are aquatic systems that exist and flow across the surface of Saturn's giant moon, Titan. The Cassini mission was in orbit around the ringed planet for 13 years before it plunged into the planet once its nuclear fuel was exhausted. During Cassini's time at Saturn, it surveyed many of the planets moons and used it radar system to peer through Titan's thick, smoggy atmosphere.
During these orbits, the probe captured images of rivers and lakes on Titan, consisting of liquid ethane and methane at -290F, but deltas seemed to be missing. The mystery of why liquids on Earth and Mars carried sediments which formed deltas when deposited and not similarly on Titan, may be due to temperature differentials between the moon's liquid hydrocarbon rivers and the seas they empty into. It is also possible that some geological or chemical issue is at work there that is unique to Titan. More investigations of this strange moon are needed to solve the mystery. WHB
Titan Lakes (credit: JPL/Caltech/NASA/Cornell))