Where on Mars did the largest river once flow?

Question

It is generally accepted that once there was running water on the surface of Mars.

Can it be determined from remote sensing data from space where the biggest, largest and longest ones were?

Answer 1

I suggest that the Valles Marineris–Chryse Planitia complex is the largest former Martian river system.

Valles Marineris has many features that appear to have been created by water flow and the outflow channels leading from it to the former ocean^a,b,c,d,e,f in Chryse Planitia are clearly visible and bear many features in common with river systems on Earth.

Although Valles Marineris bears a striking similarity in structure to the Grand Canyon on Earth, its probable primary formation mechanism¹ is currently believed to be "rifting, strike-slip faulting, [or] subsurface mass removal."^g2 That said, there is clear evidence for the historical presence of liquid water within it³. Some authors have posited the existence of meltwater-/spring-fed lakes^h,4 in Valles Marineris while others have shown evidence of long-term precipitation feeding into the valleyⁱ; either mechanism could support outflow rivers into Chryse Planitia over extended periods of time. Other authors^j,k,l disagree and say that the outflow channels were created by cataclysmic outburst flooding via dam failure⁵ with geothermally-melted subsurface ice^l being suggested as a water source for the impounded lakes. Said lakes would have been located within Valles Marineris. The actual mechanism is likely to have been a combination of the above^h.

Regardless of the mechanisms and timescales involved, I suggest that with a total length in the neighborhood of 5,000 km, the Valles Marineris–Chryse Planitia complex is the largest former river system on Mars. Its route and structure can clearly be seen in remote-sensing data (see image below).

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¹ At least at the large scale. Smaller features such as the inflow valleys as well as general erosion may well have been created via hydraulic action. Some authors^l, however, do posit a purely hydraulic genesis for the valley. The overwhelming consensus though is that the outflow channels to the east and north-east were created entirely by flowing water.

² The paper cited promotes strike-slip faulting as the probable primary formation mechanism.

³ The primary source of contention is the precise mechanism(s) leading to liquid water having being present in Valles Marineris.

⁴ Note that said lakes are unlikely to have come anywhere close to filling Valles Marineris to the brim.

⁵ Evidence of cataclysmic outburst flooding can be seen on Earth in, e.g., the Channeled Scablands of south-eastern Washington State (USA) that were created by the repeated catastrophic failures of glacial ice dams holding back Lake Missoula.

⁶ The zero altitude or "sea level" on Mars was previously defined as being the elevation at which there is 610.5 Pa of atmospheric pressure. This pressure corresponds with the triple point of water; given sufficient temperature, pure liquid water⁷ can potentially exist below this elevation. In 2001 newly-acquired laser altimetry data (from MOLA) was used to set a more precise datum^m. The 610.5 Pa pressure is found at approximately -1.6 km relative to the new datum (the precise value varies by 1.5 to 2.5 km depending on location and season). The map shown is based on MOLA data and uses the newer datum (the approximate level of the old datum is green on the map).

⁷ Salt water and brine can be liquid at lower temperatures than pure water.

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Topographic map of the Valles Marineris–Chryse Planitia complex based on MOLA altimetry data:
_{(Valles Marineris is the long, linear, and fairly horizontal feature near the bottom-center, Chryse Planitia is the basin extending north from the top-right of the image)}

^{Source: NASA / JPL-Caltech / Arizona State University - JMARS via Wikipedia, Public domain (full-size image)}
Image width: ~6,500 km, Elevation⁶ scale:
^{Source: NASA / Goddard Space Flight Center via Wikipedia, Public domain}

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Cited papers:

^a Linda M.V. Martel (2001), Outflow Channels May Make a Case for a Bygone Ocean on Mars, Hawai'i Institute of Geophysics and Planetology, http://www.psrd.hawaii.edu/June01/MarsChryse.html, web page, retrieved 2018-12-23.

^b Carr, M. H., and J. W. Head III (2003), Oceans on Mars: An assessment of the observational evidence and possible fate, J. Geophys. Res., 108, 5042, doi:10.1029/2002JE001963, E5.
(full paper; gratis PDF download on linked page)

^c Fabio Vittorio De Blasio (2014), Possible erosion marks of bottom oceanic currents in the northern lowlands of Mars, Planetary and Space Science, Volumes 93–94, April 2014, Pages 10-21, ISSN 0032-0633, doi:10.1016/j.pss.2014.01.014.
(Abstract; no freely-available version)

^d Boyce, J. M., P. Mouginis‐Mark, and H. Garbeil (2005), Ancient oceans in the northern lowlands of Mars: Evidence from impact crater depth/diameter relationships, J. Geophys. Res., 110, E03008, doi:10.1029/2004JE002328.
(full paper; gratis PDF download on linked page)

^e Lorena Moscardelli, Boulders of the Vastitas Borealis Formation: Potential origin and implications for an ancient martian ocean, GSA Today, 24(2), 4-10, doi:10.1130/GSATG197A.1.
(PDF)

^f Lorena Moscardelli, Tim Dooley, Dallas Dunlap, Martin Jackson, Lesli Wood, Deep-water polygonal fault systems as terrestrial analogs for large-scale Martian polygonal terrains, GSA Today, 22(8), 4-9 doi:10.1130/GSATG147A.1.
(PDF)

^g An Yin (2012), Structural analysis of the Valles Marineris fault zone: Possible evidence for large-scale strike-slip faulting on Mars, Lithosphere, 4 (4), 286–330, doi:10.1130/L192.1.
(PDF)

^h Dohm, J. M., J. C. Ferris, V. R. Baker, R. C. Anderson, T. M. Hare, R. G. Strom, N. G. Barlow, K. L. Tanaka, J. E. Klemaszewski, and D. H. Scott (2001), Ancient drainage basin of the Tharsis region, Mars: Potential source for outflow channel systems and putative oceans or paleolakes, J. Geophys. Res., 106(E12), 32943–32958, doi:10.1029/2000JE001468.
(full paper; gratis PDF download on linked page)

ⁱ Kite, E. S., S. Rafkin, T. I. Michaels, W. E. Dietrich, and M. Manga (2011), Chaos terrain, storms, and past climate on Mars, J. Geophys. Res., 116, E10002, doi:10.1029/2010JE003792.
(PDF)

^j Keith P. Harrison, Mary G. Chapman (2008), Evidence for ponding and catastrophic floods in central Valles Marineris, Mars, Icarus, Volume 198, Issue 2, 351-364, ISSN 0019-1035, doi:10.1016/j.icarus.2008.08.003.
(PDF)

^k Mars Space Flight Facility, Arizona State University, Valles Marineris, a Martian Rift Zone, https://themis.asu.edu/vallesspecial, web page, retrieved 2018-12-23.

^l McKenzie D, Nimmo F (1999), The generation of martian floods by the melting of ground ice above dykes, Nature, 1999 Jan, 397(6716), 231-233, doi:10.1038/16649.
(PDF)

^m Smith, D. E., et al. (2001), Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars, J. Geophys. Res., 106(E10), 23689–23722, doi:10.1029/2000JE001364.
(full paper; gratis PDF download on linked page)

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Further reading:

• Baker, V R, The Channels of Mars, Austin, TX: Univ. of Texas Press, 1982.

• Burr, Devon & McEwen, Alfred (2002). Recent extreme floods on Mars, IAHS-AISH Publication, 101-106.
  (Abstract, gratis PDF of full paper available from abstract page)

• Oded Aharonson, Maria T. Zuber, Daniel H. Rothman, Norbert Schorghofer, Kelin X. Whipple, Drainage basins and channel incision on Mars, Proceedings of the National Academy of Sciences, Feb 2002, 99 (4), 1780-1783, doi:10.1073/pnas.261704198.
  (PDF)

Answer 2

A strong candidate for largest river on Mars that I have found is the Kasei Valles region of Mars.

According to the Wikipedia entry, this area is home to a geological feature similar to our famous Grand Canyon. . . except for one major difference. This guy is 300 miles wide at maximum instead of 18 miles.

^{Source: Areong via Wikipedia, CC BY-SA 4.0}

General info about Mars outflow channels. . .

Channels extend many hundreds of kilometers in length and are typically greater than one kilometer in width; the largest valley (Kasei Vallis) is around 3,500 km (2,200 mi) long, greater than 400 km (250 mi) wide and exceeds 2.5 km (1.6 mi) in depth cut into the surrounding plains. These features tend to appear fully sized at fractures in the Martian surface, either from chaos terrains or from canyon systems or other tectonically controlled, deep graben, though there are exceptions.

Specific info about Kasei Valles. . .

This huge system is 300 miles wide in some places. In contrast, Earth's Grand Canyon is only 18 miles wide. It is one of the longest continuous outflow channels on Mars. The Kasei Valles system begins in Echus Chasma, near Valles Marineris. It then runs northward, and appears to empty into Chryse Planitia, not far from where Viking 1 landed. At around 20° north latitude Kasei Valles splits into two channels, called Kasei Vallis Canyon and North Kasei Channel. These branches recombine at around 63° west longitude, forming a large island in the channel known as Sacra Mensa. Some parts of Kasei Valles are 2–3 km deep.

Like other outflow channels, it was likely carved by liquid water, possibly released by volcanic subsurface heating in the Tharsis region, either as a one-time catastrophic event or multiple flooding events over a long time period. Others have proposed that certain landforms were produced by glacial rather than liquid flow.

These are all quotes from Wikipedia. . . I am just now learning of the Martian Grand Canyon.

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There are bigger chasms on Mars. . . but he specifically asked about rivers. Valles Marineris is a much deeper chasm possibly having housed lakes or oceans — this was likely formed by tectonic activity then deepened by water and carbon dioxide flow. This means that it was not formed by river flow exclusively, and likely was never full.