Post by Dr Neil Coleman, University of Pittsburgh.
A group of Martian craters formerly contained lakes, some of which overtopped and breached the crater rims to cause flooding and channel erosion.
![Image 1: View of Morella Crater and the complex of Elaver Vallis channels eroded by floodwaters released when the crater rim was breached. The distal reaches of Elaver Vallis were obliterated by the southward expansion of Ganges Chasma, which is 5 km deep. The chasma as seen today did not exist during the Elaver flood, otherwise high groundwater pressures would have been relieved by breakouts in the walls and floor of the chasma [graphic is a mosaic of THEMIS daytime infrared (IR) images].](https://planetarygeomorphology.files.wordpress.com/2014/03/image-1.jpg?w=600&h=328)
Image 1: View of Morella Crater and the complex of Elaver Vallis channels eroded by floodwaters released when the crater rim was breached. The distal reaches of Elaver Vallis were obliterated by the southward expansion of Ganges Chasma, which is 5 km deep. The chasma as seen today did not exist during the Elaver flood, otherwise high groundwater pressures would have been relieved by breakouts in the walls and floor of the chasma [graphic is a mosaic of THEMIS daytime infrared (IR) images].
Techniques to study dam failures on Earth can be used to analyze some of these floods. Elaver Vallis, located south of Ganges Chasma, was carved by catastrophic drainage of a paleolake from the 78-km-wide Morella Crater (Image 1). The lake dimensions and the water volume drained can be reconstructed using MOLA topographic data. The drainable volume was 2.216 × 1012 m3, similar to the combined volumes of Lakes Erie and Ontario. Peak flood discharges likely exceeded 1.9 × 107 m3 s-1, more than the maximum Missoula Flood stages in the Spokane Valley of Washington state.
The source of the lake water was from a confined aquifer that flowed from a deep depression inside Morella Crater (Ganges Cavus). The lake overtopped the crater rim at ~1771 m, the minimum elevation for the groundwater potentiometric surface which reflects the confined aquifer pressure. The crater breach formed rapidly because two large channels were eroded and a broad region was inundated. The ancient Morella impact severely fractured the underlying and peripheral rock units. Disrupted and overturned beds would have been more susceptible to flood scour and deep channel incision.
![Image 2: View of the erosional features on the floor of Elaver Vallis, including longitudinal ridges in the deeper channels and numerous flood-carved scarps or terraces in southern half of frame. Blue arrows show Elaver Vallis flow directions. The southern channel is a hanging valley that formed early in the flooding. The northern channel eventually captured all the flow and continued to deepen [THEMIS daytime IR image I01488001]. Outline of this image is shown in Image 1.](https://planetarygeomorphology.files.wordpress.com/2014/04/image-2.jpg?w=434&h=1024)
Image 2: View of the erosional features on the floor of Elaver Vallis, including longitudinal ridges in the deeper channels and numerous flood-carved scarps or terraces in southern half of frame. Blue arrows show Elaver Vallis flow directions. The southern channel is a hanging valley that formed early in the flooding. The northern channel eventually captured all the flow and continued to deepen [THEMIS daytime IR image I01488001]. Outline of this image is shown in Image 1.
The morphology of the Elaver channels (Image 2) is consistent with megaflood features on Earth. These features include scabland topography, streamlined hills, residual uplands separating channels, low sinuosity, exhumation of resistant pre-flood topography, longitudinal grooves, hanging valleys, cataracts, scarps indicating high-water marks, and scour marks and moats around flow obstacles. In addition, multiple chaos zones occur in the channels, likely produced by outflows triggered by deep channel incision that thinned the cryosphere. The margins of the initial overland flooding outside the incised channels remain visible billions of years later. Therefore surface processes were not effective enough to remove high-water marks, and dust veneers remain thin enough that the Thermal Infrared properties of eroded surfaces are distinguishable.
Further Reading:
Burr, D. M., P. A. Carling, and V. R. Baker [Editors] (2009), Megaflooding on Earth and Mars, 319 pp., Cambridge University Press, New York.
Christensen, P. R., N. S. Gorelick, G. L. Mehall, and K. C. Murray (2013) THEMIS Public Data Releases, Planetary Data System node, Arizona State Univ., Source
Coleman, N. M. (2013) Hydrographs of a Martian flood from a breached crater lake, with insights about flow calculations, channel erosion rates, and chasma growth, J. Geophys. Res., 118(2), doi:10.1029/2012JE004193.
Walder, J. and J. O’Connor (1997) Methods for predicting peak discharge of floods caused by failure of natural and constructed earthen dams, Water Resources Res., 33, 2337–2348.
Waythomas, C. F., J. S. Walder, R. G. McGimsey, and C. A. Neal (1996) A catastrophic flood caused by drainage of a caldera lake at Aniakchak Volcano, Alaska, and implications for volcanic-hazards assessment, Geol. Soc. Amer. Bulletin, 108, 861-871.
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