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.
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.
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.
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