This paper summarizes state-of-the-art models for water flow and sediment transport
and suggests implications for the sediment grain size distribution, transport process, and
delta formation. The flow velocity in Martian outflow channels is commonly calculated
from the Manning equation, which is dimensionally incorrect and masks the large
uncertainty of the reconstructed flow velocity. More modern friction predictors based on
surface grain size distribution are tested on 190 rivers on Earth including moderately
catastrophic events. The uncertainty for the flow velocity is a factor of 3β4. The sediment
transport is commonly assumed to amount to 40% of the water flux (hyperconcentration),
but this is only true for special conditions. A debris-flow origin of the channels is unlikely.
Application of modern sediment transport models to typical Martian conditions
indicates orders of magnitude smaller sediment fluxes dominated by bed load transport,
resulting in much larger timescales for sediment emplacement in crater lake deltas and in
the potential northern ocean. This is in part caused by the unexpected grain size
distribution of the sediment derived from observations of landers and of delta
morphologies. The implied duration of hydrological activity and channel and delta
formation is of the order of 103β106 years, which is still very short on the geological
timescale of Mar
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