Modes of Sediment Transport in Channelized Water Flows with Ramifications to the Erosion of the Martian Outflow Channels

Depending on their grain sizes (settling velocities), sed iments are t ransported in rivers as bed load, in suspens ion , or as wash load. The coarsest material rolls or bounces along the bot tom as bed load whereas finer material is placed into suspens ion by the water turbulence. The finest sed iments are t ranspor ted as wash load. evenly distr ibuted through the water depth and effectively moving at the same rate as the water. The criteria for quanti tat ively determining which grain-size ranges are being t ransported in terrestrial rivers as bed load, suspended load and wash load are applied to an analysis o f sediment t ransport in the large Martian outflow channels , a s suming their origin to have been from water flow. Of impor tance is the balance of the effects o f the reduced Martian gravity on the water flow velocity versus the reduction in grain settling velocities. Analyses were performed using grain densit ies ranging from 2.90 g / c m '~ (basalt) to 1.20 g / c m a (volcanic ash). The results show that the Martian flows could have t ranspor ted cobbles in suspens ion and that nearly all sandsize material and finer would have been t ranspor ted as wash load. Washload transport requires little or no net expendi ture of the water-flow power, so the sands and finer could have been carried in nearly unlimited quanti t ies. A compar ison with terrestrial r ivers indicates that concent ra t ions as high as 60-70% by weight of wash-load sediment could have prevailed in the Martian flows, resulting in the very rapid erosion o f the channels .