Most sediment doesn’t reach the sea

Posted on July 25, 2012

Most of the massive amount of sediment that is transported globally by stream action is carried by large rivers that cross broad coastal plains.  Although it’s often assumed that much of this sediment eventually ends up in the sea, case studies suggest otherwise.   Most river sediment is deposited on the floodplain upstream of the estuary or is effectively retained in the estuary, deltas and coastal wetlands.  It’s been estimated that less than 5% of sediments carried by rivers on the Atlantic coast of the USA ever reach the sea.  Rivers are often more sluggish, with very low slopes, as they approach the coast.  Although rates of discharge can be high in these stretches of river, their flat slopes mean that stream power is very low, and this creates a bottleneck in sediment transport capacity.  Biased perceptions about sediment transport have been encouraged by the fact that rates of sediment delivery to the coast are typically estimated by reference to monitoring stations that are situated well inland, where slopes are higher – for example, gauging stations on  the Sabine, Nechos, Trinity, Brazos and Colorado rivers, among the largest rivers in Texas, are 54-98 km upstream of the river mouth.

Reference:  Slattery, M.C. & Phillips, J.D.  2011.  Controls on sediment delivery in coastal plain rivers.  Journal of Environmental Management 92, 284–289.