Δ DELTAS

 

Several studies have noted the temporal coincidence between shoreline erosion around some major deltas (e.g., Nile, Mississippi, Ebro), and the reduction of stream sediment loads due to reforestation, soil conservation practices, and trapping of river sediment behind dams. There are, of course, excellent reasons to suspect a causal link, but the link itself has not, in my view, been fully established.

First, in many cases it has not been established that reduction of soil erosion within river basins and sediment trapping behind dams has reduced sediment delivery to the coastal zone. In some cases, particularly where rivers cross a coastal plain, there are sediment “bottlenecks” that limit sediment delivery to the coast. While the upstream changes do indeed reduce fluvial sediment loads in parts of the river, and may well reduce input to the bottlenecks upstream of the deltas/estuaries, it may be that even with the reduced loads there is still as much sediment as the river can carry in its lower reaches. Lower reaches of coastal plain rivers are often characterized by extensive sediment storage space, frequent overbank flow that delivers sediment to these storage zones, and very low stream power. It is often difficult to assess changes in sediment actually getting to deltas and estuaries because the downstream-most gaging station is well upstream of the coast, and of any lower-river transport bottlenecks (for more extensive discussions and case study evidence, see this, this, this, thisthis, and that).

Some studies also don’t pay attention to other factors that influence delta erosion and deterioration, such as (accelerated) sea-level rise, subsidence, compaction, and human modifications that either limit sedimentation (e.g., flood control levees), prevent avulsions that may be important in deltaic sediment distribution (e.g., channel stabilization), or contribute to delta wetland loss (dredging, canals, etc.). The point is that in some cases deltas could be experiencing net erosion even where river sediment supply is not decreasing.

False-color LANDSAT image of the Mississippi River delta. 

It is well established that human settlement and expansion dramatically increased soil erosion and river sediment loads in many cases due to deforestation, agriculture, mining, etc.  To the extent reforestation, soil conservation and sediment controls reduce erosion, and dams trap sediment, this could be viewed as returning the sediment regime to its pre-modern situation. Thus, if we want to say that choking off this excess anthropic sediment is starving deltas, shouldn’t we also observe that the excess anthropic sediment was resulting in delta progradation before?

I did a quick-and-dirty literature review on this. I was not thorough enough to draw any conclusions, but I did see enough to know this: the evidence is mixed and varied. There are some cases where accelerated soil erosion can be shown to have led to delta growth, some cases where this is unclear, and some where no evident delta growth occurred.

Resolving this question means evaluating each river and delta on a case-by-case basis, accounting for the possibility of sediment bottlenecks, and considering the other factors that influence delta growth or erosion. 

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