Measuring delta health on a global scale
I have previously noted that the major river deltas of the world are in a dynamic balance between growth and shrinkage. Recent human alterations of the watersheds on which deltas depend (and sea level rise via climate change) are rapidly altering this balance of power in complicated ways.
A deltaic landscape expands when river sediments increase and shrinks when sediments decline and/or sea level rises. Readers of this blog are aware that the Mississippi/Atchafalaya River delta system is shrinking dramatically. It is instructive to recognize how the changes in our backyard relate to deltas around the world.
Clearing land for agriculture and development increases sediment runoff and expands deltas but dam construction reduces sediment runoff and shrinks deltas. Rapid rise in sea level from climate change (currently at 3 mm/year and accelerating) threatens to virtually eliminate deltaic ecosystems.
Compare these two satellite views of the Yellow River delta in China put on the web by a geology graduate student named Brian R. (no last name given).
He offers these thoughts on the dramatic changes:
Compared to the earlier image, the 2000 shot shows an increase in developed land and a slight decrease in vegetation. The most striking difference between the two images, however, is the shape of the coastline. A steady supply of sediment expanded the Yellow River delta until it pushed into Bo Hai like a giant hook. Sediment colored the coastal waters bright blue. This delta-building process has added several hundred square kilometers of land to China’s coast.
I wonder if the change from a “smoother” coastline to the more “birdsfoot” river-dominated morphology has mostly to do with the land use changes? That is, a more restricted river course would preclude frequent avulsions and construction of the distributive dispersal pattern. A more restricted river course would result in more spatially focused growth. I don’t know … just thinking out loud.
Editor’s note: Brian R’s hypothesis on the deveopment of a “bird’s foot” configuration conforms with the historical changes to the modern delta of the lower Mississippi that accompanied the historic restriction of the river course.
A new study compares the environmental health of major deltas around the world, including our own Mississippi system. This study, published in the Bulletin of Atomic Scientists, provides an excellent global overview of the complicated interplay between riverine watersheds and ocean forces, and the ephemeral deltaic landscapes that reflect this interplay. One of the authors is well-known Louisiana coastal authority John Day, Professor Emeritus at LSU.
The article notes that increases in freshwater use for agriculture throughout riverine watersheds results in a reduction in the productivity of deltaic vegetation (wetlands) and a shift toward the marine side of the equation. This conclusion is relevant to both the ongoing issues of gulf hypoxia and the prospect of large scale river diversions to nourish our coastal wetlands.
The authors note that from a global perspective over two billion humans, almost one third of everyone alive today, live on deltaic systems including ours. To put this statistic in perspective we know that about two million Louisianans who live south of I-10 are at risk. Small comfort that we represent only 0.001 % of the world population facing similar risks!
A complementary paper published in the Proceedings of the National Academy of Sciences and summarized in Phyzorg.com describes a methodology for collecting sediment cores offshore of major delta systems as a forensic tool to quantify human alterations to watersheds. The key authors, both pre-Katrina Tulane faculty members, are Tom Bianchi, now at Texas A&M and Mead Allison, now at the University of Texas, Austin.
Bianchi and Allison interpret the sequence of sediment layers in these cores to measure past changes in nitrogen application in the watersheds from agricultural fertilizers, records of past flooding and hurricane events, all useful for predicting effects of climate change and other management issues.They have collected samples from deltas all over the world, including the Mississippi delta and the (Huanghe) Yellow and Yangtze in China.
By using their methodology, human activity in some watersheds can be traced back more than 5,000 years ago to some of the first cities in Mesopotamia, along the Nile and in regions of China. The authors note that the world’s largest 25 rivers drain about one-half of the Earth’s surface and transport 50 percent of the fresh water and 40 percent of particulate materials into the ocean.
Chinese river deltas are currently threatened as the country attempts to manage severe water shortages, over-grazing and desertification for a growing population by manipulating natural water sources from their major rivers through damming and diversions. Over the last 20 years, China has become the world’s largest consumer of fertilizers and two of its rivers, the Yellow and the Yangtze, are among the top five in the world in terms of sediment discharge.
The comparison images of the Yellow River delta above show the effects of 21 years of this discharge. In contrast, south Louisiana is suffering from a massive decline in river-borne sediments, thanks to dams on the Missouri River.
In summary, the world’s major deltas reflect geological processes and human alterations over vast watersheds in the context of climate change. This means that saving coastal Louisiana requires thinking far beyond the footprint of the deltaic “veneer” on which we live. Unfortunately, for two decades we have focused on the veneer and devoted only a miniscule effort thinking beyond (and beneath) our coastal zone. It’s as though we ‘ve hired painters to restore a house with termite-ridden beams.