A new analysis of data from more than 4,000 rivers around the world indicates humans are having profound and conflicting effects on the amount of sediment carried by rivers to coastal areas, with consequences for marine life and pollution control, according to a University of Colorado at Boulder environmental scientist.
The report, "Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean," appears in the April 15 edition of Science Magazine. Lead author James Syvitski, director of the Institute of Arctic and Alpine Research at CU-Boulder, said 15 billion tons of sediment are transferred to coastal areas around the world every year.
"In other words, the world's rivers are carrying and depositing enough sediment each year to cover the state of Texas with one inch of sediment, or the state of Colorado with three inches of sediment," Syvitski said.
The report found that humans are stirring up much more sediment than expected, about 2.3 billion metric tons annually, through agriculture and other soil erosion activities. However, manmade reservoirs are simultaneously reducing the flux of sediment reaching the world's coasts by about 1.4 billion metric tons per year.
"We're churning up our landmasses, and if not for the reservoirs we'd be flooding the coastlines with sediment," Syvitski said. "As an example, the Nile reservoir holds back 98 percent of natural sediment from that river's coastal region." The report estimated that more than 100 billion metric tons of sediment and carbon are now sequestered in reservoirs built mostly in the last 50 years.
"Take either part of the two-sided human influence away, and there would be drastic consequences for sediment transfer," Syvitski said.
Sediment transfer levels have many effects on coastal zones, according to Syvitski. "If we add to the sediment load, land mass can grow and accumulate at river mouths. This affects harbors and makes more dredging necessary to keep shipping lines open. Also, coastal fish farms and coral reefs can be severely impacted by too much sediment. The same concept applies to coastal wetlands and sea grass communities," he said. "Natural productivity could go way down if these areas were drowned in sediment.
"On the other hand, if too little sediment reaches the coast, the coastline will retreat from ocean storms," said Syvitski. "Sediment flux is also tied to nutrient transfer, particularly carbon, so reduced sediment transfer means less nourishment for marine communities in coastal areas. Additionally, if new sediment isn't deposited in coastal areas, buried pollutants can be churned up by wave action."
Syvitski worked with scientists at the University of New Hampshire and the Netherlands' Delft University of Technology to create a scientific model capable of globally consistent estimates of sediment flux near river mouths.
The team was able to develop the model so they could determine what sediment loads would naturally occur without human activity. "It was particularly useful to be able to 'strip away' dams, reservoirs and other human impacts to make comparisons," Syvitski said.
The study also highlighted geographical differences in human influence on sediment transfer. "Some regions of the world have a bigger problem than others. There have been huge changes since ancient times near inland seas like the Mediterranean and the Black Sea. Areas like the Arctic haven't seen much change. The greatest impacts have occurred where more people live," Syvitski said.
African and Asian rivers carry a greatly reduced sediment load, while Indonesian rivers deliver much more sediment, according to the report.
The report was completed for the International Geosphere Biosphere Programme, a large-scale effort by scientists to study how humans have been and will continue to affect the entire planet.
"It's one of the largest science programs in the world, with representatives from nearly every country," Syvitski said. He has been a member of the steering committee for the IGBP's Land Ocean Interaction in the Coastal Zone division for the last six years.