19552

This article, on the deleterious effect of dams on coastal systems, contains a major conceptual error. It states that “another important cause of the ground sinking is the waning of sediment deposition by the Mississippi River.” But over the past 100 million years, the northern Gulf Coast region has been subsiding because of excessive sediment loading produced by the Mississippi and its ancestors. Since the sediment supply is now waning, the subsiding will also begin to diminish, albeit at an extremely slow rate. It will take a long time to achieve equilibrium. What is true (and noted in the story) is that because of the decreased sediment supply, beaches, deltas, and sandbars are being starved and have been eroding for some time. However, this is very different from the statement quoted above.

Bruce Bartleson
Gunnison, Colo.

Your article says the topographic data is measured on a grid with 0.5°-by-0.5° cells. You go on to say that the data define drainage basins with areas greater than 100 square kilometers. But since each cell is roughly 3,000 km ² (at least at the equator), there isn’t sufficient resolution to detect 100 km ² features.

Mike Speciner
Acton, Mass.

While some data, such as soil type, are described only at the 0.5°-by-0.5°-grid size, other parameters are available at a 1-kilometer spacing, says James P.M. Syvitski of the University of Colorado in Boulder .—S. Perkins The article states that “people today mobilize about 15 times as much sediment as natural processes do.” Yet, the article also states that the annual worldwide total of sediment reaching the ocean is estimated to have been about 15.5 billion metric tons in the prehuman era and would be about 17.8 billion tons today if no dams existed to trap the sediment. If the latter statements are accurate, the added sediment load due to human activities would be only about 15 percent, not a 15-fold increase. Which is it?

Elliot Tramer
Whitehouse, Ohio

Although scientists estimate that agricultural erosion may mobilize as much as 75 billion tons of soil per year, only a small fraction of that reaches the ocean. So, as odd as it seems, both statements are true .—S. Perkins