Sediment in the Bay of Biscay
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Sediment in the Bay of Biscay

April 7, 2011
In early spring, the rivers of France often carry sediment-laden water from the mainland into the Bay of Biscay, discoloring the blue water with tints of tan and green. Coastal upwelling also occurs in spring and summer, bringing sediment from the floors of the continental shelf towards the water surface. As nutrient rich water rises to the surface along with sediment, phytoplankton soon begins to bloom, adding to the bright display.

On April 1, 2011, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Aqua satellite captured this true-color image of jewel-tone and tan colors swirling in the Bay of Biscay, off the shores of France (east) and the Iberian Peninsula (south).

The colors extending from the rivers and estuaries are from sediment-laden plumes which flow into the Bay. As sediment sinks, it scatters light, and appears to change color. Typically, sediment near the surface, such as the plumes as they exit the estuaries and rivers, appears brown or tan, while sinking sediment fades to green, then blue and finally black. In some cases, especially where chalky-white calcium carbonate sediment is churned from the ocean floor, sediment alone can create a bright and colorful palette of many hues.

In this image, the largest plume of sediment comes from the Gironde Estuary in southwest France. This estuary is formed by the confluence of the Dordogne and Garonne Rivers, just north of the city of Bordeaux, which can be clearly seen as a large gray circle.

The Gironde is the largest estuary in western Europe, covering approximately 245 square miles (635 sq km). The Garonne flows from the Central Mountains of France, approximately 360 miles to the Gironde Estuary and then into the Bay of Biscay. The Dordogne is nearly as long, flowing from the Pyrenees Mountains that divide France and Spain.

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