English: Bright white clouds hung over green forests and muddy-tan water poured into the North Atlantic Ocean off the coast of northeastern Brazil in mid-June 2023. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite acquired a true-color image of a sunny day over the Amazon River Delta on June 19.

The widespread, fluffy clouds are low-altitude cumulus clouds, and often are called by the nickname “popcorn clouds”, most likely because they resemble popped kernels of popcorn scattered over the landscape. They are common over forests at certain times of the year. They form when warm, humid air rises from the forest and cools as it rises, resulting in the development of the clouds. They don’t form over rivers or oceans because those waters—and the air above them—are cooler, so there is less moisture rising into the air.

The Amazon River travels a tremendous distance across South America, picking up sediment along the way, including the foothills of the Andes Mountains in Peru and Bolivia near its source. Hydrologists estimate that erosion from the mountainous far western part of the river basin contributes about 85 to 90 percent of all the sediment that reaches the Atlantic Ocean.

By the time it reaches the Amazon Delta, the muddy waters are loaded with nutrients (such as nitrogen and phosphorus) and organic matter. These nutrients help to nourish a rich marine ecosystem. The estuary region at the Delta is known for its many freshwater and marine catfish and croakers. Many of the commercial fisheries in the estuary target piramutaba and marine shrimp.