Peru's coast is a long strip of desert cut in more than 50 places by seasonal rivers that flow from the Andes to the Pacific Ocean. But the coast has not always been arid. After South America broke away from Africa and the rest of Gondwana, the west coast was marshy. The uplift of the Andes Mountains, however, cut off the flow of moist air from the east, creating a rain shadow - an area where rainfall is scant to non-existent - and turning the coast into an arid strip of land with ocean on one side and mountains on the other. Chile's Atacama Desert is one of the driest places on earth. The Peruvian desert, which is the northern extension of the Atacama, receives only about half an inch of precipitation annually. But while little moisture crosses the Amazon, the western Andean foothills do receive some moisture in the form of fog known as garua, creating an unusual ecosystem known as lomas.

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The cold Pacific or Humboldt Current is the reason why Peru's coast is so dry. Moisture evaporating from the ocean is absorbed by the warmer air over land, keeping clouds from forming. In winter, however, when the air temperature drops slightly, heavy fog, called garua, forms on the coast and drifts inland against the western foothills of the Andes Mountains. On some of these hills, especially those that face south, the direction of the prevailing winds, the mist condenses on rocky outcrops and tree leaves and drips onto the ground. By September, these hills, called lomas, are covered with green vegetation and blooming flowers. Some of these lomas are home to endemic bird species (those found nowhere else on Earth) and other wildlife takes advantage of the abundance of food during the winter and spring. The lomas are under threat, as cities and farms expand and people cut trees for fuel. One of these areas, the Lomas de Lachay, north of Lima, has been set aside as a natural reserve.

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Along the Peruvian coast is a string of rocky islands and peninsulas that are home to guanay cormorants, boobies, and pelicans. The birds leave the islands every morning to feed on the fish, especially anchovies, that are plentiful in the Humboldt Current. They return in the afternoon. There is no vegetation on the islands, so they build their nests out of guano and feathers, adding a new layer every year. By the time Europeans first saw the islands, the guano was more than 100 feet deep in places.

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The coastal ecosystems and the civilizations that flourished on the Peruvian coast have been shaped by Pacific currents. The Peruvian or Humboldt Current flowing northward along the coast of South America, carrying cold, nutrient-rich water from the ocean around Antarctica almost to the Equator, where it veers eastward toward the Galapagos Islands. An upwelling off the coast of Peru brings this layer of water to the surface, providing nutrients for an abundance of plankton that provide food for a large number of fish, especially anchovies and sardines. This marine ecosystem also includes sea birds, dolphins, porpoises, sea lions and fur seals that feed on the fish. Many sea birds nest on rocky capes and islands where guano collects.

Read more: Coastal current