The alpine lakes of the Tibetan Plateau are some of the most remote in the world. This mosaic of astronaut photographs, taken along a single International Space Station orbit track, depicts Lake Puma Yumco during the winter season. The lake is located at an elevation of 5,030 meters above sea level (16,503 feet), and is considered ultraoligotrophic, meaning that nutrient concentrations in both the water column and lake sediments are extremely low. Water in such lakes tends to be blue to blue-green and to have high clarity due to low levels of photosynthesizing organisms such as phytoplankton. Color change in these lakes is usually due to shallowing of the water—blue indicating deeper water. A depth transition is visible along the west-southwest shoreline of Lake Puma Yumco at the lower left of image. The most striking feature of the image mosaic, however, is the intricate ice block pattern on the lake surface.
Lago de Valencia (Lake Valencia) is located in north-central Venezuela and is the largest freshwater lake in the country. The lake was formed approximately 2-3 million years ago due to faulting and subsequent damming of the Valencia River. The lake has been completely dry during several discrete periods of its geologic history. Since 1976 Lake Valencia water levels have risen due to diversion of water from neighboring watersheds—it currently acts as a reservoir for the surrounding urban centers (such as Maracay).