Why Two Colorful Lakes Meet in Patagonia: An Astronaut Photo Reveals a Glacial Land Bridge

This striking astronaut photo shows where a glacial lake in Patagonia is separated by a rocky land bridge, creating two neighboring bodies of water with dramatically different colors.

Why do Lake Pueyrredón and Lake Posadas have different colors?

Lake Pueyrredón, on the left in the astronaut photo, is the Argentine half of a roughly 65-kilometer (40-mile) glacial lake that stretches across the border between Argentina and Chile. The Chilean half is known as Lake Cochrane and is not visible in this image.

Patagonia is a region of South America known for its diverse landscapes, including mountains, fjords, deserts and rainforests. The deep, clear water in Lake Pueyrredón reaches depths of up to 500 meters (1,600 feet), giving the lake a dark blue appearance when viewed from above.

Thousands of years ago, a thick sediment barrier formed across the southern part of Lake Pueyrredón. This created a separate body of water known as Lake Posadas.

Lake Posadas is about 8 kilometers (5 miles) long. Its brighter blue-green color comes from its shallower water and the large amounts of sediment carried into it by nearby rivers, according to NASA Earth Observatory.

A rocky barrier shaped by glaciers and rivers

Other rivers flowing from nearby hills have deposited large amounts of sediment along the western shore of Lake Pueyrredón, visible at the bottom of the astronaut image. This sediment has formed three fan-shaped landforms known as deltas, which increase in size from north to south, or from left to right in the photo.

Wind-driven currents and waves have moved some of the sediment away from the outer edges of two of the smaller deltas. Over time, this created long, narrow strips of land called spits that point downstream.

The strip of land separating Lake Pueyrredón and Lake Posadas resembles a sand spit. However, NASA Earth Observatory says it is most likely a moraine—a barrier of sediment and large rocks formed along the edges of glaciers or giant ice sheets.

Observers may also be able to spot a narrow airstrip in the smallest delta, running roughly parallel to the land barrier. Although the area is largely uninhabited, it is popular with tourists who visit the lakes for camping, hiking, fishing and kayaking, according to local travel sites.

Lake Posadas’ famous rock arch

Lake Posadas also contains a distinctive island with a huge rock arch that is popular with tourists.

Patagonia’s lakes and valleys were carved by ice

The two lakes in the astronaut photo lie in broad valleys carved by the enormous ice sheets that once covered the region. The last remnants of this ice sheet are found in northern Patagonia, about 30 kilometers (20 miles) west of Lake Cochrane.

The image was taken from space by an astronaut aboard the International Space Station.

Patagonia contains dozens of similar valleys and fjords carved by massive slabs of ice. About 340 kilometers (210 miles) southwest of Lake Pueyrredón and Lake Posadas, three valleys meet to create an unusual landscape where lakes, rivers and glaciers appear to touch.