May 16, 2013

The ebb and flow of glacial lakes

800px-Quelccaya_Glacier The Quelccaya Glacier in Peru is the largest glacier in the tropics. This glacier provides water to the city of Lima and contributes to hydroelectric power. In the last fifteen years, it has retreated nearly 195 feet per year. The increasing presence of glacial lakes around it reflects its unprecedented recession. Photo credit: Edu Bucher (

People sometimes ask us what it means if the world’s glaciers melt because of warmer temperatures. As Earth’s climate warms, the fate of the world’s shrinking glaciers matters to people who depend on them for meltwater. An increase in glacial lakes may mean more water for the present but leaves many to wonder how reliable this source of water will be if glaciers continue to recede. But what are glacial lakes and how do they form? Are they a part of a normal, healthy glacier, or evidence of glacier decline?

What is a glacial lake?

As glaciers recede more intensely due to rising global temperatures and climate change, glacial lakes are becoming more common and, in some cases, more dangerous. Glacial lakes typically form at the foot of a glacier. As glaciers move and flow, they erode the soil and sediment around them, leaving depressions and grooves on the land. Meltwater from the glacier fills up the hole, making a lake. Glacial lakes can also form from natural depressions that catch escaping meltwater. Surging glaciers, which can move up to 100 times faster than a normal glacier, may dam up meltwater as they surge beyond a lake and prevent it from draining.

Glacial meltwater drains over time in a manner similar to a plumbing system. Drainage streams on and under the surface of the glacier act as pipes by directing meltwater into the lake, while moraines and depressions work like plugs to stopper it. As glaciers wind through the landscape, they churn up rock and earth at the ice front, leaving behind mounds of earth called moraines. The moraines often act as barriers, causing meltwater to pool and form a lake. Because moraines are mounds of loose debris and rocks, they may also function as faucets which allow water from the lake to slowly drain through their permeable barrier. This water then flows into nearby rivers.

However, prolonged melting or abrupt bursts of intense melting can wreak havoc on this natural plumbing system. Too much meltwater in a short period of time might overflow a lake or burst through natural barriers, causing unexpected flooding. Lakes bound by moraines pose a serious threat because the porous moraine walls destabilize easily.  A rise in the amount of meltwater from glaciers increases the strain and water pressure on the moraine barriers, which then quickly give way. The water drains into areas of lower elevation, leaving little trace of the lake that previously rested there.

Are they normal?

Glacial lake While glacial lakes often times provide local communities with a steady supply of freshwater, without a drainage system, these same lakes may cause damaging floods. The Tsho Rolpa, one of the largest glacial lakes in Nepal, has its own drainage system to prevent a flood downstream along the Tamakoshi River. Photo credit: Rojan Sinha (

One recurring example is Chile’s Cachet II Lake, which is dammed by Colonia Glacier. In 2008, the glacier’s melting ice front failed, draining the lake in a matter of hours. The massive amount of water overflowed the Baker River, which suddenly tripled in volume, flooding communities downstream. In less than two days’ time, roads, buildings, and farms were devastated, and many livestock drowned. Places in the Andes, Himalayas, and Alaska currently lack a warning system to notify communities of impending these events, called glacial lake outburst floods. While glacial lake outburst floods constitute a growing concern, glacial lakes themselves are a normal part of the glacial process. In fact, many people depend on them as a source of water for drinking, irrigation, transportation, and other purposes.  However, in a world of ever increasing temperatures, flooding may become more common.

Persistent warming is weakening the Colonia Glacier ice front that normally dams Lake Cachet II, which has reformed and flooded at least eleven more times since 2008. Communities that lie downstream of this and other glacial lakes around the world may need to prepare for more frequent and sudden flooding.

Watch a video about researchers documenting glacial lakes in Nepal's Hongu River Valley, located high in the Himalaya Mountains.

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