[MUSIC]. There are several hazards associated with glacier runoff. In some mountains, where several glaciers flow off an ice cap in adjacent valleys. They eventually terminate in the same main, or trunk valley. This is common in large, glaciated landscapes, like in the St Elias mountains of the Yukon or in the neighboring Alaska Range. If one glacier advances or surges across the trunk valley, it may block off the flow of another glacier and form a lake. That lake will continue to rise until it becomes deep enough to burst through the glacier blocking it. This is often caused by the buoyancy of ice in water. As the lake gets high enough it will literally lift the glacier off its bed and the water will rip through it. Also, as glaciers retreat their melt water can become trapped behind their, end moraines, which are large piles of rubble that glaciers bulldoze up as they advance. If these lakes overflow, or the moraine dam bursts, it can also cause catastrophic flooding downstream. These types of events are known as glacial lake outburst floods. One dramatic example of this type of event is Neoglacial Lake Alsek in present day Kluane National Park in the Yukon. The Alsek River is the wilderness river flowing to the Yukon in Northern British Columbia into Alaska. The river flows next to the Lowell Glacier that sometimes locked off the river, and created a large lake over one hundred kilometers long and a hundred meters deep behind it. The last such blockage took place about 1850, this event is still well remembered by local First Nations peoples. Southern Tutchone descendants of these Athabascan peoples, actively use the Alsek River Valley between the Alaskan coast and interior Yukon. They gave the name Naludi, or Fish Stop, to the Lowell Glacier, because it interrupted salmon migrations when it surged across the river. When the glacial dam eventually broke, its release created a massive flood, washing away everything in its path on its way to the Pacific Ocean. In the Karakorum Range of Pakistan, glacial outburst floods have destroyed a hydroelectric dam. And in June 2013, floods destroyed the town of Kedarnath, killing several thousand people. The greatest damage and loss of lives, caused by glacial lake outburst floods, had been in the Cordillera Blanca of Peru. Here, three floods alone occurred in the 1940s, killing 10,000 people. In the Altai mountains of central Asia, outburst floods created the largest inland tidal wave ever recorded. Glacier lake outburst floods are not new, but their risk of occurrence is greatly increasing as global climate warming is accelerating. For example, Imja Tsho has been identified as one of the potentially dangerous glacial leaks in the Himalayas, located in the Mount Everest region of Nepal, at the foot the Imja glacier, the lake was first mapped as just a few ponds from a satellite image taken in 1962. Today, the lake is formed containing enough water to fill 26,000 Olympic-sized swimming pools. Only its terminal moraine where the debris that's accumulated at the terminus of the glacier it's moved, holds it back. And it continues to grow more than 75 meters in length each year. If such a flood were to occur, it will put thousands of local Sherpa communities and tourists at the staging grounds for Mount Everest at risk. Many communities are taking active measures to mitigate dangers and prevent future catastrophes associated with glacial lake outburst flooding. For example, scientists from the High Mountain Glacier Watershed Program have met with village leaders to discuss some options for managing the risks that Imja Tsho poses. These included, accepting the risk of a possible flood, relocating to higher elevations, and constructing controlled drainage canals. In some places, monitoring an early warning system, including siren towers, have been installed, to alert communities in the event of a flood. Sections of a critical transportation corridor the Nepal-China highway have been reconstructed with arched bridges, well above the 1981 glacial outburst flood levels. And in Peru, where glacial lake outburst floods have been very destructive, 34 of the most dangerous glacial lakes have been drained and dammed. Another hazard presented by melting ice is the legacy of accumulated chemicals that are no longer in widespread use. Organic chemicals produced by humans, in particular pesticides like dichlorodiphenyl trichloroethane that are known as DDT, are very volatile and quickly incorporated as a gas into the atmosphere. There, they travel to cooler regions, such as the poles and high mountains, where they condense out of the atmosphere. Any pollutants incorporated with snow that falls onto glaciers will become part of that glacier. Glacier ice in the poles and high altitude regions in the world will have relatively high concentrations of these pollutants. Because these contaminants are no longer in use, and have accumulated in glaciers, they are referred to as legacy pollutants. If the ice melts, it will release these pollutants back in the environment. This is a less discussed but very important problem associated with global warming in mountain regions. Legacy pollutants in ice are well-documented in mountain regions. For example, in an ice core taken from Snowdome Peak on the Columbia Icefield in the Canadian Rockies shows a profile through time of the deposition of these chemicals and how they have accumulated over the decades. As warming occurs, these chemicals are being released back into the environment, including Bow Lake in Banff National Park. A similar process has also been documented in Asia, where melting Himalayan glaciers are major contributors of polychlorinated biphenyls, or PCBs, And high molecular weight polycyclic aromatic hydrocarbons, or PAHs, in the surface waters of the Ganges River plain during the dry season. By remobilizing legacy pollutants from melting glaciers, climate change can enhance exposure levels over large and already heavily impacted regions of Northern India. Time to revisit your mountain world. This time along with some new mountain ranges, try to locate some of the world's great rivers, which have their head waters in the mountains. Matt has a tech tip on navigation tools and there's your end of lesson quiz. Good luck and we'll see you next time for a lesson on water's coldest expression, glaciers.
