Why Are the Deepest Lakes in the World So Narrow? Cracks in a Continent

Where the crust is being stretched, it thins and breaks along steep faults, and blocks between those faults drop downwards to form a long trough with steep sides. Because the movement is vertical and the faults are roughly parallel, the resulting depression is narrow relative to its length and can be extremely deep. Water accumulates in it, and since the floor continues to drop as extension proceeds, the basin deepens over millions of years rather than filling with sediment and disappearing as ordinary lakes do. That combination of continuing subsidence and long duration is why these lakes reach depths and ages that no other kind approaches.

A small number of lakes account for most of the world's liquid fresh water:

• •Baikal in Siberia, the deepest and oldest, holding around a fifth of unfrozen fresh surface water
• •Tanganyika in East Africa, the second deepest and second largest by volume
• •Malawi, also in the East African system, exceptionally rich in fish species
• •Turkana, Albert, Edward and Kivu along the same system
• •The Dead Sea, in a rift basin with no outlet and therefore hypersaline
• •Lake Tahoe and several others in extensional basins elsewhere

Deep lakes in warm climates develop a permanent layered structure that has substantial consequences. Surface water is warmed and stays on top, while deep water remains cold and dense, and because the temperature difference persists all year there is no seasonal overturning of the kind that mixes temperate lakes. The deep water therefore receives no oxygen from the surface and becomes permanently anoxic, so the lower two thirds or more of some of these lakes support no animal life at all. Nutrients settling into that layer are effectively removed from circulation, which limits productivity. One of these lakes holds enormous quantities of dissolved carbon dioxide and methane in its deep water, which poses a genuine hazard.

One of these lakes presents a hazard with no real parallel elsewhere. Kivu, on the border between Rwanda and the Democratic Republic of the Congo, holds enormous quantities of dissolved carbon dioxide and methane in its deep water, supplied by volcanic activity below and trapped by the permanent layering that prevents mixing. If that layering were disturbed, by an earthquake, a landslide or a volcanic eruption, the dissolved gas could come out of solution rapidly and release a dense suffocating cloud over a densely populated shoreline. Smaller Cameroonian lakes did exactly this in 1984 and 1986, killing over seventeen hundred people in the second event, and those lakes are now degassed continuously through pipes. Extraction of the methane from Kivu for power generation reduces the hazard while providing energy.

These lakes are among the most remarkable places in the world for biologists because they are old, isolated and internally varied. Age matters because species accumulate over millions of years rather than being wiped out by glaciation as northern lakes were. Isolation matters because there is nowhere for the inhabitants to go and nothing arriving, so populations diverge in place. Internal variety matters because rocky shores, sandy shores and open water offer distinct ways of living within one body of water. The result is that a single lake can hold hundreds of fish species found nowhere else, descended from a handful of colonists, and these radiations are among the standard textbook examples of how species form.

Stretching crust drops blocks between parallel faults, producing troughs that are narrow, long and deepening for millions of years, which is why these lakes are the deepest and oldest there are. Baikal alone holds around a fifth of the world's unfrozen fresh surface water. Warm ones never overturn, so their deep water is permanently without oxygen, and their age and isolation have produced hundreds of fish species found nowhere else.