Lakes

Birth

1. Glaciers

The Master Creator

It may not seem that glaciers have a lot to do with lakes. Today these masses of ice and snow are found near the poles or high up in the tallest of mountains, far from the lakes one finds in more temperate latitudes and altitudes. But glaciers once covered an enormous part of the planet, in the Northern Hemisphere covering all of what is now Canada and portions of the United States, extending as far south as Indiana. They covered most of northern Europe and the northern regions of much of Asia. In the Southern Hemisphere, an ice sheet extended outward from the current boundaries of Antarctica all the way to the edge of the Antarctic continental shelf. And, wherever glaciers spread over land, they left prodigious quantities of lakes behind.

To have a lake, you need some kind of depression in the ground, a basin with a rim on all sides high enough to hold significant quantities of water. This doesn’t happen as naturally as one might think. When mountains form, they tend to create terrain that tilts more or less uniformly downhill from peak to foot, a situation that typically does not result in the formation of a lake basin. Tectonic plates may warp, tilt, or fracture to form basins that ultimately become lakes. But this results in far fewer lakes than those made by glaciers.

Lakes can form in very flat regions, like the Great Plains. In such regions, rock beds made of limestone or other types of soluble rock can be acted upon over time to form lakes via a process called solution. Here, a small, shallow dip can become exaggerated as accumulated water becomes enriched in carbon, which in turn forms carbonic acid that can dissolve rocks such as limestone. The surface slumps as the underlying rock dissolves, resulting in a lake. Excellent examples can be found along Croatia’s Dalmatian coast and in Florida. Lakes can also be formed by volcanoes and by landslides that dam up a river valley. But, all of these types of lakes—solution lakes, lakes formed by various tectonic processes—all of them take second place to glaciers when it comes to number of lakes formed.

Most of the planet’s lakes were formed by glaciers. While estimates of the exact percentage of lakes formed in this way vary, these estimates always exceed 50 percent and range to as high as 90 percent. Thus the number of glacially formed lakes is very large simply because the total number of lakes found on Earth is enormous.

Precisely how many lakes exist is difficult to say for several reasons, not the least of which is the difficulty in agreeing upon a lower size limit for what exactly constitutes a lake. The introductions of limnology (the study of lakes) textbooks often include a good deal of fretting over the difference between a pond and a lake. But if, for the sake of simplicity, we include ponds in our inventory of lakes, matters are little helped since now the question becomes what is the lower size limit for a pond? Ultimately the decision is arbitrary and perhaps it is best to just count the number of lakes larger than a particular size. In the first chapter of The Lakes Handbook, C. S. Reynolds does precisely this, noting that if one limits the definition of a lake to a freshwater body having an area of more than 24.7 acres, existing data suggest there are 1.25 million lakes on the planet. If one then adds those bodies of water having an area between 2.47 acres and 24.7 acres, the number increases by 7.2 million, giving at least 8.45 million lakes. To put these numbers in perspective, note that a perfectly circular lake having an area 2.47 acres has a diameter of 370 feet, and a 24.7-acre circular lake has a diameter of 1171 feet. The estimates of total number of lakes only get larger if we include yet-smaller bodies of water, manmade lakes, or ephemeral lakes (ones filled during only certain times of year). In any event, no matter how the counting is done, our planet is well-endowed with lakes. This is a good thing, given how many species rely on them to survive, our own species being no exception.

Eight million is a lot of lakes, and if it weren’t for glaciers, that number would be far smaller. Glaciers are prodigious lake builders. This is an odd thing to ponder, particularly if you happen to be vacationing by a lake right now, perhaps during the summer, cooling yourself as you float upon the water surface. Perhaps you are lazing by one of the Finger Lakes in New York State, or perhaps one of the many lakes found in Minnesota, or maybe you are touring the scenic English Lake District. These lakes differ in many respects, not the least of which is their location, but they all share the same origin story—they were all formed or modified by glaciers, these mammoth sheets of ice. This thought does not naturally come to mind as you laze by the lake shore during the heat of summer.

Glaciers create lakes in a multitude of ways. Sometimes they dump ground-up rocks and silt at the end of a valley (often a mind-boggling amount of ground-up rocks and silt), damming up a river valley and creating a long thin lake; the Finger Lakes are excellent examples. Other times they may grind away at pre-existing faults or cracks in the bedrock, gouging out a basin. Other times they shatter rocks via frost-thaw cycles, then slowly, over the millennia, drag the rock away, leaving a bowl behind.

Filling the created bowl with water is easily done, and often provided by more than one source. When they recede, glaciers leave massive quantities of meltwater behind, readily filling any dips or depressions in the moraine they also leave behind. Enormous quantities of water are also left behind in underground aquifers. Rainwater and streams or rivers entering a basin also supply a lake with water.

Although our interest here is in lakes, it is important to note that in many parts of the world, the very topography of the land is the way it is because of a glacier. While volcanoes and the motion of tectonic plates may be the creators of the raw hills, mountains, and valleys covering our planet, glaciers modify these features in powerful ways, creating the topography we have today. There are few things on Earth that have the landscape-sculpting power possessed by these miles-thick sheets of ice.

To understand how glaciers form lakes (and are forming lakes), let us pause to explore what glaciers are and how they work. A glacier is, essentially, a long flowing river of ice whose source is located in an elevated area where significant snowfall occurs, and whose termination point is a lower area where snowfall is relatively sparse. In the elevated region, snow accumulates, compressing older layers of snow into ice. This is a slow process, as are most processes related to glaciers, the layers of snow growing year by year, decade by decade, century by century. This upper region is called the accumulation zone, and lacking other processes, the accumulation zone would continually thicken.

In lower areas where snowfall is sparse, a process called ablation thins the glacier. Ablation is the combined action of evaporation and sublimation (the direct conversion of solid water into water vapor). This zone is referred to as the ablation zone and, lacking any connection with the accumulation zone, the ablation zone would quickly disappear. So, lacking a key piece of physics, ice in the accumulation zone would grow without bound, and ice in the ablation zone would quickly dwindle away to nothing.

But there is a key process preventing both of these scenarios from happening, a process perhaps the most difficult to wrap one’s head around when thinking about glaciers—the simple fact that ice flows. The pressure of the growing layers of ice in the accumulation zone is simply enormous, causing the ice to flow downhill into the ablation zone, where it more or less replaces the mass of water lost via ablation. “More or less,” because glaciers can advance and retreat. When snowfall in the accumulation zone exceeds the mass of water lost in the ablation zone, glaciers advance, the ice flowing forward and extending the tip (called the snout) of the glacier. When the mass of water lost from the ablation zone exceeds snowfall in the accumulation zone, then glaciers retreat, the snout moving upstream.

None of this occurs quickly, but the flow of glaciers is inexorable. It is a kind of slow-motion violence capable of obliterating anything in its path. And it is this flow of ice that is critical to many of the ways that glaciers create lakes.

The confusion about how ice would be worthy of the word flow and all its liquidy connotations is that, simply stated, it is not true that solids do not flow. To be sure, under the forces that something like an I-beam experiences, flow (at least in any observable sense) does not occur. But, when forces become sufficiently large, even metals will flow, as is the case when, for example, a copper bullet impacts steel, the relatively soft copper flowing and deforming under the very large forces at play during the brief period of the bullet impact event. The same occurs in glaciers, the ice slowly flowing downhill under the enormous pressures exerted by an ice sheet up to a mile thick.

This type of glacial flow dominates what are referred to as cold-based glaciers, glaciers whose temperature at the interface between the ice and the ground beneath is below freezing. Here, the glacier is frozen to the ground, with little sliding at the base. The bulk of the glacier moves because it deforms and flows downhill even as the ice wedded to the earth at its base moves very little. However, the real violence glaciers do to the ground occurs when they are (or become) warm-based. The temperature at the bottom of a “warm-based” glacier is above freezing, causing a film of water to exist at the ice/rock interface. This film of water permits the glacier to slide over the ground. The pressures beneath are still large, and the motion of the ice over the rock creates a layer of partially ground-up silt, pebbles, rocks, and even boulders. This debris then serves as a kind of grit, like rough-textured sandpaper—but sandpaper under enormous pressure. Warm-based glacial motion scores, scrapes, and grinds the bedrock, leaving all of the telltale signs of glaciation.

Though glaciers move quite slowly, there is a surprisingly large range in their velocities. Particularly fast-moving portions called “ice streams” can move many hundreds of feet in a year, while the more sluggish portions may travel only a few feet per year. In some locations, a glacier can be essentially static and unmoving for long periods of time. But regardless of their speed, flowing glaciers pick up and push aside a massive amount of material. Dust and debris from the surrounding area fall upon the surface of the glacier and are incorporated into the layers of snow where they are eventually integrated into the icy body of the glacier. Rocks from the size of pebbles to enormous boulders are plucked up by glaciers and transported downstream, in the direction of the ice flow. All of this material is referred to as moraine or glacial till, and ends up somewhere once a glacier retreats. The amounts of moraine can be enormous, easily dominating a landscape. All of this rocky material can dam up water flows, or simply leave a huge layer of moraine that itself has depressions in it capable of becoming lakes.

Broadly stated, glaciers form lakes via two processes. The first is the cracking, destruction, and removal of rocks, a process that typically occurs in the upper regions of the glacier, often very close to its origin near a mountain peak. This can form, for example, cirque lakes (described below), where a bowl is gouged out by the glacier and the resulting material deposited on the downstream side of the bowl, forming a basin that becomes a lake. The second process of lake formation concerns the ultimate location of all of the material that glaciers gouge out—the moraine and glacial till—the rock, soil, silt and other detritus glaciers pick up and move about.

In the popular imagination, glaciers are often considered to be pristine tongues of blue ice, and parts of glaciers do indeed look this way. But a glacier can also be fantastically dirty, filled with all manner of material that existed in its path. At the large end, boulders as large as a house can be moved by glaciers, sometimes plucked up at the base of the glacier and moved downstream, then left behind, sometimes in the middle of a valley where no similar rock can be found nearby (these are called dropstones or glacial erratics). At the other end of the size scale, glaciers can grind rocks up to the point where all that is left is an extremely fine powder called rock flour. Rock flour is easily transported by the melted water of a glacier and gives the meltwater an odd color, a kind of milky turbidity that has been variously described as opalescent, bluish, or greenish-yellow. These colors are used by glaciologists as a telltale sign that the flow of water into a lake or bay has originated as glacier meltwater somewhere in the mountains above.

“Moraine” gets pushed either to the side, forming lateral moraines, or in front of the glacier, resulting, ultimately, in a terminal moraine. Just like an enormous bulldozer pushing earth about, glaciers can dam up valleys and leave undulating quantities of moraine. All of these can fill with water, as described earlier, and form lakes.

One type of glacially formed lake, particularly common in the northern United States and Canada, is the kettle lake. There are countless numbers of these quaintly named lakes. Perhaps the most famous is Walden Pond in Massachusetts, whose shores were graced by Ralph Waldo Emerson and Henry David Thoreau, whose book, Walden is often seen as the origin of environmentalism. These lakes are formed not from the relatively simpler process where terminal moraine is dumped at the end of a valley, thereby damming up a river, but from a decidedly messier and in fact incompletely understood process. When glaciers recede, it isn’t necessarily as one might imagine, the snout melting as a sort of uniformly receding line. In some cases, enormous chunks of ice break off as the glacier melts, staying behind perhaps for as long as a hundred years or more. Meanwhile the remainder of the glacier, the main tongue of ice, continues to recede, pulling back to a higher altitude or a more northerly location. In the simplest of cases, these blocks of ice are surrounded by moraine. Hence, as the ice block melts, the moraine serves as the bank of a newly formed lake, the water conveniently provided by the melted ice.

But the process can be more complex when the large blocks of ice aren’t simply surrounded by moraine, but are also covered by it and contain it. A bowl shape will still result when the ice fully melts, but the resulting shoreline and lake bottom can have just about any shape. The process is poorly understood, but certainly is complex, as the slowly melting ice fills a basin formed by the surrounding moraine that is certain to adjust as the ice melts. Moraine once supported by underlying ice is bound to fall and shift. Moreover, weathering of the moraine once the ice is gone will further change the shape of the lake banks. Accordingly, kettle lakes exhibit a broad range of shapes, from the smooth and rounded to the highly irregular.

A glacially formed lake common to alpine regions is the cirque lake, also referred to as a tarn. These lakes are most commonly located close to the origin of alpine glaciers and are found in every mountain range that has experienced glaciation or is currently glaciated. In these headland regions, the rock of the mountain immediately adjacent to the head of the glacier experiences large variations in temperature. The glacier itself cools the region, but during the day sunshine increases the temperature to above freezing at the upper edge, melting some of the ice. The resulting meltwater, as well as precipitation and snowmelt from higher up on the peak, flows into cracks in the rock near the glacier edge. At night, temperatures drop below freezing, and the water in the cracks freezes and expands.

Repetitions of this frost-thaw cycle shatter the rock, causing rock falls and exposing other cracks. Since this process occurs primarily at the head of the alpine glacier where the topography is already steep, these frost-thaw cycles tend to form vertical cliffs along the headland of the glacier. The pieces of shattered rock eventually make their way beneath the glacier, which uses these bits of rock to slowly grind out a curved bottom as the glacier moves downstream. The resulting glacial till continues to move downstream, and some is deposited on the far side of the cirque, creating a lip that encloses the basin. Upon glacial recession, what remains is a more or less circular lake with a steep vertical headwall on the side nearest the mountain peak and a lip composed of till on the side opposite the headwall. Cirques often resemble amphitheaters or look like giant armchairs in the upper mountain valley. Fed by runoff from precipitation and snow melt, cirques are the gorgeous mountain lakes often seen near peaks.

While cirques are usually found near mountain tops, they can also be found in lower regions of the mountain, and when this happens, they may appear as a line of round lakes distributed along the mountain valley and connected by a river, the effluent of one cirque feeding the cirque beneath it. Such lake chains are sometimes referred to as paternoster lakes, the name derived from the similarity of this chain of roughly circular lakes to rosary beads, paternoster being Latin for “Our Father,” the first prayer in the rosary. A striking example of such a lake chain can be found along the Swiftcurrent Valley in Montana’s Glacier National Park.

This description of how glaciers grind and scrape rocks, accumulating all manner of mud and silt, gives an impression of glaciers as messy things. And, indeed, the interface between the sole of the glacier and the ground can certainly be a very dirty place. Moreover, the surface of glaciers can be soiled with bands of dirt and speckled bits of moraine. But it is also true that glaciers can be extraordinarily beautiful. When one reads textbooks on glaciers, in between the terminology-filled pages and the descriptions of moraine and rock scrapes, are also lyrical passages describing the stunning beauty of glaciers. It seems these scientists, these glaciologists, could only contain their joy and wonder for so long and just had to wax poetic for a time, before returning to their scientific selves. This is nowhere more apparent than in the book Glaciers of North America, written in 1897 by Israel C. Russell, professor of geology at the University of Michigan. In describing crevasses, the enormous cracks in the ice that open up when glaciers move over uneven terrain, he writes:

The sides of crevasses are frequently hung with icicles, forming rank on rank of glittering pendants, and fretted and embossed in the most beautiful manner with snow wreaths, and partially roofed with curtain-like cornices of snow. These details are wrought in silvery white, or in innumerable shades of blue with suggestions of emerald tints. When the sunlight enters the great chasms, their walls seem encrusted with iridescent jewels. The still waters with which many of the gulfs are partially filled, reflect every detail of their crystal walls and make their depth seem infinite. No dream of fairy caverns ever exceeded the beauty of these mysterious crypts of the vast cathedral-like amphitheatres of the silent mountains.

Perhaps it is this glacial beauty and magic that imparts itself upon the equally beautiful lakes formed by glaciers.

Another thing one notices when reading books and journal articles on glaciers and the lakes they form, is the focus on the Northern Hemisphere. Regions like Canada, the northern United States, Greenland, Iceland, and northern Europe get virtually all the attention. Very rarely does the literature discuss South America or Africa or Australia. Shouldn’t there be glacially generated lakes in these regions, formed perhaps by ice that extended from Antarctica? The answer to this question is, mostly, “no” simply because, to have a lake, you must have land, preferably land near a pole. In the Southern Hemisphere there isn’t a whole lot of land near the pole, at least compared to the Northern Hemisphere.

When an ice age begins, glaciers advance in two ways. First, they originate on the peaks of mountains and advance downward, a process that happens even in mountain ranges located on the equator. The second way glaciers advance is from the poles toward the equator, a process that forms enormous continent-spanning glaciers. So, to have a very large glacier and hence a lot of glacially formed lakes, it helps to have a lot of land near the poles. With this rubric in mind, let us compare the Southern and Northern Hemispheres by observing just how much land exists between an arbitrarily chosen latitude of 45 degrees and the pole at 90 degrees. This exercise yields a surprising result. In the north, there is a lot to talk about. Virtually all of Canada exists north of 45 degrees north—the entire nation except for parts of Nova Scotia, a bit of southern Ontario, and a very small bit of New Brunswick. In the United States all of Alaska, Washington, and North Dakota are north of this boundary along with most of Montana, and significant parts of Minnesota, South Dakota, Wisconsin, Michigan, and Maine. Virtually all of Russia is north of 45 degrees along with most of France, all of Germany, Poland, Hungary, Ukraine, Denmark, the United Kingdom as well as the nations of Norway, Sweden, and Finland (and of course Iceland and Greenland). That adds up to a lot of land.

If we now turn our attention to the Southern Hemisphere, what we will see (and if someone hasn’t pointed this out to you before, this may come as a bit of a surprise) is almost nothing. South of 45 degrees exists the southern tip of New Zealand, the Falkland Islands, and a portion of the southern tip of South America—a bit of Chile and Argentina. Save for a few small island outposts such as the South Sandwich Islands or Heard Island, that’s all there is. None of Africa is south of 45 degrees. None of mainland Australia is south of 45 degrees. Indeed, if you were to take a globe and look down directly over the South Pole, your primary impression would be one of oceans. If you do the same by staring down at the North Pole, your primary impression is one of land. In short, the prodigious covering of the Northern Hemisphere with glacially produced lakes exists because that is where the land is.

If you are any kind of a student of geography, you have probably noticed that the above description of the Southern Hemisphere contains a glaring omission. What the Southern Hemisphere does have between its pole and 45 degrees latitude is something the Northern Hemisphere doesn’t—an entire continent: Antarctica. Furthermore, Antarctica is enormous and nothing if not glacially covered. But, this large continent is not a place where glaciers used to be—it is an intensely glaciated place now, with almost no exposed land. There hasn’t been the process of glacial recession and the concomitant lake formation that exists in the Northern Hemisphere. Once again, when we talk about lakes and particularly glacially formed lakes, the discussion mainly concerns the Northern hemisphere. But, there is an interesting exception.

Although Antarctica does not have lakes in the typical sense, which is to say bodies of water with a floor composed of rock or soil and a surface exposed to the atmosphere—it does have many lakes of a very different kind. Termed subglacial lakes, Antarctica’s lakes are pods of water located at the boundary between the bedrock of Antarctica and the ice sheet lying above. Some of these lakes are enormous—at least one, Lake Vostok, is as large as Lake Ontario. One might argue that these lakes are somewhat like a typical lake in winter with a sheet of ice on its surface. However, this sheet of ice can be miles thick and does not melt away in the summer. These lakes contain water that has not been exposed to the atmosphere in hundreds of thousands of years or, according to some estimates, for millions of years. Covered as they are in thick ice, the waters in these lakes are utterly dark. What exists or might exist in the black depths of these mysterious lakes deep beneath the glacial surface is a subject we will return to in a later chapter. Suffice it to say that in the cold darkness of these lakes exist life forms we are only just learning about.

The above has hopefully demonstrated the importance of glaciers in forming lakes, a process that occurred when glaciers dominated those parts of our planet that, today, experience ice only in the winter, if at all. But, while the importance of these glaciers on the formation of lakes might seem to be one of historical significance, it should be noted that the process is ongoing. Indeed, in the currently popular discussion of global warming, the fact is often lost that Earth is currently in the midst of an ice age, specifically the Cenozoic ice age, which began approximately 37 million years ago. That may seem like a long time but in geological terms, it is relatively brief. For context, note that the ice age prior to the Cenozoic, the Late Paleozoic, began some 300 million years ago.

Ice ages exhibit periods of glaciation followed by interglacial periods, time intervals when glaciers recede and the planet experiences warmer conditions away from the poles and mountain peaks. There is much debate about how long these periods lasted (and will last). There is, however, general agreement that during an ice age, glacial stages last for about 100,000 years and interglacial periods last about 20,000 years. The peak of the last glacial period is thought to have been about 18,000 years ago, and it is this period of time that colloquially is often referred to as the ice age. But it must be noted that our ice age, the Cenozoic, continues.

And, global-warming concerns notwithstanding, our current interglacial period is actually not as warm as prior ones have been. The peak of the last interglacial period was about 125,000 years ago and was a time when flora and fauna were found much farther north than today, as evidenced by fossils of beetles and trees in locations such as Baffin Island. The amount of ice on the planet during this last interglacial period was much smaller and world sea levels are estimated to have been 16 to 26 feet higher than today.