Guest Column by GeoT
Dateline: 08/09/00
Have you ever simply looked at a map with no particular point of interest in mind? Just lookin'. And then, perhaps, some feature strikes your interest like. . . lakes?
Kind of a curious worldwide distribution pattern. It seems some continents and regions have a great many more lakes than others. Hmmm. Wonder why. Now, are we 'Thinking Geographically'? Could be!
We sometimes see lakes where we'd expect to see them; and other times, we see them where we're not so sure we should. Lake Chad!? Lake Eyre?! Salton Sea?!
Mountains are an example. Mountains and highlands ought to drain quite well and many do. Like the pitched roof of a house. But, if there are pools of water up there, something rather unusual has happened. Not the normal cycle of erosion.
The something unusual could be one or more of several things. Look at eastern Africa (the highest region of the continent) and that's where most of the largest lakes are! Look at the northern and middle Rocky Mountains. Surely, as steep as they are, the water should drain; but there are lakes. Or, look at Crater Lake in Oregon, and there is a lake smack in the center of a mountain!
Click here for a map of the Grand Teton Mountains
Look how glaciers have carved valleys into this range. Look at the lakes! Some are cirque lakes in the depressions where the glaciers formed, some are tarns in small basins produced by scouring and plucking down the valley. Sometimes the tarns are in a string and called Paternoster Lakes. Look at Jackson Lake. It is behind a moraine built by the coalescing glaciers flowing from the mountains. Look at the north end of Jackson Lake --- oops!! The Snake River flows along the base of the Tetons and pools where it is impeded by the moraine.
Click here for a map of the Gros Ventre River
While we are in Teton Territory let's not miss another way a lake can form. Rockslides. Not all that common - but here's one. Look at Lower Slide Lake. See where it says Gros Ventre Slide Geological Area to the southwest? Look at the bent contours under the lettering. This is where, in 1925, an estimated 50 million tons of rock slid across the Gros Ventre valley producing a dam. Pan the map up the valley and you will find another smaller slide-produced lake! This is east of the town of Kelly, Wyoming located southeast of Jackson Lake.
Click here for a map of Hebgen, Montana
Another lake formed by a rockslide. This one triggered by an earthquake in August, 1959. Tragically, the slide hit a campground with the loss of 26 lives.
Look to the north. More lakes produced by Alpine glaciers. (Photos)
Click here for a map of Crater Lake Oregon
What we are looking at with Crater Lake is the result of a violent explosion and collapse of the top of Mt. Mazama 7700 years ago. This lake is nearly 2000 feet deep as a result of this activity. Look around the rim for evidence of pre-existing glacial valleys (contour lines show the typical U shape). Look at Wizard Island, which was built by subsequent volcanic activity. Another curious lake is in Siberia - Lake Baikal. Amid the highlands and mountains.The name Siberia means 'sleeping land' and Baikal is Yakut for 'ocean' or 'sea'.
Lake Baikal contains 20% of the earth's non-frozen fresh water. It is a cold lake too, attaining a high temperature of only around 50 degrees Fahrenheit, that would cause hypothermia even in July and August when air temperatures can be in the 90s! Swimming in Lake Baikal would be a refreshing (but brief) experience, I'd think!
Lake Baikal is the world's deepest lake, nearly 1 mile to the floor, and is said to contain enough water to fill the Great Lakes of North America. Quite a lake! Question is: what is it doing here?
The answer is found in the upper mantle. A convection cell below eastern Siberia produced a series of horizontal and vertical movements in the crust. Rifting. In some places the rock was raised to form mountains, but in others, it was dropped to form valleys. In the case of Lake Baikal, a very deep rift valley! A similar explanation is found in east Africa. Rifting there too. The Great Rift Valley runs thru the highlands of east Africa. Downdropped blocks of rock form the lake basins.
The greatest lake-former of them all has to be glacial ice. Look at the Canadian Shield and the lakes around it. This explains why a world map shows a preponderance of lakes in the northern parts of North America and Europe the areas covered by Pleistocene glaciers. But, as we have seen, there are other ways for lakes to form, too.
Next time we pack up the Oldsmobile Bravada and "head for the lake," we might just ask ourselves how that lake got there for us to enjoy!
Now. . . all this has me wondering about Lake Titicaca and Lake Poopo in the Andes mountains of South America. . .
GeoT is a long-time high school geography teacher from Illinois. In addition to geography, he enjoys railroads and model railroads, old Oldsmobiles, and gardening.
