Fresh water's best expression

 kw: book reviews, nonfiction, limnology, lakes, natural history

This is a picture of me walking (well, standing) on water. It's easy when it's frozen! This was taken at Pactola Lake in the Black Hills of South Dakota during my graduate school days. In the background waterfowl are taking advantage of the unfrozen part of the lake. Pactola is not a natural lake but a reservoir, a combination of water supply for Rapid City and flood control for Rapid Creek. The lake's area is about 740 acres (300 hectares), which makes it a middle-sized lake. While it is several miles long, it is at most just over half a mile wide, which makes it perfect for canoeing. That's something some friends and I did a few times.

According to John Richard Saylor, in Lakes: Their Birth, Life, and Death, there are about one-and-a-quarter million lakes on Earth, of 10 hectares (24.7 acres) or greater extent. Some limnologists count lakes larger than 1 hectare, saying there are some 8.5 million.

Right away I am going to fault the author for utterly ignoring metric units. I suspect many readers will wonder, "What's special about 24.7 acres?" A simple foot note or parenthetical note could clear that up. Of course, 10 hectares is arbitrary, but nearly everywhere except America it's at least understandable. There isn't a well-agreed-upon way to distinguish a lake from a pond, so a line has to be drawn somewhere.

Limnology is the study of fresh water in all its forms, although glaciology has its own niche when studying frozen water. However, this book says little about rivers, except as feeders or drains of lakes. The book follows a simple classification scheme. Glaciers produce the most lakes by far, either by gouging out basins or by depositing moraines. 

A few years before moving to South Dakota I spent a few weeks doing geology in an area called 20 Lake Basin, above Yosemite in the Sierras. This image from Google Earth has 15 labels, but two of them are "...Lakes", and there are several lakes visible with no label. Most of them are gouged-out lakes. The basin is surrounded by glaciers. I swam in nearly every one of the lakes shown.

Landslides sometimes form lakes by blocking a stream. Such lakes seldom last long, but there are a few that have persisted for centuries. Then there's Quake Lake near Yellowstone in Montana, formed by an earthquake in 1959. It's about six miles long, and seems to be here for the long haul. Time will tell.

I don't know if people make more dams than earthquakes and landslides do, but there are thousands of artificial dams and their attendant ponds or lakes. Some are huge, such as Lake Powell (half dried up at present) on the Colorado River: 25-40 miles wide, with an area around 160,000 acres (65,000 hectares).

There is a kind of allure about damming up a stream. As a child, I was like many of my friends, in that we sometimes dammed up a narrow spot in a local creek, to see how high we could get the water to rise. It was typically no more than an inch or two. I spent a few otherwise idle hours moving stones and sand into place to patch up first one, then another, "escape route" the water would take as it rose.

On the Wikipedia page for Lake Powell, it is noted that sediment with a volume of 11 billion gallons settles in the lake every year. Since 1969, capacity has been reduced 7%. This is part of the ordinary life cycle of any lake. Once formed, it will be gradually filled with sediment. During its "active lake" phase, a lake will host wildlife that varies according to the local climate. 

All lakes eventually fill up, or are drained in some way (often by human activity in recent centuries). Given time, a lake that avoids being drained turns into a marsh, then a bog, then a meadow, and in time, it may leave little trace. 

The author points out a few lakes that are millions of years old (Lake Baikal in Russia comes to mind). He does a quick study with us about what it takes to get a lake to last so long, or longer. While some areas of Earth are up to a few billions of years old, the erosion cycles they have been through would have erased any lakes that formed in that time. It's likely that a tenth of a billion years is about the limit.

It's an enjoyable excursion into the subject. Much recommended!