Letter 12: Lakes of the Great Rift Valley

I understand that you're learning about plate tectonics, the movement of the plates that compose the top layer of the Earth, plates that are pushed about by the upwelling of lava from deep in the Earth. You have probably learned that the eastern side of Africa is being split off from the rest of Africa. For about 30,000,000 years now, a huge plume of lava has been pushing up from below. It has raised a huge plateau a mile high. Along the eastern and western edges of the plateau the plate has cracked, making deep rifts. Along the rifts, there are numerous volcanoes. Here's a map of the Great Rift. Someday, the ocean will flow into this 3000-mile-long crack in the continent of Africa.

Our travels in Kenya and Tanzania took us back and forth across the Great Rift Valley. You can see part of the Rift Valley in this somewhat hazy photo (smoke from fires in Uganda was limiting our visibility). Note the abrupt vertical walls left when the plate rifted and the nearer part dropped down. (When you see a waterfall like this one, it's a sign that the landscape has been changing; the river hasn't had time to cut its bed down to the lower level.)
Everything we saw was impacted by the Rift. The great grasslands of the Serengeti, for example, don't grow trees because the volcanic ash that has rained down on the land has formed a layer that is too hard for tree roots to penetrate. Only the acacia trees (the ones with the huge white thorns) can grow there (and only because they have very shallow root systems).
One of the most fascinating aspects of the Rift Valley is its lakes. In the western rift, there are several very deep lakes, which we didn't get to visit. In the eastern rift are a dozen or more long narrow lakes aligned with the rift, all of them quite shallow. Those that have outlets are fairly fresh water, but those that do not have outlets have become very salty. They contain large amounts of dissolved sodium carbonate from the ash of the volcanoes, which makes them "alkaline" (that is, basic or caustic, the opposite of acidic). Some of them are so caustic that they can dissolve human skin.

Great Rift Valley
Kenya
(Click on any of the photos to enlarge them)

We spent a couple of days visiting one of these alkaline lakes, Lake Nakuru. The mud there was snow white from the sodium carbonate, which is why this mud-covered Black Rhinoceros is so white.
Though you can't see it in the picture, this rhino was being followed by a baby, also snow white.

Black Rhino covered in white mud
Lake Nakuru

We saw many wonderful animals around Lake Nakuru, including a male Lion that had climbed way up in a tree for some reason (which isn't easy for big males to do). But for us, the most interesting of the creatures of the alkaline lakes were the flamingos.
The whitish ones with the mostly pink beaks are Greater Flamingos. The pink ones with black beaks are Lesser Flamingos.

Greater and Lesser Flamingos
Lake Nakuru

We saw mostly the Lesser Flamingos. There were thousands and thousands of them.
They can walk about in these alkaline lakes that would quickly burn you and me, because they have very tough skin on their legs. They walk and swim out into the middle of the lake at night to eat the cyanobacteria that grow in the water. They feed by filtering the cyanobacteria through their peculiar bills. These cyanobacteria are red, which is what colors the flamingos' feathers pink; the flamingo babies are pale grey when they hatch.

Flamingos
Lake Nakuru

In the morning, the flamingos come back to the edge of the lake to places where fresh-water streams are feeding into the lake, so that they can drink the fresh water and bathe away the salt that has accumulated on their feathers.
They are a marvelous sight flying about.

Flamingos
Lake Nakuru

Here's a short video clip that you might want to watch. It shows the Lesser Flamingos nesting at Lake Natron in northern Tanzania, another of the alkaline lakes of the eastern Rift Valley. There are about two million Lesser Flamingos in the world today, and every one of them was hatched on a salt island in the middle of Lake Natron. The temperature there often gets to 120 degrees, and the pH can reach an alkalinity of 9 to 10.5. It's a very hostile environment, but it protects them from their main predators, Hyaenas and Maribou Storks (both of which we saw on the shores of Lake Nakuru keeping an eye out for the chance to eat a flamingo).

Flamingos and Pelicans
Lake Nakuru

Sadly, their greatest danger is not the hyaenas or the storks. It is humans. The Tanzanian government wants to build a soda plant to mine the salts from Lake Natron, which it is feared will so disrupt the breeding cycle of the Lesser Flamingos that their population will crash. That would be a very sad thing.

Aunt Melinda

Our travels in Kenya and Tanzania took us back and forth across the Great Rift Valley. You can see part of the Rift Valley in this somewhat hazy photo (smoke from fires in Uganda was limiting our visibility). Note the abrupt vertical walls left when the plate rifted and the nearer part dropped down. (When you see a waterfall like this one, it's a sign that the landscape has been changing; the river hasn't had time to cut its bed down to the lower level.) Everything we saw was impacted by the Rift. The great grasslands of the Serengeti, for example, don't grow trees because the volcanic ash that has rained down on the land has formed a layer that is too hard for tree roots to penetrate. Only the acacia trees (the ones with the huge white thorns) can grow there (and only because they have very shallow root systems). One of the most fascinating aspects of the Rift Valley is its lakes. In the western rift, there are several very deep lakes, which we didn't get to visit. In the eastern rift are a dozen or more long narrow lakes aligned with the rift, all of them quite shallow. Those that have outlets are fairly fresh water, but those that do not have outlets have become very salty. They contain large amounts of dissolved sodium carbonate from the ash of the volcanoes, which makes them "alkaline" (that is, basic or caustic, the opposite of acidic). Some of them are so caustic that they can dissolve human skin.	Great Rift Valley Kenya (Click on any of the photos to enlarge them)
We spent a couple of days visiting one of these alkaline lakes, Lake Nakuru. The mud there was snow white from the sodium carbonate, which is why this mud-covered Black Rhinoceros is so white. Though you can't see it in the picture, this rhino was being followed by a baby, also snow white.	Black Rhino covered in white mud Lake Nakuru
We saw many wonderful animals around Lake Nakuru, including a male Lion that had climbed way up in a tree for some reason (which isn't easy for big males to do). But for us, the most interesting of the creatures of the alkaline lakes were the flamingos. The whitish ones with the mostly pink beaks are Greater Flamingos. The pink ones with black beaks are Lesser Flamingos.	Greater and Lesser Flamingos Lake Nakuru
We saw mostly the Lesser Flamingos. There were thousands and thousands of them. They can walk about in these alkaline lakes that would quickly burn you and me, because they have very tough skin on their legs. They walk and swim out into the middle of the lake at night to eat the cyanobacteria that grow in the water. They feed by filtering the cyanobacteria through their peculiar bills. These cyanobacteria are red, which is what colors the flamingos' feathers pink; the flamingo babies are pale grey when they hatch.	Flamingos Lake Nakuru
In the morning, the flamingos come back to the edge of the lake to places where fresh-water streams are feeding into the lake, so that they can drink the fresh water and bathe away the salt that has accumulated on their feathers. They are a marvelous sight flying about.	Flamingos Lake Nakuru
Here's a short video clip that you might want to watch. It shows the Lesser Flamingos nesting at Lake Natron in northern Tanzania, another of the alkaline lakes of the eastern Rift Valley. There are about two million Lesser Flamingos in the world today, and every one of them was hatched on a salt island in the middle of Lake Natron. The temperature there often gets to 120 degrees, and the pH can reach an alkalinity of 9 to 10.5. It's a very hostile environment, but it protects them from their main predators, Hyaenas and Maribou Storks (both of which we saw on the shores of Lake Nakuru keeping an eye out for the chance to eat a flamingo).	Flamingos and Pelicans Lake Nakuru