Playas

Playas are the most prominent landforms in deserts. The word itself names a shallow lake, but what you see on the ground is usually a flat, salty plain. The key to understanding a playa is to see it as the end point of desert drainage — the place where water goes to die.

In desert basins surrounded by mountains and hills, all the drainage flows towards the centre of the basin. As this water moves down from the basin margins, it carries sediment with it. Over time, this sediment is deposited gradually, layer upon layer, and the centre of the basin gets filled in. The result is a nearly level plain forming right at the heart of the basin.

Now, this plain does not always look like a plain. When there is sufficient water — after rains, for instance — the plain gets covered by a shallow water body. This shallow lake is the playa itself. But the water does not stay long. Because deserts have high evaporation rates, the water is retained only for a short duration. It evaporates quickly, leaving the flat surface exposed again.

Here is where the salt comes in. The water that collects in the playa carries dissolved salts from the surrounding rocks and soils. When the water evaporates, the salts are left behind. So quite often, playas contain a good deposition of salts. When the playa plain is covered up by these salts, it gets a special name: alkali flats.

So the sequence to remember is this:

• Drainage from surrounding hills and mountains converges towards the basin centre.
• Gradual deposition of sediment from the basin margins builds a nearly level plain.
• In times of sufficient water, a shallow water body covers this plain — that shallow lake is the playa.
• High evaporation means the water is retained only briefly.
• Evaporation leaves behind dissolved salts, giving the playa a good salt deposit.
• A playa plain covered by salts is called an alkali flat. …