Earth Science 7.2 Deserts

Geological Processes in Arid Climates Desert landscapes reveal the affects of running water and wind. These combine in many ways to provide a wide variety of desert landscapes.

Geological Processes in Arid Climates Deserts are very different than landscape is humid areas of the world. Humid areas generally have rounded hills and curving slopes. By contrast, deserts generally have angular rocks, sheer canyon walls, and surfaces covered in sand and small pebbles. Why rounded vs. angular?

Geological Processes in Arid Climates In humid regions, well developed soils support a continuous layer of plant growth on top. In these areas, slopes and rock edges are rounded from chemical weathering.

Geological Processes in Arid Climates In contrast, in a desert much of the debris of rock is from mechanical weathering. The minerals that make up the rock debris are unchanged chemically. Because of the dryness of the climate, the rock debris do not break down into rich soil as they do in humid climates.

Geological Processes in Arid Climates Chemical weathering is not entirely absent from a desert however. Over long spans of time, clays and thin soils do form. Many of the iron-rich silicate minerals oxidize producing rich rust colors in the landscape.

Water in Desert Climates In humid or temperate climates, streams that run year round are a normal fixture. In contrast, in deserts many streams are “ephemeral” lasting only a short time. They run usually only after a rare heavy rain washes over the landscape. We call these ephemeral streams. In the western states, people call these dry creeks washes or arroyos

Water in Desert Climates These ephemeral streams are especially dangerous because of flash floods that occur. During heavy rains, waters falls so fast that the ground can not absorb it. The lack of vegetation allows the water to run quickly off the land to fill these dry creek beds. Flash floods are very dangerous in canyons and dry river beds with steep sides.

Water in Desert Climates The floods end as quickly as they start but the amount of erosion that can happen during one can be impressive. The lack of plants to hold the soil together means these flash floods can carry much away in a short period.

Water in Desert Climates Because arid regions lack permanent streams, they have interior drainage. This means these temporary streams do not flow out to the ocean but end within the desert itself.

Water in Desert Climates In the United States the Dry Basin and Range is an example of this. This area includes southern Oregon, all of Nevada, western Utah, southeastern California, southern Arizona, and southern New Mexico.

Water in Desert Climates When the occasional runoff of water from rainstorms does happen; it is heavily loaded with sediment from the large amount of erosion that happens in a short span of time.

Water in Desert Climates Emerging from the mouths of the canyons, the runoff spreads over the slopes at the base of the surrounding mountains and loses it’s velocity and force. When this occurs the water deposits all of it’s sediment suddenly, dropping it within a short distance of space.

Water in Desert Climates Most of load is dumped within a short distance of leaving the canyon mouth. The result is a cone of debris known as an alluvial fan at the mouth of the canyon.

Water in Desert Climates On rare occasions of abundant rainfall or snowmelt in the mountains, streams may flow across the alluvial fans to the center of the basin, converting the shallow basin floor into a shallow “playa lake”. Playa lakes last only a few days or weeks before evaporation and infiltration dry them up. The dry flat lake bed that remains is called a playa.

Water in Desert Climates Most areas of the country have stream and river systems that drain the land and lead water back to the ocean. In desert areas, streams dry up long before water reaches the ocean. Water quickly disappears between evaporation and infiltration into the soil.

Water in Desert Climates Some permanent streams and even rivers manage to cross arid areas. The Colorado River, Arkansas River, and the famous Nile River all begin in mountains with abundant rain. These rivers are large enough and with enough flow that they lose little in their crossing of the arid desert landscape.

Water in Desert Climates The Nile River, for example, leaves the lakes and mountains of central Africa and crosses over 3000 kilometers of the famed Sahara desert without a single tributary joining it’s flow.

Water in Desert Climates The Nile River, for example, leaves the lakes and mountains of central Africa and crosses over 3000 kilometers of the famed Sahara desert without a single tributary joining it’s flow.

Water in Desert Climates The point to remember about water in a desert environment, is that although it is rare, it is a major force of change through erosion. Most desert erosion results from running water.

Computer lab: Assignment Go to the computer lab and use the internet to research one of the following topics and write a short 3 paragraph report on the subject. Put the ideas in your own words; you may not copy, cut or paste your report. You may choose from one of the following Nile River Colorado River Lake Powell Zion Canyon Grand Canyon Sahara desert Death Valley Gobi desert