Arid Landforms
I.	Climatic setting
	A.	Potential evapotranspiration exceeds precipitation. Deficits are chronic.
	B.	When rainfall does occur, it is often convective in nature. Heavy rain showers of brief duration.
II.	Vegetation and soils
	A.	Sparse shrub cover means soil surface is exposed to rain splash. This compacts surface pore space and inhibits infiltration. Thus, surface runoff after heavy rains can be substantial and rapid.
	B.	Results in erosive loss of fine textured sediments at surface, leaving behind a characteristic gravelly surface, called a desert pavement.
III.	Drainage networks in arid landscapes
	A.	Generally low discharges.
			Streams are intermittent in nature. Most of the time, channels are dry.
	B.	When it does rain, lack of vegetation cover combines with desert pavement to produce rapid overland flow and flash flooding in normally dry stream channels.
		1.	This rapid delivery of overland flow to stream channels produces a burst of elevated discharge, which temporarily increases stream capacity relative to sediment load. The result is downcutting of small canyons, called washes or arroyos.
IV.	Basin and Range Topography
	A.	Alternating uplifted and down-dropped normal fault blocks can produce a landscape of mountain ranges separated by broad basins.
	B.	If such tectonic activity occurs in arid regions, internal drainage networks are common.
			Not enough water flowing through streams to provide for downcutting of drainage divides. Water does not flow to the sea, but rather collects in basins, or low points in the landscape.
	C.	In the mountains, orographic precipitation and cooler temperatures (reduced evapotranspiration) combine to produce higher discharge into stream channels.
		1.	Streams flowing from bedrock-confined canyons carved in the mountains have a high capacity.
		2.	as this channelized water flows from a mountain canyon down onto the adjacent basin, the water spreads out laterally as it escapes the confining canyon walls.
			a.	this dramatic change in channel shape allows shallow water to wash over a large fan-shaped surface. The wetted perimeter of the unconfined flow is much greater, resulting in friction that reduces stream velocity and triggers aggradation.
			b.	the result is an alluvial fan, a fan-shaped wedge of sediments deposited at the mouth of canyons. This is exactly the same aggradational process that causes deltas to form (decreased velocity --> decreased capacity relative to load).
		3.	where a long, linear mountain range has many adjacent canyons, alluvial fans will build and coalesce with neighboring fans, producing a broad apron of sediments at the base of the mountains known as a bajada.
		4.	as shallow water flows over alluvial fan or bajada surfaces, it infiltrates, gradually reducing discharge and stream transport downslope.
			a.	This causes coarser sediment load to be deposited near the top of the bajada surface, with finer textured materials remaining in suspended load and washing out into the adjacent basin.
			b.	clay-rich soils develop in the middle of these internally drained basins.
		5.	Also, water pools in lowlands after heavy rains, then slowly evaporates. A surface layer of salts (formerly part of the dissolved load of nutrients in the stream water) is left as an evaporite.
				These salt-encrusted low points in the center of drainage basins are called alkali flats, or playas.
		6.	The Great Salt Lake is the base level of a large, internally drained basin in Utah and Nevada. Its salt content, several times higher than the oceans, is the result of concentration of salts from materials dissolved and eroded from its surrounding uplands.
				It was much larger (and less saline) at the height of the last Ice Age. As it has shrunk in size, it has left behind tell-tale evidence of its former larger size and greater depth.
				a.	linear beach ridges can be seen carved into the hillslopes on the surrounding mountains.
				b.	the Bonneville Salt Flat is part of an extensive playa surface that was formerly below lake level, but is now exposed as a large saline evaporite deposit.
V.	Slope processes in arid landscapes
	A.	Lack of vegetation, rain splashed soil surface favor sheetwash, or overland flow as the primary slope erosion agent. Soil creep is not very active, so slopes are commonly straight, not rounded.
			This is reinforced by basal sapping of weaker rock units by differential erosion, which leaves behind exposed cliff faces that are gradually undercut and mass waste as rock falls or landslides.
	B.	In areas of pronounced crustal upwarping of horizontal sediments, alternating resistant and weak rock layers can produce canyon country with deeply incised canyons.
		1.	this yields landforms dominated by cliffs of resistant caprock over straight sloping terrain that rests near the angle of repose.
		2.	the Grand Canyon of the Colorado River is a stunning example of this kind of topography.
		3.	interestingly, limestone is a common resistant caprock in arid areas. Although calcium carbonate readily dissolves in humid environments (where surpluses create sinkholes and caves in a karst landscape), it is resistant to physical weathering in a dry setting.
VI.	Wind as a denudational agent.
	A.	Surprisingly, even in arid landscapes, water is the dominant hillslope erosion agent and carver of the landscape.
	B.	However, in some areas, loose sediments that are exposed by the thin vegetation cover can be eroded, transported, and deposited by the wind.
	C.	Like water, wind represents flow of a fluid. Of course, it is much less dense and viscous, so it only erodes where surface mineral soils are exposed and where wind speeds are of a high enough velocity to dislodge, lift, and carry sediments.
		1.	Wind erosion can produce deflation features, small to medium-sized depressions in areas of exposed soil or regolith that are hollowed out by the wind.
		2.	Duststorms and sandstorms are common meteorological events in desert landscapes. They are the product of high winds with little or no rain, eroding exposed mineral soils. The transported sand grains cause much damage by abrasion as they strike surfaces such as plants, buildings, or animals (including people).
		3.	Wind deposition results in piles of sediment on the earth's surface. Where the winds transport sand, the resulting depositional features are sand dunes.
			a.	These commonly form in windy areas of high sand concentration (along ocean and lake shores, in unvegetated desert areas).
			b.	They can accumulate to great depth where prevailing winds concentrate them in a region of decelerating wind flow (rapid reduction in wind transport capacity). This has formed areas like the Great Sand Dunes, at the base of the Sangre de Cristo Mts. in south-central Colorado.
	D.	Winds can also transport silt and clay. In fact, since these are smaller particles, they can be carried more easily and for greater distances.
		1.	Deposits of wind-blown silt were commonly formed downwind of major glacial drainage ways during the Ice Ages.
			a.	Since sediment load choked stream channels, there was lots of exposed glacial sediment in broad floodplains.
			b.	Strong winds along the margin of the glacier (due to concentrated temperature gradient) would erode these from the valley and deposit them on the downwind side of the channel.
			c.	the result is loess deposits. Sometimes, these can be many meters thick, such as the loess bluffs along the eastern edge of the Mississippi River floodplain from Wisconsin to Mississippi.
		2.	Other source areas of wind-blown sediments are volcanic eruptions. These have produced a wind-blown veneer of silt deposits downwind of the volcanic Cascade Range in eastern Oregon and Washington.
		3.	Thanks to the silty texture, loess and wind-blown volcanic materials form very fertile soils. They are young, rich in basic ations, and have ideal water holding capacity.