Have you ever heard of the term “arroyo”?
In Spanish, the word translates to “brook,” but it’s also used to refer to dry creeks and streambeds.
This unique feature of the Southwest is often seen in states like Arizona, New Mexico, Colorado, and Utah.
They could be wet or dry depending on the recent weather.
Here’s everything you need to know about arroyos, how they formed, and what they’re doing to the environment.
1. What is an arroyo?
An arroyo is a nearly vertically walled, flat-floored stream channel that forms in fine, cohesive, and easily eroded material.
They can cut as deeply as 65 feet into the valley floor, and they are often wider than 165 feet.
Arroyos typically fill with water during the rainy season.
Because they are prone to flash-flooding and erosion, this landform can be dangerous.
You don’t want to be caught in one unexpectedly in inclement weather.
If your land has arroyos, you’ll want to identify them ahead of time to prevent this from happening.
2. How do you pronounce arroyo?
The term arroyo is Spanish in origin.
It’s pronounced “uh-roy-oh.”
3. Where are these features found?
Arroyos are most commonly found in arid and semi-arid climates in the Southwest.
Arroyos becoming wider and deeper over time has been a challenge in the U.S. since western settlement in the mid-1800s.
They constantly change the structure of the physical environment while also having expensive impacts.
4. What are the three causes of an arroyo formation?
There are three primary causes of arroyo formation.
Cause #1: Climate
The climate is responsible for arroyos in many scenarios.
Flooding caused by heavy rain is often what prompts arroyos to form.
Studies have shown that unusually heavy rainfall in Tucson, AZ is responsible for the formation of arroyos in the area.
What causes the rainfall?
Strong and frequent El Nino Southern Oscillation (“ENSO”) events that bring heavy rain.
If unusually wet ENSO conditions occur immediately after a period of below-normal precipitation, then drainage areas in the Southwest become vulnerable to arroyo cutting.
This effect is coupled with the fact that sparse vegetation doesn’t have its normal capacity to protect soil from the impact of rain or to slow/absorb runoff.
Cause #2: Land use
When Western expansion occurred, migrants introduced livestock (cattle, sheep, and horses) to the area.
During a 20-year period (1870 to 1890), the number of livestock increased from 300,000 to 2,300,000.
Similar increases were seen in other Western states as well.
Now, why would changes in land use suddenly produce arroyos?
Here are the top reasons.
- Valley floors became overgrazed
- The soil also became compacted and susceptible to erosion
- Trails were created along streams and hillsides by both humans and livestock, which formed ditches and left the land surfaces susceptible to arroyo formation
However, there were also earlier periods of arroyo formation that predated the introduction of livestock.
This indicates that overgrazing cannot be solely responsible.
It’s thought that factors like climate change have also played an important role in their formation.
Cause #3: Natural internal adjustments
For an arroyo to develop, it’s thought that there needs to be both external force and climate change to trigger incision.
The channel system must be primed for incision and receive random, heavy rain or flood events.
So, for example, if a cloudburst occurs over a drainage area, then it can deeply erode a single channel.
This would leave hanging valleys where tributaries enter.
When future runoff occurs, the tributaries would cause incisions at their mouths and the arroyo would elongate through upstream migration.
If the slope at a tributary becomes extreme enough, it can trigger further arroyo formation as the channel splits laterally along the stream bed.
5. What are the impacts of this phenomenon?
Arroyo cutting has a variety of unfortunate effects on the environment.
Draining of marshes and swamps
Since before 1865, marshes (known as Cienegas) containing beaver ponds, fish, and tall grasses have been drained by arroyos.
Altering of flora and fauna in the area
The draining of marshes and swamps alters the flora and fauna of an area.
When arroyos are formed, they widen and deepen the original stream channel, and they displace the water that was originally nurturing plants and animals.
Decreasing agricultural productivity
Agriculture is often destroyed by arroyos.
When arroyo cutting begins, the surrounding water table is lowered, which makes irrigation difficult.
Arroyos also remove as much as 25 percent of their valley floor and cover downstream agricultural land with unwanted flood-borne sediment.
This sediment harms fertility (and therefore productivity) because it contains large quantities of both sand and gravel.
Arroyo formation often causes excessive deposits of sediment.
This decreases flood protection by reducing the natural regulatory functions of stream channels.
The sediment fills in the spots that would otherwise hold water.
Arroyos prompt a loss of land.
As a result, humans often have to flee the area where they form.
Arroyos can also damage roads, railroads, bridges, culverts, fences, and irrigation.
6. What are examples of famous arroyos?
Two examples of famous arroyos include the Gobi Desert in Mongolia and San Lorenzo Canyon in New Mexico.
7. What does an arroyo look like?
Not all arroyos look the same, but they often appear as shallow stream beds covered in sand or mud.
They may be dried with cracks in the surface.
You may also see arroyos that are cut deeply (20+ feet) with a narrow channel in the ground.
8. Are all arroyos the same?
No, arroyos come in a variety of shapes and sizes.
Some arroyos are more depressions where channel runoff occurs during precipitation.
Other arroyos are deep ravines with steep sides.
This steepness is often prompted by erosion.
When there’s a heavy desert thunderstorm, water will carve out space over time.
And then others take the shape of shallow canyons.
9. Can an arroyo be dangerous?
Yes, when combined with flash flooding, an arroyo can be dangerous.
Most people know the desert (where an arroyo occurs) to be very dry.
However, heavy rain still falls in the desert, and when it does, the hard, dry ground cannot absorb the water quickly enough.
Thus, flooding happens quickly and easily.
This is where the term “flash flooding” comes from.
Water will rush into an arroyo, and it’ll take everything in its path with it.
If you’re in an area with flash flooding, you may see a warning.
Be on the watch for this!
10. What’s the difference between an arroyo, a wash, and a wadi?
Are an arroyo, wash, and wadi the same thing?
You may hear some of these words used interchangeably.
In the Western U.S., a wash is the dry bed of a stream.
It flows only occasionally, and when it does, it’s into a ravine or canyon.
A wadi is a term used in Africa or Asia for a valley, gully, or streambed.
In northern Africa and southwest Asia, a wadi typically remains dry except in the rainy season.
For the most part, these words mean the same thing.
It’s just a regional difference in what people choose to say.
10. What can property owners do if they have arroyos on their land?
Properly caring for land with arroyos depends on several variables.
The location of the land within the drainage basin
The slope of the land
The microclimate and vegetation
Future use of the land
Often, keeping as much precipitation and vegetation on the land can help.
This will help to slow erosion and prevent the movement of sediments off the land.
You also want to avoid altering the gradient of the slopes and stream channels with soil removal.
We recommend avoiding check dams.
While they can temporarily trap sediment and decrease erosion, they often worsen the problem of entrenchment and sediment buildup.
11. What are erosion control techniques for property owners?
As noted above, decreasing erosion on your property is one of the best ways to prevent arroyos.
Here’s a list of erosion control techniques for property owners.
Contouring is helpful for property owners engaged in agriculture.
Contour farming uses row patterns that run nearly level with the hill.
This technique can help reduce soil erosion by as much as 50 percent from up and downhill farming.
It reduces sediment and runoff and increases water infiltration.
A swale is a depression or ditch that you can dig to move rainfall to your desired location.
We recommend swales near trees, shrubs, or nonxeric plants as water will collect wherever they’re dug.
A boomerang is a small half-circle mound of dirt on the downhill side of shrubs or trees.
Adding boomerangs to your land allows water to be trapped in areas where plants can use it.
Otherwise, the water would run right downhill and take soil or vegetation with it as it traveled, effectively eroding the land.
A surge basin collects large amounts of rainwater.
The water is often channeled from downspouts and runoff from hardscapes (driveways, patios, etc.).
Surge basins are intended for high-capacity rainfall or flows of water, so they must slow down the water’s speed.
We recommend locating a surge basin away from your home’s foundation and lining it with a geosynthetic fabric.
This fabric will prevent the water from undercutting the soil beneath the decorative rock or another suitable material.
Rocks can also create friction and slow the moving water.
If the water is already slow in your area, then you can consider skipping the fabric and using an organic mulch at the bottom of the basin instead.
Mulch is often used as an erosion control material.
This material not only absorbs water but also acts as a tiny damn to slow the water’s speed.
We recommend fibrous, woody mulches as these hold up the best in rare, heavy downpours.
Lighter mulches (crushed nut shells or finely shredded bark) often wash away.
Sponges are buried organic materials that absorb water and retain it where it’s most beneficial to plants.
Dig a trench that’s 8-12 inches deep and 4-6 inches wide along your contours.
Make sponges from straw, compost, or newspaper.
Fill the trench with these sponges along with the original soil.
This will help to control runoff and provide a better environment for planting for years to come.
These are channels that allow rising water in surge basins to have an appropriate and controlled exit.
Overflows must be placed deliberately so the water goes into your landscaping or another useful area.
You can also line your overflows with fabric and rock to further prevent erosion.
Overgrazing dramatically reduces the land’s ability to absorb water.
When land has denuded of grass, it is more susceptible to flooding and erosion.
For this reason, it’s good practice to use rotational grazing and other sustainable agricultural techniques to protect soil health.
13. What are geologists, engineers, and soil scientists doing about arroyos?
These parties are working hard to develop technology to predict runoff, stream flow, and sediment transport within arroyos.
The modeling techniques that are used for this require quantitative knowledge of factors like topography, rocks, vegetation, and climatic variability.
Geologists and soil scientists also help by testing the results of the models and monitoring landscape changes related to weather events, longer-term climate fluctuations, land-use changes, etc.
When there’s more knowledge on these types of topics, it’s easier to bring awareness to the public.
When people (especially buyers and sellers of land) are conscious of their behavior in erosive or flood-prone areas, it can make all the difference to the protection of land.
Have you ever seen an arroyo?
Landowners in the Southwest may notice these washes the most.
While a variety of factors dictate the best course of action, it’s critical to decrease erosion, counter overgrazing, and stop disturbance on the land.
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Disclaimer: we are not lawyers, accountants or financial advisors and the information in this article is for informational purposes only. This article is based on our own research and experience and we do our best to keep it accurate and up-to-date, but it may contain errors. Please be sure to consult a legal or financial professional before making any investment decisions.