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The Pantano Wash: Investigating an Ecosystem

Daniel

Most people have heard the old saying, "When it rains, it pours." This is literally true where I live in southern Arizona. It rains so much at one time here that flash floods can result. Over time, huge dry riverbeds have been carved out of the earth by the water. When I say "dry," I mean that they are dry most of the time, but during a rainstorm they fill up with water. These natural formations, called "washes," are about 20 feet deep and 100 feet wide. During the rainy season, the washes can have up to 15 or 16 feet of water in them, flowing very swiftly. You might think that this would kill any plants in the wash, but grass, bushes, and trees are able to survive and even thrive in this harsh environment.

Growing up in Tucson, I have always been intrigued by these ecosystems. One wash in Tucson, called the Pantano Wash, flows through my neighborhood. I have always had questions about this wash. How do the many trees and shrubs that live in the bottom of the wash survive the flash floods? How does the water flow through the wash? Does this have anything to do with how the plants survive? I decided to investigate the wash to see if I could answer some of my questions.

 
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Plant biodiversity found in a wash


THE FOUR LEVELS OF THE WASH

On my expeditions, I was surprised by the amount of diversity inside the wash. I observed many different varieties of grasses, shrubs, and even small trees. Over time I began to notice that the wash was divided into four layers, which I decided to call the Top Level, the Upper Level, the Lower Level, and the Bottom Level, respectively.

 

Soil samples from all four levels. (Click to enlarge)


I. The Top Level

The Top Level is the term I am using for the land surrounding the wash (see picture ). I included this level as a layer because its vegetation is so different from the other layers. From my research I identified some of the trees in this area: the Prosopis (mesquite) tree and the Parkinsonia florida (palo verde) tree. Both of these trees have unique adaptations to survive in the desert. The mesquite tree has extremely tiny leaves—their size makes it harder for the water to be evaporated from the leaves. And the palo verde tree has green bark, which helps it to perform photosynthesis, the process plants use to make food. I took two soil samples from each of the four levels and examined them under a microscope to see if I could find any reasons why these plants grow better in one area than in another.

 
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The upper level


Photo and drawing of Baccharis sarothroides (Click to enlarge)


II. The Upper Level

The Upper Level is directly below the Top Level. There are many plants growing at this level. Here, I identified 20 female and 10 male Baccharis sarothroides (desert broom) trees. Desert broom is an unusual plant because the female and male flowers grow on separate trees. In most plants, both male and female flowers grow on the same plant. From my research I discovered that Baccharis sarothroides can grow to be 10 feet tall, but the ones I saw were about six or seven feet tall on average. I also identified mesquite trees and many large shrubs growing in this level. When I took a soil sample from this level, I observed that the soil was very moist compared to the other soil samples I had taken. In a later soil sample I took here, I noticed that the soil was dry but was packed into dirt "clods." From these observations I concluded that the soil had been moist at one time, but then the water had dried out and made the soil form into clods.

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The lower level


 

III. The Lower Level

The Lower Level is directly below the Upper Level. Not as many plants grow in this level as in the higher levels. The vegetation in this layer is mostly small shrubs and grass. These plants grow sparsely throughout the entire level. The soil from this level is almost all dried-out clods. It is brown in color. Since this is the second-lowest level of the wash, it is the second level that water will reach.

 
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The bottom level


IV. The Bottom Level

In the Bottom Level there are hardly any plants at all, except for the occasional sparse patch of grass. I noticed a marked difference between the soil on this level and the soil on the other levels: the samples I took from this level consisted mostly of pebbles. I think the reason for this is that the floods carry the sand and soil downstream, leaving only rocks and pebbles behind. The rocky consistency of the earth is probably part of the reason that plants don't grow very well in this level; few plants grow well in rocky soil. This is the layer through which much of the floodwater flows.

 
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This photo shows evidence of water flow.


Water Flow

During my investigations, I discovered that water flowed north through the wash. I determined this fact from the various clues I found: ripples in the sand near rocks, and the height of the ground (water always flows downhill). When I made my first observation of the wash on October 21, 2004, we had not gotten any precipitation since September 19, 2004. (On September 19 we received 1.30 inches of rain. This was the third largest rainfall we had received in 2004.) When I observed the wash on the morning of October 21, I noticed that the sand in the Bottom Level of the wash was rough and had many "craters" in it that could have been caused by people or animals. Later in the day on October 21, we received 0.5 inches of rain. When I came back to the wash to do my second observation on October 23, I noticed that a two-foot-wide section of the bottom layer had been washed smooth by the rain. This was the only area of the wash that I observed to be affected by this particular rainfall.

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A pattern in sand


While on one of my expeditions, I noticed an interesting pattern in the sand about 100 feet south of the bridge. I observed that the water had flowed down over a sloped area of sand and had carved miniature gullies and canyons in it about one inch deep. I could understand how these were formed—water flowing over a slope takes the shortest path and carves through the slope—but I didn't understand why the water that made these formations was flowing west, perpendicular to the main water flow. After I investigated a little, I discovered that about 20 feet east of the miniature gullies and canyons, inside the Upper Layer, was a natural dam, located three feet above the Bottom Level.

Map (Click to enlarge)


This dam was created when sticks and branches were washed down to this spot, where they got stuck in the trees. This then caused a natural barrier to water flow. However, the water gradually seeped through the holes between the branches. These trickles of water then ran down the slope I described above, creating the miniature gullies and canyons. The water then flowed into the Bottom Level, where it joined the main stream (See Map). The water level would have to have been about three feet high to reach the dam. I also noticed a patch of cracked dirt on the uphill side of the dam. This shows that some of the water was never able to flow through the dam and evaporated, leaving a patch of mud that dried out and cracked.

 

CONCLUSIONS

Plant growth
Plants seem to grow well in the Top Level of the wash. The floods never reach this level, so the plants are safe from uprooting. The plants here get most of their water from the rains. In the Upper Level, plants also seem to grow very well. I think that this results from several factors: the soil is not rocky, the roots of the larger plants are able to get water from the lower levels, and the floodwater rarely reaches this level, so they are safe from uprooting. The plants that grow in the Lower Level are smaller and grow farther apart. I think that this is because the water reaches here more often then the higher levels and is sometimes high enough to kill off some of the plants in this level. The plants then have to start growing all over again until the next flood. Hardly any plants at all grow in the Bottom Level. The soil here is too rocky for plants to survive in, and when any water at all comes down through this layer, it kills any new seedlings that somehow managed to survive in the rocks.

Water Flow
Using all this information, I came up with an idea of how the water actually flows through the wash. I think that when only a small amount of water flows through the wash, it flows in the lowest area of the Bottom Level. (My observations about the flattened sand in the Bottom Layer after a storm are what led me to this conclusion.) This small amount of water would not do any damage to the plants in the wash. However, when more water flows through the wash, the water levels are high enough to reach the Lower Level or even some parts of the Upper Level. But water cannot flow as easily through the upper levels because of the vegetation—and obstacles like natural dams that form around vegetation—so most of the water runs down into the Bottom Level after the water there has flowed downstream. The water that spills over to the upper levels might uproot a few small shrubs, but otherwise does little harm to the plants in the wash.

However, when there is a major flood, I believe that there is too much water to be stopped by dams or trees. I think that this is the amount of water that would destroy shrubs and even harm trees. Luckily, we hardly ever get the amount of rain necessary to do this much damage.

Summary
From my expeditions into the Pantano Wash, I discovered a lot about where plants grow and the effect of flooding on plant survival. I learned that plants can thrive in seemingly impossible conditions. And most of all, I learned that when you stop and investigate the world around you, you can discover many new things that you never even imagined.

 

References

Ayers, Tina, Tom Kolb, and Elizabeth Taylor. Native Plants of Arizona. Northern Arizona University. Retrieved 22 November 2004 from the World Wide Web.
http://www.for.nau.edu/azproject/main.html

Buchholz, Daryl D. How to Get a Good Soil Sample.  University of Missouri. Retrieved 18 November 2004 from the World Wide Web.
http://muextension.missouri.edu/explore/agguides/soils/g09110.htm

Craig, Palmer H. "Loadstone." The World Book Encyclopedia. 1967.

Fiero, Brad. Desert Plants of Tucson, Arizona. Pima Community College. Retrieved 20 November 2004 from the World Wide Web.
http://wc.pima.edu/~bfiero/tucsonecology/plants/list_cn.htm

How to Take Soil Samples. Oregon State University. Retrieved 18 November 2004 from the World Wide Web.
http://eesc.orst.edu/agcomwebfile/edmat/ec628.pdf

Peterson, John A., et al. Baccharis sarothroides Fact Sheet. Virginia Tech Forestry Department. Retrieved 22 November 2004 from the World Wide Web.
http://www.cnr.vt.edu/dendro/dendrology/Syllabus2/bsarothroides.htm

Plant Profile for Baccharis sarothroides. United States Department of Agriculture. Retrieved 20 November 2004 from the World Wide Web.
http://plants.usda.gov/cgi_bin/plant_profile.cgi?symbol=BASA2

 

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