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Arroyo de los Pinos Last year we had an unusually wet spring which inspired me to fill in the bare spots that had developed in my yard. Utilizing SFBG hoop house plants that were sold in the spring, I planted hardy specimens such as penstemons, flax, and maximillians, and spread native grass and wildflower mix seeds. In an area that receives roughly 14 inches per year in a good year with half of that possible in one month, I was really focusing on minimal water use plants. The flax and the penstemons were incredibly beautiful, and elsewhere in our area wildflowers and grasses had also sprung up with gusto and were a delight to the eye. I have often thought because of my busy schedule, a community garden would be a great way to get my hands in the dirt and share the experience as well as the responsibility of gardening with others. As a recent participant with the Santa Fe Botanical Garden I felt that I could fulfill those aspirations by helping with the Garden in Santa Fe. There is a feeling of wonder, sharing, and responsibility in planning for a public garden where visitors and locals alike can wander through, learn about, smell, and see plants. The site for the public garden is at the Arroyo de los Pinos, the drainage located north of the Museum Hill complex. This arroyo is ephemeral; it carries water mostly as a result of storm events. It is fairly deep in some places, “entrenched” meaning during flood stage the water does not have access to its banks and the “flood prone area” outside those banks. At present there is a poor distribution of plants, a few scraggly piñon pines and junipers (they can grow anywhere) and, thankfully, grama and muhley grasses. While the granitic soils that are present drain fairly well, there is almost no organic material as part of the soil structure. In places the bedrock is exposed where erosion has taken its toll. Plans to address these problems include creating a viable ecosystem that will retain soil and slow down the water. One such process is called induced meandering. Rather than having a straight shoot of water coming down the canyon we can lengthen the bed by increasing the sinuosity. An example of this principle is imagining two skiers, one traveling down hill while another goes through a mogul run. In this example the slope is the same, but in the second case the velocity is reduced because of the turns and the added roughness. A site design has been created for the arroyo by a local firm, Earth Works Institute. To create the design, staff collected data, including a survey of the channel. This survey resulted in a profile of the thalweg, the deepest part of the bed, where water would most likely flow (longitudinal profile). The information gathered provides us with existing slope and sinuosity. The middle reach, between the property boundary and Camino Piñones will be the main focus for restoration and has the most severe erosion. This will be ameliorated through the use of native materials, rocks and wood, to create structures that will slow down the flow of water, retain sediment, (making the water cleaner) and create an environment where vegetation will survive. Cross-sectional information was also collected. A cross-section is defined as taking elevation readings perpendicular to the flow of water. This describes the channel shape. Because of the ephemeral nature of the arroyo, the stresses put on the channel are extreme. Rainfall in the summer can be torrential, and water has great capacity for carrying anything in its path. Anything not tied down is fair game, roots are great at holding on to soil, but during dry years vegetation may become sparse. Another factor that increases velocity is impervious surfaces upstream; including roads, roofs and parking lots. Our goal is to get vegetation to grow to hold the soil which in turn holds the water that allows the vegetation to grow. With vegetation growth it becomes easier to retain sediment/soil, increase water quality through filtration (the grasses and forbs act as a screen) and enhance the ecosystem – not only the vegetation, but in the soil. A wonderful circle when it works. Through induced meandering we hope to increase the sinuosity within the arroyo so that a new floodplain will develop and aggrade the bed. The meanders will be created by installing channel deflectors called baffles. The function of a baffle is to deflect the flow of water toward the opposite bank. This lengthens the stream path and reduces the energy. As the water slows, its capacity to carry material is reduced, dropping off soil, pebbles and at times boulders on these structures. Point bars will develop and soil will build up. These become microclimates which are hospitable to vegetation. Another structure-type that is planned for the arroyo is weirs. As with baffles they can be versatile in their construction. Two most often used types are rock dams and wicker weirs. The one rock dam is placed in the bed only one rock tall and can be several rocks wide. The height of the structure is determined by the cross-sectional data and should not be tall enough to impede water flow. The goal of these weirs is to aggrade the stream bed to a desired elevation and to provide roughness. Eventually a weir should evolve into a riffle zone if it is properly placed and sized. If a weir is designed too tall erosion may occur on the bank, either widening or splitting the flow. An example of rock dam in a series. Using these tools will result in a better ecosystem, and in future years sustain the plants that will please the eye, educate and always provide changes. Drainage systems, whether rivers, streams or arroyos are dynamic, always moving and changing; it is their nature. Channelization and armoring often do not succeed because of this; water can get beneath structures and through movement or freeze/thaw actions impact the integrity of hard structures.
Nina Wells is currently a member of the SFBG Board of Directors. She works with landowners on watershed issues through the New Mexico Environment Department Surface Water Quality Bureau as an Environmental Scientist. She can be contacted at 505.827.0572 or nina.wells@state.nm.us.
Visit www.santafebotanicalgarden.org to learn more about how Santa Fe Botanical Garden celebrates, cultivates and conserves the rich botanical heritage and biodiversity of the region. |
