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<br />SECCION IV - GEOHIDROLOGfA
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<br />charge and diverting it for beneficial use. Conveying the water of streams
<br />through areas of ripariau phreatophyte growth hy means of pipes or liued
<br />ditches also reduces consumptive waste:.
<br />The efficieucy of water consumption may he increased hy suhstituting plants
<br />of higher economic value for plants of low economic value. In the United States
<br />this has heen done, fo.r example, hy suhstituting alfalfa for an association of
<br />greasewood, rahhithrush, and saltgrass (White, 1932), As yet, the practice is
<br />not widespread. Before replacement can he accomplished, however, it is necessary
<br />to rid the area of the existing vegetation-not as easy task, especially when deal-
<br />ing with aggressive plants such as saltcedar.
<br />An entirely different method, hut one that may prove feasihle, is increasiug
<br />the efficiency of the water consumptiou through the development of a higher
<br />economic use for the existing phreatophytes, This method would he most valu.
<br />ahle in areas where the water has a high mineral content and is not suitahle
<br />for most uses. Some examples of possihle higher economic uses are: rahhithrush
<br />as a source of rubber, saltcedar as a source of tannins, and mezquite as a source
<br />of high.quality charcoal. Intensive research doubtless would reveal other uses.
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<br />RELATION OF PHREATOPHYTES TO FLOOD AND SEDIMENTATION
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<br />Some species of phreatophytes grow so dimsely ou the flood plans of streams
<br />as to form a partial barrier or dam to flood flows. As a result, in times of
<br />flood the water spreads over and inundates areas that normally would not be
<br />flooded, endangering lives and demaging property. Saltcedar is a prime of.
<br />fender in this respect, followed closely by willow and other riparian vegetation~
<br />Examples of. streams that present flood hazards as the result of phreatophyte
<br />growth in the United States are the Salt and Gila River in Arizona, and the
<br />Rio Grande in Texas and New Mexico and along the International Boundary
<br />hetween the United States and Mexico.
<br />Floodwaters are nearly always silt laden. The damming or ponding effect
<br />of dense phreatophytic growth reduces the velocity of the water, and at the
<br />same time reduces its power to carry the full silt load, As a result there is de,
<br />position of silt in areas of phreatophyte growth. Such deposition as occurred,
<br />for example, on the flood plain of the Rio Grande and the Pecos River in New
<br />Mexico and the Gila River in Arizona, In the delta area ahove Lake McMillan
<br />a man.made reservoir on the Pecos River, it is estimated that in places as much
<br />as 15 feet of silt may have heen deposited as the result of saltcedar growth. In
<br />this situation the dense growth acted as a desilting agent to reduce the amount
<br />of silt entering Lake McMillan and at the same time to prolong the life of the
<br />reservoir. The effectiveness of the saltcedar as a desiIting agent is shown by "_:::'>."
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