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Show year for the entire basin, to 1 million acre-feet70 a year for the lower part of the basin. The latter is about two-thirds of the water due Mexico under treaty obligations. If the greater estimate is cor- rect, more water is used annually than flows down the river. It is believed to be high, however, as it is based on an estimate of weed cover occupying 10 percent of the total area in the basin, probably a generous estimate. The introduction and spread of salt cedar into the basin have aggravated this situation. This species is aggressive, rapidly invades sand banks and deltas, and crowds out other vegetation that uses lesser amounts of water. It can survive both drought and swamping, reproduces abundantly from both seeds and cuttings, and seems immune to fire and herbicides. Its peculiar tolerance for saline soils make salt cedar a particularly aggressive invader in. the Southwest. While successfully adapting itself to waste areas, it has also started to encroach on agricultural lands. Salt cedar and cottonwood can use 5 to 7 acre-feet of water a year. For this water cost, little value is received. Salt cedar is not effective in controlling gullies or bank erosion. Although dense stands at the head of reservoirs reduce the amount of sedi- ments deposited in the reservoir, at the same time they clog channels so effectively that damage occurs. A typical example is in the combined stretch of the Salt and Gila Rivers between Gillespie Dam and Granite Reef Dam. In some reaches of the channel a combination of dry river channel and high water table has provided ideal conditions for salt cedar. The channel is now so choked with vegetation that the occurrence of a flood could dis- place the rrver and seriously damage the neighbor- ing highly improved rural and urban areas. In about the same locality it was found that over a 46-mile stretch of the river, bordering vegetation used about 25,000 acre-feet of water. This amount was drawn from 9,300 acres of land and was enough to irrigate 9 ,,000 acres of farm land. Over 75 per- cent of the water was used by salt cedar.71 "* * * Qn ^g Dasis of available data, it is esti- mated that the water used by phreatophytes in this "Department of the Interior, The Colorado River (1947) H. Doc. 419, 80th Cong., 1st sess., p. 233. "Allred, B- F., Sheep and Goat Raiser, 1950. Also, Pacific Southwest Federal Inter-Agency Technical Com- mittee, Minut«s of 7th and 8th meetings. 440 region is not less than 200,000 acre-feet and may be as much as 350,000 acre-feet per year." 72 No concerted attack on the salt cedar problem has been made. State and Federal agencies have tried different kinds of control, but on a small scale. First results were generally disappointing. In the middle 1930's, the Civilian Conservation Corps cleared several hundred acres in Safford Valley, Ariz., by hand grubbing. Regrowth the next spring was even denser than the original growth. Similarly, regrowth following burning and mowing was often 3 to 8 feet in height a year. Various herbicides tried thus far have not been suc- cessful. Leaves and limbs are killed, but resprout- ing from roots persists. The application of high volume sprays, which cover the thousands of leaves on each plant, generally has been more effective than low-volume airplane application. On areas suitable for cultivation, salt cedar has been removed with heavy equipment at a cost of $30 to $100 an acre. Here, control of regrowth is not difficult as clean tillage keeps it down. This raises a question, however, as to whether or not downstream water users can be persuaded to con- sent to control by tillage. Generally, heavy salt cedar infestations are above reservoirs, and down- stream irrigation districts strongly oppose expan- sion of irrigated acreage above the reservoir.73 On uncultivated lands, costs of known controls are excessive. In addition, if the work is not main- tained the brush grows back in 4 to 5 years. The Fish and Wildlife Service has found that clearing salt cedar in the spring and planting mil- let for duck feed suppresses sprouts, the millet overtopping and shading the cedar sprouts. In the fall the fields are flooded during the flight period. This treatment seems to kill the original plants, including all roots. However, wind, water, or birds reseed the fields and young seedlings must be cleared each spring. Where salt cedar covers large areas it has been suggested that it be removed in alternate strips and a grass cover established on the cleared area. After the grass is established, the remaining strips would be removed. This procedure would pre- vent wind and water erosion. n MacDonald, H. R., Wolcott, H. N., and Hem, J. D., Geology and Ground Water Resources of the Salt River Valley Area, Maricopa and Pinal Counties, Ariz. Un- published report of U. S. Geological Survey, February 4, 1947. n Allred, B. F., op. cit. |
