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<br />Tamarisk. a salt.tolerant brushy tree introduced to the West sometime in the late 1800s <br />(Robinson. 1965). is the most common non-native species. Once established. vegetation <br />can directly influence the impact of high flows by increasing channel roughness and <br />decreasing flow velocities (Graf, 1978). <br />The Salt is a tributary ofthe Gila River. Arizona's largest water course. Much of the <br />Gila's course is lined with tamarisk. Burkham (1972a) did a detailed study of the Gila in the <br />1960s and 1970s, Moderate-sized floods were observed on three occasions. before and <br />after dense thickets of tamarisk were intentionally removed from the banks of the river. Not <br />surprisingly. when the channel was clogged with tamarisk. it could not efficiently handle <br />even moderate flood. As the water sieved through the brush. the river slowed and dropped <br />its load of sediment. The floodwaters rose higher above the river banks and spread farther <br />out from the banks where the channel was obstructed by tamarisk. Since Burkham's work. <br />large floods on the Gila in 1972, 1978 to 1979, 1983, and 1993 have ripped most. but <br />not all. of the tamarisk from the floodplains. <br />Mesquite thickets. called bosques. are common along the Salt and Gila Rivers <br />(Minckley and Brown. 1982). Mesquite uses less water than other species. such as tama- <br />risk. and can grow at higher elevations above the level of the river. Mesquite also can grow <br /> <br />Kayaker in <br />Ledges Rapid <br />on the So/t River <br /> <br /> <br />on rocky substrate in addition to sandy floodplains. Periods of small floods. such as the <br />middle of the 20th century (Webb and Betancourt. 1992), allow bosques to encroach upon <br />the floodplain. As with tamarisk thickets. floods wreak havoc on mesquite bosques.ln one <br />channel bend between the bedrock walls of the canyon. floods on the Gila between 1972 <br />and 1979 halved the size of one bosque (Minckley and Clark. 1984). Most of the destruc- <br />tion was caused by lateral erosion of the channel banks. as is the case with tamarisk. but <br />mesquites can better withstand flooding because of their toehold in the rocky slopes. <br /> <br />14 <br />