|
<br />1492
<br />
<br />W. L. GRAF
<br />
<br />e
<br />
<br />gions, as described by Turner (1974), Smith
<br />(1976) showed that vegetated stream banks
<br />are several thousand times more resistant to
<br />erosion than bare unconsolidated materials,
<br />and studies by Dietz (1952) demonstrated
<br />
<br />that seedlings at high-water lines are impor-
<br />tant in the formation of islands and bars,
<br />Most of the major streams of the Co!orado
<br />Basin exhibit extreme variability in their
<br />discharges, so that surfaces exposed during
<br />
<br />
<br />" '-,-
<br />co, , .... - ,-,';-
<br />~'- "/'5i,,.~,~,
<br />......... - -- -,. "
<br />,'" ','. .~,,... ,'~ '"
<br />~i" :t;' ~" -.~- ... "-
<br />~:'.. ~ '-~"~:~" , ~>. "~~.'" <. ,
<br />, , :}~,' ',,'7, -,'~<~
<br />"'"..~~-
<br />. , ~ .~ -"
<br />.,.", ,
<br />
<br />
<br />'j'l i
<br />:i I J
<br />1::.,,1
<br />;,;11-:)
<br />!tU Figure 1. Floor of Grand Canyon, Co!orado River, Arizona, 1 km above mouth of Es-
<br />,if:; calame Creek, looking downstream to southwest. Coronado Butte on center skyline, Zuni
<br />,: 'IJ ~oint on l~ft, and Solomon Temple on right. Top: 1871 photo by J. K. Hillers. Note bare
<br />I, 'fj nver margms. Bottom: 1968 photo by E, M, Shoemaker and H. G. Stephens shows exten-
<br />
<br />II, ',:r · tion Office.
<br />! ,- ,~
<br />,t' ,
<br />Ii
<br />il
<br />"
<br />1~
<br />
<br />low-water periods may provide seed beds;'
<br />for establishing new vegetation, Before thel
<br />introduction of tamarisk, these new plants;
<br />usually salt grass and dwarf willow, were~
<br />swept away by subsequent floods, but;
<br />tamarisk was hardy enough to withstand,
<br />such inundation, to continue to grow, tl),
<br />stabilize the underlying surface, and to in-;
<br />duce sedimentation (Hadley, 1961}..:
<br />As the tamarisk continues to grow, thei~!0
<br />lands and bars that it occupies become in-!
<br />cre<\singly stable and grow by overbank!
<br />deposition, resulting in progressive restric-'
<br />tion of the channel. Although many authors,
<br />suggest that tamarisk grows only above the:'
<br />annual high-water stages, botanical studies:
<br />prove that the plant is capable of surviving'
<br />periods of inundation as long as 6 weeks. 1
<br />(Horton and others, 1960; Warren an~ <~
<br />Turner, 1975), If relatively low-water con':'
<br />clitions prevail for 2 to 4 yr, allowing
<br />tamarisk seedlings to mature, the exposed
<br />surfaces on which they grow are likely to be
<br />stabilized and to form new channel land-
<br />forms. lbe final geomorphic product of the'
<br />invasion of tamarisk in arid canyons of the
<br />Colorado Plateau is the suit of landforms il-
<br />lustrated in Figures 2, 3, 4, and 5.
<br />
<br />Study Area
<br />
<br />
<br />Previous researchers have analyz
<br />tamarisk invasions in centra! Arizona a
<br />New Mexico (Robinson, 1965; Turn
<br />1974), where the t1uvia! landscapes
<br />dominated by broad alluvial valleys. In
<br />sian of valley floors by tamarisk has
<br />suited in the development of thickets sever
<br />thousand hectares in area that constr'
<br />channels and lower water tables. The f1uvi
<br />landscapes of the Colorado River basin'
<br />the Colorado region are dominated by can'l
<br />yons, however, and conclusions based onll
<br />research in open terrain are of quesrionab~j
<br />value in the plateau region. This study is
<br />restricted to canyons of the Colora
<br />Plateau in general, with intensive study
<br />respresentative section (Fig. 6) of the Gr
<br />River in and near Canyonlands Natio
<br />Park, Utah, The widespread distribution
<br />sandstone'provides a steady source of san
<br />sediments that accumulate as islands a
<br />bars in the streams (Hunt, 1956, 1999)",
<br />The availability of data has restricted]
<br />present analysis of the spread of tamarisk, .
<br />the Colorado and Green RiverJi~
<br />streams, which flow through canio'~
<br />much as 330 m (1,000 ft) deep for tb.OS,
<br />their lengths, Elevations of the can
<br />floors range from 830 m (2,500 ft) to~,
<br />m (6,000 ft). The floors, becauseo(
<br />',Jt;!'i\
<br />relatively low positions relative tc>: itO
<br />J;:]H
<br />[Ill
<br />
|