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D (70 ({7 cr )uvial adjustments to the spread of tamarisk h the Colorado Plateau region ,Ie rn se ,;~';i WILLIAM L. GRAF Department of Geography, University of Iowa, Iowa City, Iowa 52242 I \' ~ i ld )1- le- 'n- Ise ABSTRACT Tamarisk, a shrub or low tree that was '" artificially introduced into the American !lSouthwest in the late 1800s, has spread ';:'r.r,oughout the Colorado Plateau region by !OfX\lPying islands, sand bars, and beaches .Iong streams. Historical photographs ~!$bow that tamarisk spread from northern ~Fona to the upper reaches of the Col- ~'orado and Green Rivers at a rate of about "20 kmIyr, Detailed studies on the Green River in Canyonlands National Park, Utah, Show that the plant has trapped and 'stabilized sediment, causing an average re- duction in channel width of 27%. Photo- 'grammetric analysis of historical ground photography, including photos from John Wesley Powell's 1871 expedition, and re- tent aerial photographs supplemented by field surveys provided quantitative data. I Expanded islands and channel-side bars : exhibit allometric relationships as they Idtange, apparently maintaining a balance I between turbulence and friction. Overbank lOooding is common on the tamarisk- '~tabilized features. ;sa In- se, 76, iak ~es ac- n f:: ler- 1 cli-!I uic ;0. , of H )pe (1 les: :1 >1m ij H i1NTRODucnON hi ~ H ;1 A review of historical ground photog- ',, :1SPhy taken by explorers, surveyors, and ;'!iprofessional photographers indicates that '"within the past century the most dramatic :jlchange in the fluvial landscape of the Col- . do Plateau has been in the riparian vege- rion, The changes shown in.Figure 1 are ical of most of the major water courses the Colorado Plateau, where shifting nd bars, beaches, and islands that were ce sparsely vegetated are now covered by e thickets of tamarisk, In most cases, 'thickets consist of a few fully developed risk trees 10 to 12 m (30 to 36 ft) taU, rounded by closely spaced plants 5 to (15 to 18 ft) tall, that exclude other .~. By the late 1960s, tamarisk proba- bly covered 500,000 ha (1.3 million acres) in the American Southwest (Robinson, 1965), Because tamarisk colonizes moist sand surfaces, grows rapidly, and stabilizes sed- iment, it is an effective geomorphic agent, The early photography, beginning with John Wesley Powell's second expedition, in 1871, shows substantial growth of stable surfaces near channels as a result of the tamarisk invasion (Shoemaker and Ste- phens, 1969). Baars and Molenaar (1971), Lohman (1974), and Shoemaker and Ste- vens (1975) offered examples of compara- tive photographs, Islands have become longer and wider, channel-side bars have widened, and some alluvial-fan surfaces have expanded, accompanied by concomit- ant reductions in channel widths. The largest changes in near-channel landforms have taken place in canyons with relatively gentle gradients and floors that are at least two channel widths wide. Changes are much smaller in canyons with steep gra- dients, rapids, and floors that are almost completely occupied by the river channel. The net result of channel restrictions in the canyons is increased frequency of over- bank flooding and increased roughness of near-channel surfaces that serve as channel perimeter during high-water periods (Had- ley, 1961). An appreciation of fluvial re- sponses to the growth of tamarisk in areas already infested will permit accurate pre- diction of changes in areas yet to be in- vaded. Additionally, the responses of fluvial systems to biotic disruptions may shed some light on the general nature of such sys- tems and may be useful in the analysis of other types of disruption in the canyons of the Colorado Plateau, 1ncreasing recre- ational use of the canyons necessitates a clear understanding of environmental sta- bility for effective land management, In this paper, I discuss the introduction and spread of tamarisk in the Colorado River system and the adjustments of the gica! Society of America Bulletin, v, 89, p. 1491-1501,13 figs.," tables, October 1978, Doc, no, 81005, p ~' 1491 fluvial geomorphic systems of the Colorado Plateau to the growth of tamarisk. Tamarisk The accepted common names of tam- arisk, salt cedar and sa!tcedar, are usually applied to many of the 90 species of the plant, but in the southwestern United States and for purposes of this paper, the term "tamarisk" applies to the species Tamarix chinensis (Lour.) (Horton and Campbell, 1974). When it was introduced into the south- western United States in the mid-1800s, tamarisk was cultivated as an ornamental shrub in California and later as an erosion- control plant in New Mexico (Robinson, 1958), but after escaping cultivation in the late 1800s the plant occupied moist sand surfaces with little competition from native plants (Horton, 1962, 1964), The cause of the spread of the plant throughout the drainage systems of the region has been a matter of controversy, Most previous workers have relied on written accounts and field collections (for example, Christen- sen, 1962), which are useful but sometimes less accurate than photographic evidence. Bowser (1960) reported that the plant was growing under natural conditions along the Virgin River in southwestern Utah in the 1890s, while Christensen (1962) stated that the plant was not grow- ing in the Green or Colorado systems before 1925, Christensen (1962) suggested that the most rapid spread of the plant was from 1935 to 1955, but Harris (1966) and Clover and Jotter (1944) contended that the plant was common in most major drainage systems of the Southwest after 1920. Photographic evidence resolves some of these issues, The geomorphic effects of plant growth near channels is significant in humid re- gions, as shown by Zimmerman and others (1967) and Thornes (1970), and in arid re-