8262

• UCREFRP Catalog Number: 8262
• Author: Graf, W. L.
• Title: Fluvial Adjustments to the Spread of Tamarisk in the Colorado Plateau Region
• USFW Year: 1978
• USFW - Doc Type: Geological Society of America Bulletin
• Copyright Material: YES

TAMARISK IN THE COLORADO PLATEAU REGION

t~! ': '
kphicsurroundings, have climates that
, n\ot'e arid than nearby terrain. In the
ntralpart of the Colorado Plateau, rain-
" all may be as little as 7 S cm (3 in.) annu-
l,'I"aUy, so that the major rivers derive most of
i\their discharge from mountain areas out-
j,'side the plateau region.
" Although general assessment of the
f' spread of tamarisk was accomplished for
I the Colorado Plateau, analysis of the
I geomorphic impact of the plant was re-
I stricted to a representative study area com-
I posed of the lower Green River in and near
1 Canyonlands National Park, Utah (general
j location shown in Fig, 6, detail shown in
i Fig. 7). In its final 187 km (117 mil, the
! Green River flows through a meandering
I course entrenched several hundred metres
I", into horizontally bedded Paleozoic and
, Mesozoic sandstone (Lohman, 1974). The
1 rock gorges are partiallyfiUed with upper
: Pleistocene sediments (Cooley, 1962).

. MElHODS

Sources of Data

Herron (1917) produced a series of de-
railed maps (scale 1:5,000) of the channel
and near-channel landforms on the Green
*er in the study area. Because the maps
"e produced from surveys during the

period 1914-1916, they represent a record
of conditions prior to the invasion of
tamarisk. Plan and profile maps of the river
were published in sheet form by the U,S.
Geological Survey in 1924. Topographic
maps (1:62,500) of the study area produced
'during the early 1950s are representative of
conditions after the invasion of tamarisk,
but their scale precludes the amount of de-
tail required for exacting analysis of chan-
nel changes, More recently, Belknap and
Belknap (1974) provided detailed maps of
the river, but field checks revealed that they
were not accurate enough for analytic use in
the present study,
Ground photographs provide the most
accurate long-term record of channel and
landform changes. Ground photographs
can be analyzed to provide quantitative
data using photogrammetric techniques
originally designed for oblique aeria!
photographs (American Society of Photo-
grammetry, 1960; Malde, 1973; Faig,
1976), If the optical characteristics of the

Figure 2. Tamarisk seedlings growing on lowest or annual
)Cnch at TIdwell Bottom, mile 51.9, Green River, Utah. Plants av-
'rage 2 em in height.

Ute 3. Full-grown tamarisk growing on terrace 3 m above
'IV-water level at Tidwell Bottom, mile 52.4, Green River, Utah.
~nts average 7 to 8 m in height,

1493

camera are unknown, benchmark mea-
surements must be obtained by field survey
or by modern photographs taken with a
camera of known characteristics. The first
photographs of the study area were taken
by John Wesley Powell's assistant in 1871,
followed by a series of lesser known river
travelers who frequently photographed the
same locations (Table 1), A US. Geological
Survey team reoccupied the original Powell
sites in 1968 (Shoemaker and Stephens,
1969), The photographic record shows
conditions at 18 sites in the study area,

Field Studies

Of the 18 sites in the study area covered
by photography before and after the inva-
sion of tamarisk, 13 were further analyzed
in field studies in 1976. At each site,
Brunton-transit surveys of the channel and
near-channel landforms provided quantita-
tive measures of postinvasion dimensions.
Landmark measurements were made with

Figure 4, Upper terrace, 3 to 5 km above low-water !cvcl with-
out tamarisk at Tuxedo Bottom, mile 26.8, Green River, Utah,

Figure 5. Junction of upper terrace with valley walls, Tuxedo
Bottom, mile 26.6, Green River, Utah. Near site of settler's cabin
that shows no sign of flooding.

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