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OCR for page 178
8
Historic Changes in Vegetation Along the
Colorado River in the Grand Canyon
R. ROY JOHNSON, University of Arizona, Tucson, Arizona
ABSTRACT: Glen Canyon Dam has irreversibly changed the Colorado
River in Grand Canyon. Prior to the dam, sediments and associated
nutrients were transported into the canyon from upstream, with a large
percentage of those sediments and nutrients then flushed from the sys-
tem as output, supporting downstream systems. These predam sedi-
ments and nutrients supported aquatic and terrestrial organisms of the
river corridor throughout Grand Canyon. The dam has drastically re-
duced upstream input, making the Colorado River ecosystem in Grand
Canyon more nearly a closed system. Downstream systems, e.g. the
fishery in Lake Mead, have also been impacted by the loss of nutrients
trapped in Lake Powell.
Both riparian and aquatic ecosystems have been impacted by postdam
flow regimes. Prior to the construction of Glen Canyon Dam the com-
ponents of the natural riverine ecosystem of the Colorado River in Grand
Canyon were driven by high energy, pulsing events typical of south-
western rivers. Three pulsing features of the river affected by the dam
included, in decreasing importance to the riparian ecosystem (1) seasonal
flow patterns and maximum/minimum flows (2) nutrient and sediment
transport and turbidity and (3) fluctuations in water temperature. Ripar-
ian vegetation was dependent on moisture, nutrients, and sediments sup-
plied by the river. Flooding is a natural phenomenon in riparian habitats,
thus riparian species are generally flood adapted. However, in the Grand
Canyon predam high flows and sediments transported by those flows
scoured vegetation from the riparian zone on a regular basis.
178
OCR for page 179
HISTORIC CHANGES IN VEGETATION...
Riparian ecosystems are ecotonal, interfacing between the upland zone
of the terrestrial environment and the adjacent aquatic environment of
the river. As such, species richness and population densities both tend
to be higher in riparian ecosystems than on adjacent uplands because of
the edge effect. The term naturalized ecosystem has been used for the
riparian ecosystem in Grand Canyon, an ecosystem in which native
processes have not been appreciably interrupted and/or nonnative (ex-
otic) species have become naturalized, causing no loss of native spe-
cies. By contrast, the aquatic ecosystem in the Grand Canyon is an
exotic ecosytem, an ecosystem that has been so extensively modified
that it consists of a humanly created environment in which native pro-
cesses have been interrupted, nonnative species abound, and native spe-
cies have been extirpated.
Early investigators in the Southwest applied the concept of desertifi-
cation to "water projects," notably reclamation projects and other river
management activities. Desertification, leading to increasingly xeric con-
ditions, has resulted in losses of 90% to 95% of the riparian habitat
throughout the Southwest lowlands. By contrast, a more constant post-
dam water supply has resulted in the riparian environment in Grand
Canyon becoming increasingly mesic. The Colorado River in Grand
Canyon is the only major riverine ecosystem in the lowland Southwest
where there has been an appreciable increase rather than a decrease in
riparian vegetation and associated animal populations during the l900s.
The importance of ripanan habitat to wildlife and humans has been
extensively documented. The riparian zone in Grand Canyon is used
extensively by whitewater recreationists, hikers, and a large number of
native animals. Thus, the proper management of riparian vegetation in
the canyon is of primary importance.
INTRODUCTION AND OBJECTIVES
179
The Grand Canyon, one of the original seven natural wonders of the
world, is located in northern Arizona, on the Colorado Plateau Province, a
region called by the noted geologist Hunt (1967) "easily the most colorful
part of the United States." The canyon is of great interest to resource
managers, recreationists, scientists, and politicians. At least two ex-presi-
dential candidates have written about the canyon Arizona Senator Barry
Goldwater (1940) and former Governor Bruce Babbitt (1978~. It serves as a
textbook example for stratigraphic studies in its more than a billion years of
exposed geologic history. Grand Canyon National Park receives more than
three million visitors annually, approximately 22,000 travel down the Colo-
rado River, considered by many to be the world's premiere whitewater
river. Approximately 100, 000 additional recreationists use the river be-
tween Glen Canyon Dam and Lees Ferry, half as fishermen, the others on
raft trips. The region's biological importance was assessed thusly by one of
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180
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
this century's greatest mammalogists (Hall, 1981), "Concerning subspeciation,
which is a step toward the formation of species, the southwestern quarter of
the United States...has a larger number of subspecies than any other conti-
nental area of equal size in the world...The reasons seem to be high degree
of relief...deeply cut, steep, rock-walled canyons (the Grand Canyon of the
Colorado, for one), resultant wide range in temperature, sharply marked
zonation of plants, and tremendous diversity of types of soil." It is this
environmental diversity, so important to both natural and cultural systems,
and the impacts of water management systems on this diversity that we
address here.
Glen Canyon Dam has irreversibly changed the Grand Canyon environ-
ment. Interactions between humans, vegetation, and flow levels are often
complicated. Some plants and animals are adapting to postdam conditions
(Carothers and Dolan, 1982), others are not. Some changes are local, e.g.
the gradual downhill displacement of sand into the river as a result of
camper's footsteps (Valentine and Dolan, 1979~. Prior to construction of
the dam, this lost sand would have been replaced almost on an annual basis
by high spring flows. Other changes are large scale, impacting not only the
riverine system within Grand Canyon but also affecting ecosystems hun-
dreds of miles downstream. For example, the Colorado River formerly
transported sediments and associated nutrients into the canyon from tens to
hundreds of miles upstream. The dam has drastically reduced this input,
making the Colorado River system in Grand Canyon more nearly a closed
system. These predam sediments and nutrients supported aquatic and ter-
restrial organisms of the river corridor. A large percentage of those sedi-
ments and nutrients were flushed from the canyon as output, supporting
downstream systems. Thus, the dam has also impacted systems further
downstream, e.g., the fishery in Lake Mead that suffers from a loss of
nutrients trapped in Lake Powell (Evans and Paulsen, 1983; and numerous
other papers by Paulsen et al., in Adams and Lamarra, 1983~.
The possible loss of integrity of the ecological fabric of the region, or
even modification of critical features of the region, is of interest to us here
as we evaluate vegetational changes, specifically changes in riparian veg-
etation along the Colorado River. Johnson and Carothers (1987) called the
riparian ecosystem in the Grand Canyon a naturalized ecosystem. A natu-
ralized ecosystem is one in which native processes have not been apprecia-
bly interrupted and/or nonnative (exotic) species have become naturalized,
causing no loss of native species. Attempts to reconstruct lists of the predam
biota during studies in the 1970s and 1980s have failed to demonstrate the
loss of riparian species since the construction of Glen Canyon Dam in 1963.
Instead, the addition of nonnative species along the river corridor has in-
creased species richness within riparian biotic communites, apparently without
the deleterious effects often associated with nonnative invaders. However,
OCR for page 181
HISTORIC CHANGES IN VEGETATION.
181
thirty years is an infinitesimal length of time in the evolution of natural
communities such as those in the Grand Canyon. By contrast, the aquatic
ecosystem in the Grand Canyon is an exotic ecosystem, an ecosystem that
has been so extensively modified that it consists of a humanly created envi-
ronment in which native processes have been interrupted, nonnative species
abound, and native species have been extirpated.
Riparian ecosystems are ecotonal, interfacing between aquatic and terres-
trial ecosystems. As such, species diversity and population densities both
tend to be higher in riparian ecosystems than in adjacent upland ecosystems
because of the edge effect Johnson, 1979; Odum, 1979~. The newly cre-
ated rivenne environment in the Grand Canyon consists of a series of subecotones,
or subzones, even further expanding the edge effect and increasing biotic
diversity and population densities. Since the terms subecotone and subzone
are not in common usage, the riparian zone of the Colorado River in Grand
Canyon has, by convention, been further divided into even smaller "zones."
The terms Old High Water Zone and New High Water Zone are so widely
used that they are commonly abbreviated OHWZ and NHWZ (and will be
so designated throughout this paper). This loose use of terminology, or
perhaps better, nonheirarchical use of a nontechnical term, zone, leads to
confusion if one does not realize that the NHWZ and OWHZ are actually
subcategories of the riparian "zone."
In examining the riparian ecosystem of the Colorado River in Grand
Canyon, vegetational studies are critical. Plants making up riparian vegeta-
tion serve as processors of environmental conditions, biological computers
if you wish. Unlike animals, plants cannot move from one location to
another, thereby escaping changes from day to day or year to year. There-
fore, they respond to changes in temperature, soil, moisture, slope, aspect,
and the multitude of other climatic, edaphic, and related conditions of their
environment. In turn, vegetation serves as a substratum for animal life,
including humans prehistorically serving a critical subsistence function,
today a greatly sought after recreational function. Thus, vegetation is prob-
ably the best component for assessing the "health" or condition of the ripar-
ian segment of this riverine ecosystem. By correctly interpreting changes in
vegetation over time we can thereby evaluate the past and present "health"
of the ecosystem and postulate what is to come.
Objectives
Although our task here is the discussion of historic changes in riparian
vegetation before the 1980s, projections for the future of species and pro-
cesses along the river corridor necessitate some discussion of research dur-
ing the 1980s, especially recent findings from the Glen Canyon Environ-
mental Studies (GCES). Thus, in order to make our story complete, within
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182
COLORADO RIVER ECOLOGY AlID DAM MANAGEMENT
space limits, we can stay neither entirely within that time period, the ripar-
ian zone, nor vegetation. Other essential areas of discussion include floristics
since riparian vegetation of the river corridor is composed of individual
plants of numerous species. And finally, a basic ecological principal, sim-
ply put, states that in nature everything is connected to everything else.
This, then necessitates our addressing the interaction of plants with water
and other components of the aquatic ecosystem as well as the interdepen-
dence of terrestrial animals, and to a lesser degree aquatic animals, on
riparian vegetation.
HISTORY AND BACKGROUND OF BOTANICAL EXPLORATION
ALONG THE COLORADO RIVER IN GRAND CANYON
The first humans cognizant of riparian vegetation along the Colorado
River in Grand Canyon were prehistoric groups to which certain floristic
components of the vegetation were of subsistence or ceremonial impor-
tance. Since these early canyon inhabitants left no written record we must
rely on plant and animal lists from archeological excavations—for summa-
ries see Fowler, et al. (1969), Euler (1984) and Jones (1986~. Paleontologi-
cal records provide additional biological information from riverside sites
(for summaries see Euler, 1984) but most palaeoecological studies deal with
upland vegetation, especially those analyzing fossilized plant remains from
packrat middens (Cole, 1990~. Thus, 3,000 to 4,000 year old split-twig
figurines recently excavated from riverside Stanton's Cave (Euler, 1984)
that had been made from obligate riparian willow and cottonwood are a
good indication that those species grew along the river andlor its tributaries
at that time. (Scientific names of plants in text are listed in Appendix A;
animals are given parenthetically the first [and usually only] time the spe-
cies is mentioned.) However, human proclivity for carrying things around
and the movement of plant materials by wind and water make both paleon-
tological and archeological analyses of limited value for our purposes.
It is difficult to extrapolate back in time from limited information, espe-
cially for determining changes in an area such as along the Colorado River
in Grand Canyon, one of the most inaccessible regions of temperate North
America. Unfortunately, we find extrapolation necessary not only for pale-
ontological and archeological information but for much of the historic record
as well, noted more for its gaps in information rather than for usable bio-
logical information. Checklists for the flora and fauna of the river corridor
were incomplete until intensive surveys of the 1970s. For example, Dicoria
brandegei ("Unknown Compositae #1" on my early 1970s collecting expe-
ditions along the river) is a common and widespread native plant about the
size of a small, nonnative Russian thistle. Dicoria commonly grows as a
pioneer species on sandy deposits along with Russian thistle, especially
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lIISI ORIC ClIANGES IN VEGETATION...
183
along the upper reaches of the canyon. After being left from early plant
lists for Grand Canyon, it was finally included in the latest checklist (Phillips
et al., 1987) as "common on sand dunes along the Colorado River from
CRM [Colorado River Mile] 38-232." The species was not listed by MacDougall
(1947) in the Grand Canyon Natural History Association's third edition of
the park plant checklist (first edition in 1932) nor by Clover and Jotter
(1944~. Since the species could hardly be overlooked it is presumably a
post-dam invader, adapted to sandy deposits above the current normal high
water mark, sites that were often scoured clean of fluvial sands, vegetation,
and seeds by predam high flows. However, in addition to excluding dicoria,
McDougall (1947) also excluded tamarisk from his checklist although Clo-
ver and Jotter (1944) had previously listed tamarisk as common at several
localities. Although some botanical purists do not list nonnatives, e.g.
tamarisk, McDougall listed numerous other nonnative species, e.g. clovers,
sow thistle, etc., with annotations that they were exotics so we are left
with the uncertainty of an unsolvable mystery in relation to dicoria's estab-
lishment in the Grand Canyon.
PHOTOGRAPHY AND THE BOTANICAL RECORD
The first systematically collected vegetational information from the Grand
Canyon was by photographers. Earlier explorers, e.g., Powell, commonly
took notes on the geology and even Indians but little was mentioned in their
journals about vegetation. The following information about photographic
expeditions were taken largely from Graf (1978), Turner and Karpiscak
(1980), and Stevens and Shoemaker (1987~.
The earliest scientific records for the Colorado River in Grand Canyon
were accompanied by thousands of photographs that have been recently
used for a series of comparisons with more recent photos. The first of these
were by the Powell Expeditions of 1869 and 1871-72, especially the second
expedition. Later expeditions of the late 1800s and early 1900s included
the Wheeler Expedition of 1871-73, the important Brown and Stanton Rail-
way Survey of 1889-1890, and miscellaneous still photography discussed
by aforementioned authors. Photographic records from the early 1900s in-
clude the first motion pictures by E.L. and E.C. Kolb in 1911. The U.S.
Geological Survey (USGS) photographed potential damsites during the early
1920s and sponsored Stephens and Shoemaker's special expedition through
the canyon in 1968 (Stephens and Shoemaker, 1987), one year before the
Powell Centenniel, for comparing photos with those taken a century earlier
by Powell's photographers. These photos show and occasionally mention
vegetation but they were largely concerned with geology.
Although these numerous earlier expeditions had photographed vegeta-
tion along with the geology of the canyon the botanical value of most of
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COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
these photos was generally limited and even the vegetation in these photos
was not critically examined until the 1970s. Turner and Karpiscak's work
in the 1970s, sponsored by the USGS and MNA (Museum of Northern
Arizona) (Carothers and Aitchison, 1976; Turner and Karpiscak, 1980) is
the most complete photo comparison of vegetation before the GCES of the
1980s. Thus, although photography was generally accomplished with little
botanical intent, in retrospect much of this earlier photography has served a
useful function for vegetational comparisons through time.
Itecent Research and Plant Collections
The first systematic plant collections and floristic notes containing veg-
etational information was by Clover (1942) and Clover and Jotter (1941,
1944~. The Clover and Jotter Scientific Expedition (or Nevills' Expedition
of 1938) consisted of three cataract boats that left Lee's Ferry on July 12,
1938. In addition to being the first biologists to conduct research along the
river Elzada Clover, 41 year-old botany professor from the University of
Michigan and her young lab assistant, Lois Jotter, were the first women to
travel through the Grand Canyon. At Phantom Ranch the expedition was
joined by Emery Kolb, boatman and aforementioned motion picture photog-
rapher. The expedition reached Lake Mead, more than 250 miles down-
stream, in late July. Clover spent part of the following year collecting at
localities along the canyon that she could reach by car or foot. Botanical
specimens and information collected on those two trips constituted the only
critical examination of the biology of the canyon until the 1970s. Even
with the impending construction of Glen Canyon Dam in the l950s and
1960s all biological "salvage" investigations were conducted upstream of
the damsite in a riverine ecosystem that was to be inundated by Lake Powell
(Woodbury, 1959~. The possibility of large scale changes that were to
occur in downstream aquatic and riparian ecosystems were not considered
until they became obvious, after the completion of the dam.
In the late 1960s and early 1970s there was increasing interest in the bio-
logical, physicochemical, and socioeconomic parameters of the Colorado River
in the Grand Canyon. There was much concern about the"carrying capacity"
for campers on the alluvial terraces, or "beaches," along the river and interre-
lationships between campers, vegetation, and other components of the riverine
environment (Dolan et al., 1977; Johnson et al., 1977~. In response to these
concerns research planning was instituted through the Grand Canyon Research
Advisory Board, formed under the auspices of the Arizona Academy of Sci-
ence in cooperation with the National Park Service COPS). The board con-
sisted of an interdisciplinary consortium of scientists, knowledgeable about
the canyon, that advised NPS on research needs for the river. Beginning in
1970 a series of river expeditions was sponsored by this group and the Grand
Canyon Natural History Association. Research expeditions varied somewhat
OCR for page 185
HISI ORIC CHANGES IN VEGETATION...
185
in their basic missions, usually traveled by raft, and were composed of scien-
tists from several different disciplines. Issues regarding recreational demands
and environmental concerns, combined with a court case filed by river recreationists,
eventually reached such a level that in July, 1973, the Washington, D.C. office
of NPS funded a research trip through the canyon. A Colorado River craft (37
ft. pontoon raft) with two boatmen carried eight scientists and two assistants
who spent 16 days on the river evaluating almost 400 campsites (Borden et al.,
1975~. For the larger campsites evaluations included physical characteristics
and capacity (numbers of campers), shoreline (sand vs. rock, landing condi-
tions, bathing conditions, etc.), vegetation, photography available and other
factors.
Scientists involved in these earlier expeditions formed a core of research
leaders that have contributed much of the information in this current paper,
either in published papers, manuscripts, or by personal communication. By
1976, thirty projects and numerous river research expeditions had been funded
by NPS under the Colorado River Research Program. The studies were coordi-
nated from 1973 to 1976 by NPS Research Scientist R. Roy Johnson and the
earliest riparian studies were conducted by a research team from the Museum
of Northern Arizona under the leadership of Steven W. Carothers, Curator of
Biology. NPS allocated more than 3/4 of a million dollars for studies and
eighteen research reports published by NPS Western Region Office Johnson,
1977~. Also, during the 1970s and 1980s a great proliferation in publications
for the Colorado River accompanied these research efforts. Numerous other
reports and papers were published in other series and the open literature.
Most publications through the 1970s were referenced in a bibliography (Spamer
et al., 1981~. In addition to referencing the more important summary and
synthesis papers from that bibliography, throughout this paper we also refer-
ence works from throughout the 1980s that are most important to our discus-
sions of vegetation (see also USDI, 1988~.
DISCUSSION
Prior to the construction of Glen Canyon Dam the riparian and aquatic
components of the natural riverine ecosystem of the Colorado River in
Grand Canyon were driven by high energy, pulsing events typical of south-
western rivers. Three pulsing features of the river affected by the dam
included, in decreasing importance to the riparian ecosystem (1) seasonal
flow patterns and maximum/minimum flows (2) nutrient and sediment transport
and turbidity and (3) fluctuations in water temperature (Carothers and John-
son, 1983~. Riparian vegetation was dependent not only on moisture from
the river but also on nutrients, and sediments carried by the river's water.
Although fluvial sediments provided an ideal substratum for riparian veg-
etation, high flows and sediments transported by those flows also scoured
vegetation from the riparian zone on a regular basis.
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186
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
Increasing salinity of western rivers is one of our most serious problems
(Pillsbury, 1981), not only from a standpoint of human usage but also be-
cause of its impacts on natural riverine ecosystems. However, little is
known of salinity and other critical riverine issues in Grand Canyon. The
roles of impoundments and associated evaporation and dissolution processes
in increasing salinity of water and soil further downstream are well known
(Paulson and Baker, 1983~. However, total dissolved solids (TDS) have not
increased appreciably in Grand Canyon and below Hoover Dam (Paulson
and Baker, 1983) during recent monitoring. Presumably most of these salts
have settled out in reservoirs above the dam but too little time has passed to
evaluate long term effects. Paulson (1983) advocates the use of dams in
controlling salinity in rivers. The interrelationships of riparian plants with
soil and water salinity are being studied for our area by Stevens (ms and
personal communication) who finds salt levels in soils of the newly estab-
lished riparian zone along the river (NHWZ) of approximately 1/10 the
level of predam fluvial terraces.
In addition to interrelationships between riparian and aquatic ecosystems
the riparian zone is also closely linked to adjacent upland terrestrial ecosys-
tems. Riparian ecosystems throughout the Southwest present a verdant
ribbon of vegetation, contrasting sharply with more arid, upland landscapes.
These cooler, more mesic environments are heavily utilized by hikers, whitewater
recreationists, and wildlife. Riparian ecosystems of arid regions are noted
for their high recreational and wildlife values (Johnson and Carothers, 1982~.
The riparian zone along the Colorado River in Grand Canyon is no excep-
tion. The green of tamarisk, willows, seepwillows, mesquite and other
riparian species often ends abruptly at the talus or desert uplands where it is
replaced by browner and yellower hues of creosotebush, ocotillo, barrel
cactus, numerous species of Opuntia, other cacti and other typical desertscrub
species.
PREDAM AND POSTDAM VEGETATIONAL COMMUNITIES
Prior to construction of Glen Canyon Dam spring floods resulting largely
from melting of headwater snow in the Rocky Mountains and Colorado
Plateau attained an average high of 86,000 cubic feet per second (cfs) (2,430
cubic meters per second [ems]), but commonly exceeding 100,000 cfs (2,830
ems). At least one historic flood approximated 300,000 cfs (8,490 ems) at
Lee's Ferry (Dolan et al. 1974~. These floods, and the heavy silt load that
had given the river its Spanish name, Rio Colorado (Colored or Red River),
scoured vegetation from the rocky canyon sides and replaced sandy terraces
and their vegetation with new, unvegetated sand. Woody riparian plants
were provided with adequate water but generally grew above the scour zone
at the OHWZ.
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HISI ORIC CHANGES IN VEGETATION...
187
A few years after the completion of Glen Canyon Dam in 1963, a prolif-
eration of woody riparian vegetation became apparent. However, this veg-
etation was situated lower on the Colorado's banks in the NHWZ, or post-
dam fluvial sediments of Dolan et al. (1974~. This new relationship between
flow levels and vegetation regimes was first discussed by Dolan et al. (1974)
and more recently by Johnson and Carothers (1982~. Prevalent species of
the OHWZ, or high predam flood terraces of Dolan et al. (1974) are shown
in Figure 8-1.
CLASSIFICATION OF DESERTS AND PLANT COMMUTITIES
The upland vegetation of the Inner Gorge of the Grand Canyon consists
of Great Basin Desertscrub and Mohave Desertscrub. This was discussed
early by Bennett (1969), one of the first to examine postdam riparian plants.
The river corridor was mapped as Great Basin Desertscrub from Lee's Ferry
downstream to approximately 10 miles below the Little Colorado River and
as Mohave Desertscrub from there downstream past the Grand Wash Cliffs
(Brown and Lowe, 1980~. Detailed maps of this riparian vegetation were
prepared in 1977 by the Museum of Northern Arizona staff at a scale of
1:9600, with contour intervals of 200 feet (Table 8-1~. The Grand Canyon
region is best known in the natural community literature as the site used by
C. Hart Merriam (1890) in formulation of his Life Zone concept. Riparian
ecosystems from Lee's Ferry to Lake Mead all occur in the Upper and
Lower Sonoran zones.
coo
80
UJ 60
11
40
20
.........
......................
,. . ..
........
.......
,.. .....
. ....
..... .
........
,. ... ..
........
,.....
........
. .... .
,.. . . ...
.... .. .
..........
...................... 1. ~
.,.... ~
TALUS _~
CANYON WALL
UDSZ
/
IL
OHWZ
O-
i
IFDZ
UDSZ Upland Desertscrub Zono - Unintiuenr7sd by river regime
(stabb community)
OHWS Old High Wator Zono - Woody vegetation: Prosopis. Acacia
Fallugia [U,oper Canyonl. Bacchans sarathroidos Down
Canyon] (Stable Community)
IFOZ Intormodiato Flood Disturbance Zone (Second overbanh
torraco) - Short lived invasive species: Albagi Sakoh,
Doscuraini~, Blames runs (Unstable Community)
NHWZ Now High Wator Zono - Tamanx, Salix. Pluchoa. Bacchans
Jpp., Equisotum (Rapid Proliferation)
NHWZ
Zone of ~
Human Impact i
FLOOD ZONE At_ RIVER
POST DAM HIGH
WATER RELEASES RIVER
FIGURE 8-1 Profile of the vegetative zones of the Colorado River floodplain in
the Grand Canyon prior to the construction of Glen Canyon Dam (after Johnson and
Carothers, 1982~.
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188
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
TABLE 8-1 Vegetation types and common genera of the 1977 MNA-
NPS map.
VEGETATION TYPE
COMMON GENERA
Winter~eeiduous orthophyll forest
Deeiduous selerophyll forest
Deeiduous selerophyll woodland
Evergreen slerophyll scrub
Deeiduous orthophyll scrub
Mierophyllous deciduous thom scrub
Closed deciduous scrub in scattered trees
Seasonal orthophyll marsh
Season sclerophyll marsh
Desert scrub
Salt bush desert
Microphyllous deciduous desert scrub
Seasonal & evergreen desert herb vegetation
Acer, Fraxinus, Salix
Tamarind
sparse Tamaruc
Pluchea
Baccharis, Brickellia, Salix
Acacia, Prosopis
Fallugia, Cercis, Celtis
Epipactis, Mimulus
Phragmites, Typha
Agave, Larrea
Atriplex
Acacia
Abronia, Eriogonum
SOURCE: Museum of Northem Anzona, 1977. On file, CPSU/UA, NPS, Tucson.
Riparian plant communities are simpler than upland communities; that is,
fewer riparian plant communities occur along a given elevational or geo-
graphic gradient than along the same gradient in adjacent upland communi-
ties. This is due, at least in part, to the greatly ameliorating affect of water
in the adjacent stream course (Johnson and Haight, in press). However,
microhabitats and associated niche availability to animals are generally greater
in riparian environments, especially in arid regions where more synusia
(vegetational strata) are commonly present. Plants are also more closely
spaced in riparian communities than in adjacent upland communities. Ad-
equate moisture in riparian communities results in differing community dy-
namics from adjacent uplands where underground competition for moisture
serves as a critical selection factor (Johnson et al., 1989~.
Miller et al. (1982) added "Colorado River Beach," in dealing with am-
phibians and reptiles of the Grand Canyon. This was an additional riparian
biotic community to Hoffmeister's (1971) "Riparian of Inner Gorge." Since
riparian means, most simply, "streamside" (Johnson and Haight, in press),
vegetation is not a requirement of a riparian environment and in the Grand
Canyon much of the riparian zone consists of bare soil, rocks, or sand.
THE COLORADO RIVER AS AN EXCEPTION TO
RIPARIAN DESERTIFICATION
The concept of desertification was originally applied to desert upland
regions, usually in third world countries, notably Africa. Early investiga-
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COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
Reptiles in general are adapted to xeric environments, and while many of
them are nonriparian or facultative riparian species others occur in riparian
habitats either as obligate or preferential riparian species (terms after John-
son et al., 1984~. Rattlesnakes, for example, are often considered inhabit-
ants of xeric, often rocky habitats. However, the best known of the canyon's
reptiles, the Grand Canyon (or pink) rattlesnake (Crotalus viridis abysses)
frequently occurs in riparian habitats (personal observation; Miller, 1982~.
Warren and Schwalbe (1985) listed seven species of lizards for the riparian
zone along the river. Stabilization of river flows has probably resulted in an
increase in herp populations through greater vegetational biomass, provid-
ing in turn more cover and an increase in insect availability, as discussed
below for birds.
Birds
The importance of riparian habitat along this corridor to the avifauna of
the region has been documented by a series of recent studies by Brown,
Carothers, Johnson, and associates. During the Colorado River studies of
the 1970s Carothers and Johnson (1975) published new distributional records
for 20 birds in the Grand canyon region, seven of them additions to previ-
ously compiled faunal lists. By 1978 the number of species known for the
Grand Canyon region had grown to 284 (Brown et al., 1978), adding more
than 100 additional species to the 180 in the first Grand Canyon checklist
(Grater, 1937~. By 1987 this had increased to 303 avian species (Brown et
al., 1987) of which 250 (83%) have been recorded from the Colorado River
corridor (Brown et al., 1985~.
The avifauna is the best studied biological component of the Colorado
River. The high visibility of birds provides relative ease of scientific study,
and the interest of amateurs (including river runners) results in a larger
number of records than for other animals. Thus, several recent findings
about the avian ecology of the river may be used to partially illustrate the
role of vegetation in the riparian ecosystem. Since 1964 and publication of
The Birds of Arizona (Phillips et al., 1964), nesting populations of at least
six riparian species have become established in the Grand Canyon (Johnson
and Carothers, 1987) and populations of several other nesting species have
greatly increased. The gradual upstream movement of Bell's Vireo with
increasing riparian vegetation was documented by Brown et al. (1983~. Popu-
lations of nesting birds in tamarisk approaches the highest numbers for
temperate North America (Brown and Johnson, 1989~.
The most extensive general studies with riparian birds in the Grand Can-
yon have been by Brown (1987, 1989~. His findings, broadly generalized,
are that in the Grand Canyon obligate riparian nesting birds select tamarisk
over native plant species for nesting habitat. Additionally, individual nests
are preferentially placed in a tam arisk bushes instead of native plants, e.g.,
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HISTORIC CHANGES IN VEGETATION...
197
coyote willow, arrowweed, or seepwillow, even when other species are
equally available.
Proliferation of vegetation in the postdam riparian ecosystem is largely
responsible for the increase in numbers and species of birds. The reasons
for selection of tamarisk over native species are not so clear. However,
studies by Stevens (1976,1985) have shown a disproportionately large number
of individuals and species of insects supported by tamarisk. Since nearly
all breeding avian species along the Colorado River are insectivores these
high insect populations presumably provide an abundant avian food source.
However, this still does not explain the preferential selection of tamarisk
over other species for nesting.
Mammals
Ruffner and Carothers (1975) published new distribution records for twelve
mammalian species, including two new species for the park since the publi-
cation of Mammals of Grand Canyon (Hoffmeister, 1971~. Many of these
additions were from the river riparian zone. Some of these species, as with
herps and birds, have undoubtedly increased in numbers and/or distribution
in the region because of proliferation of riparian vegetation with resultant
increased cover and food availability. Of the 78 species of mammals listed
by Hoffmeister (1971), Ruffner and Carothers (1975), Ruffner et al. (1978),
and Suttkus et al. (1978), Jones et al. (1982) and Stevens (1989c); 40 (51%)
occur in the Inner Gorge. Most occur in the riparian zone, either as obligate
riparian mammals, e.g. beavers, or some lesser category of usage, e.g. desert
bighorn sheep that only use it on occasions. An outstanding treatment of
mammals of the area is found in Hoffmeister (1986~.
The importance of vegetation to animals, as food and shelter, has been
emphasized earlier. In some situations, e.g., for herbivorous mammals,
animals may, in turn, have a profound influence on vegetation. Burros
caused notable damage to riparian vegetation by foraging and trampling
(Carothers et al., 1976) and reduced populations of some plant species by
selective foraging prior to the removal of these feral mammals from the
canyon. One of the major factors in the lack of establishment of Goodding
willows and Fremont cottonwoods along the river is their utility to beavers
(personal observation). Beavers also eat the smaller, coyote willow and
even tamarisk. At a small beach at Tapeats Creek beavers totally stripped
coyote willows and tamarisk, leaving nothing but seepwillows by the fall of
1989 (L. Stevens, personal communication). Such foraging could result in
significant biogenic succession, producing a '`beaver disclimax" on some
beaches, if beaver populations reach adequate levels. One of the more
poorly understood vegetational parameters for most woody riparian species
is seedling establishment. Ecesis (germination and establishment) of ripar-
ian species in the canyon is being studied by Stevens (1989a). Findings
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198
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
from these studies and subsequent monitoring programs are critical to the
management of the riverine ecosystem in Grand Canyon.
THREATENED AND ENDANGERED (T & E), RARE,
AND ENDEMIC SPECIES
Riparian
The Colorado River corridor, with its linear ribbon of open water and
riparian habitat, comprises the largest amount of this type of habitat in the
region. Moisture furnished by the river serves as an ameliorating influence,
providing a series of more mesic environments in this generally arid land-
scape. Still, the Grand Canyon region is notably short on endemic, rare,
and T & E species for such a large, diverse area. Two previously undescribed
species of flowering plants, FlaYeria mcdougallii and Euphorbia aaron-
rossii were mentioned earlier in the paper (Carothers and Aitchison, 1976;
Phillips et al., 1987~. This small number of special status species is prob-
ably due in large part to the Colorado River's running the entire length of
the region, bisecting and connecting the various sections of the Colorado
Plateau and providing a relatively uniform environment that serves as a
dispersal route for many species of plants and animals.
Of as great interest as the presence, or lack thereof, of special emphasis
species are species that are lacking. The house mouse (Mus musculus), for
example, is missing as are species of old world rats (Rattus) that are found
in many of the more "civilized" parts of the United States. Feral burros, on
the other hand, built up to such high populations along the Colorado River
during the 1970s that they unfavorably impacted both the natural riparian
environment and recreational activities (Carothers et al., 1976~.
Endangered Species
Two endangered birds of the riparian zone are the Bald Eagle (Haliaeetus
leucocephalus) and Peregrine Falcon (Falco peregrinus). Bald eagles have
become regular winter visitors since the mid-l9SOs (Brown et al., 1989) in
the vicinity of Nankoweap Creek and the river where they feed on fish.
Numbers have increased to more than 50 individuals (B. T. Brown; L.
Stevens, personal communication). Recent studies by S.W. Carothers and
B.T. Brown (personal communication) have demonstrated that the Grand
Canyon supports the highest concentration of breeding Peregrines in the
lower 48 states. A total of 71 different Peregrine breeding areas were
documented during partial investigations of the region in 1988 and 1989
(Brown, 1990~. A large percentage of the Peregrine's diet consists of birds
caught over the riparian and aquatic zone of the inner canyon.
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HISTORIC CHANGES IN VEGETATION...
199
AQUATIC
Aquatic ecosystems are extensively discussed elsewhere in this volume.
However, this discussion of the riparian zone would be incomplete without
some mention of the interactions with the adjacent river with which it so
intimately communicates. For example, riparian birds are largely insectivo-
rous, with different species foraging to varying degrees on insects that have
aquatic stages in their life cycles. Although insects have been studied in the
Grand Canyon (Stevens, 1976, 1985), more definitive information is needed
to ascertain the role of vegetation in insect life histories, and the role played
by insects as food for riparian as well as aquatic organisms. In addition to
sedimentological studies discussed elsewhere in this volume, the contribu-
tion of sediment, nutrients, and detritis from the riparian zone to the aquatic
food chain needs to be further investigated.
The Lee's Ferry area is one of the nation's finest rainbow trout fishery.
By the early 1900s, the transplanting of nonnative species into the Colorado
River had become common largely because of the advent of trains, the tank
car, and other efficient means of transporting fishes. "Game fish" were
common enough at Lee's Ferry by the 1950s to attract fishermen (S. Carothers,
personal communication). As mentioned earlier, approximately 1/2 of the
100,000 annual visitors to the Lee's Ferry area are fisherman that use the
riparian zone extensively for camping, shelter, and bank fishing.
The role of these introduced fishes in reducing and/or extirpating native
fishes within the Grand Canyon is not clearly known. Johnson and Carothers
(1987) have proposed that predation by nonnatives has played a larger role
in extirpation of these native species than some have noted. Many of these
species are voracious predators on young of larger native species or adults
of smaller natives. Additionally, carp had arrived in Arizona prior to 1885
and although they generally do not feed on small fishes they do eat fish
eggs (Minckley, 1973~. Thus, during most of this century native fishes of
the Colorado system have had to compete with these introduced species
(Minckley and Deacon, 1968~.
Carothers further proposes that, based on his studies of the fishes of the
Colorado River in Grand Canyon (Carothers and Minckley, 1981), several
native species were nearing extinction before the construction of Glen Can-
yon Dam (see also Minckley, this volume).
SUMMARY AND CONCLUSIONS
The Colorado River in Grand Canyon, as other riverine systems through-
out the arid Southwest is characterized by a close interrelationship between
aquatic and riparian components of the ecosystem. Riparian ecosystems
provide nearly classroom examples of ecotones, occurring at the
Interlace
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200
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
of aquatic and te'Tes~ial ecosystems. As such, they demonstrate the edge
effect by showing an increase in species richness and population densities.
We have here discussed both riparian organisms and processes as well as
aquatic-riparian ecotonal interactions along the Colorado River in Grand
Canyon.
The importance of this riparian habitat to the nesting avifauna of the
lowland Southwest, other wildlife, and humans has been widely documented.
The riparian zone in Grand Canyon is used extensively by whitewater
recreationists, hikers, and a large number of native animals. Since these
riparianlands are used by transients and wintering birds as well as breeding
birds, the Colorado River riparian ecosystem constitutes one of the most
important avian resources in the North American Southwest.
Riparian desertification is a major problem throughout the arid South-
west. However, the riverine environment in Grand Canyon has become
increasingly mesic, contrasting with increasingly xeric conditions of most
riverine ecosystems of the Southwest. Thus, riparian vegetation in the
canyon is uniquely valuable since the Colorado River in Grand Canyon is
the only major riverine ecosystem in the Southwest where there has been an
appreciable increase rather than a decrease in riparian vegetation and asso-
ciated animal populations during the 1900s.
Continuing scientific investigations in the Grand Canyon need to exam-
ine the interrelationships between water releases from Glen Canyon Dam
and the riparian and aquatic ecosystems of the downstream Colorado River.
Biological components of these ecosystems need to be closely examined in
relation to the physicochemical environment. Discussion of some of the
research needs in examination of the intricate interrelationships between the
riparian and aquatic ecosystems of the Colorado River in Grand Canyon are
mentioned in the aquatic section, above. Of equal importance is the deter-
mination of socioeconomic factors and the examination of carrying capacity
and visitor satisfaction as discussed by numerous investigators (Heberlein
and Shelby, 1977~. Although the native riverine ecosystems are no longer
extant, strategies for the proper monitoring and management of the existing
naturalized riparian ecosystem and exotic aquatic ecosystem must be ad-
dressed if the viability and utility of the Colorado River in the Grand Can-
yon is to be maintained.
ACKNOWLEDGMENTS
A number of scientists contributed freely of their time and knowledge in
assisting me with information for this paper. My understanding of the
Canyon's ecology was enhanced by discussions with Bryan Brown and Steven
Carothers for riparian systems and Steve Carothers, Gerald Cole, and Wendell
Minckley for aquatic ecosystems. I attained a better depth of knowledge of
OCR for page 201
HISTORIC CHANGES IN VEGETATION...
201
the vegetation and flora during early discussions with Paul Martin and Ray
Turner and, more recently, with Peter Bennett and Laxity Stevens. Lois
Haight assisted with numerous technical suggestions, and Larry Stevens and
Dave Wegner critically reviewed the manuscript in its entirety.
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COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
APPENDIX A
SCIENTIFIC NAMES OF PLANTS IN TEXT
Apache Plume - Fallugia paradoxa
Arrowweed - Tessaria (Pluchea) sericea
Bennuda Grass - Cynodon dactylon
Brome, Foxtail or Red - Bromus rubens
Bulrush - Scirpus americanus and S. validus
Cactus, Barrel - Echinocactus polycephalus and
Ferocactus acanthodes
Camelthorn - Alhagi camelorum
Catclaw, Catclaw Acacia - Acacia greggii
Cattail - Typha domingensis and T. Iatifolia
Clover, Sweet - Melilotus spp.
Cottonwood, Fremont - Populus fremontii
Creosotebush - Larrea divaricata (tridentata)
Desertbroom - Baccharis sarothroides
Dicoria - Dicoria bra ndegei
Horsetail - Equisetum spp.
Mesquite, Westem Honey - Prosopis glandulosa
Ocotillo - Fouquieria splendens
Russian-olive - Elaeagnus angustifolia
Seepweed - Suaeda torreyana
Seepwillow - Baccharis salicifolia (glutinosa)
Seepwillow, Emoryils - B. emoryi
Tamarisk, Salicedar - Tamarix ramosissima
Thistle, Russian - Salsola iberica
Thistle, Sow - Sonchus asper and S. oleraccus
Waterweed - Baccharis sergiloides
Willow, Coyote - Sal~c exigua
Willow, Goodding - S. gooddingii
Representative terms from entire chapter:
colorado river
