time of recurring glaciations and at the end of each glacial cycle, greatly increased runoff from the Rocky Mountains would have resulted in giant floods with enormous erosive power.
Second, the Colorado River is crosscut by multiple faults, where the side of the fault that descends coincides with the downhill side of the river. Each time the fault moved, the downhill side of streambed would suddenly have been lowered, creating a step termed a knickpoint. A river’s erosive power slowly moves knickpoints upstream, in the process deepening the riverbed.
A second geologic event also created large and probably temporary knickpoints: There were large lava flows that dammed the Colorado River approximately 640,000 years ago. Although probably short-lived, these dams would have allowed enormous reservoirs to form along the Colorado River’s course. When the dams broke or were eroded upstream, the water stored behind the dam would have been instantaneously released, providing massive erosive force.
AN EVOLUTIONARY STORY
The physical conditions that create the rock strata you traverse are repeated again and again, creating multiple layers of limestone, shale, or sandstone. However, the Grand Canyon’s sedimentary strata were deposited over 500 million years, and the fossils in successive strata record much of the evolutionary history of life.
At the time the Bass Limestone was deposited, the only lifeform was single-celled colonial bacteria called stromatolites, visible in the rock as wavy bands. Multicellular, shell-bearing, aquatic animals evolved by the start of the Cambrian era, 542 million years ago; the Bright Angel Shale contains abundant trilobites, an early shell-bearing creature. Worm burrows, termed trace fossils, are also abundant. By the time the Redwall Limestone was deposited, different invertebrates dominated the seas, and crinoids (a stalklike relative of starfish) and brachiopods are abundant in this layer. Unlike at the time of the Bright Angel Shale’s deposition, plants and animals now colonized the land. Along the South Kaibab Trail, plant fossils are visible in the Hermit Formation; some specimens of ferns are on display on the west side of Cedar Ridge for all to observe. The Coconino Sandstone preserves reptile footprints. Stop and consider that these critters didn’t exist when you are just a couple of miles farther down the trail.
VEGETATION
Most hikers will consider their walk from the Grand Canyon’s rim to the Colorado River—and back up again—to be simply “desert.” Spring, summer, and fall temperatures are hot, the humidity is low, and the vegetation is sparse and prickly. However, you will in fact pass through four vegetation zones, each existing due to a specific combination of elevation, moisture availability, and temperature and each dominated by specific species. They are:
Pinyon-juniper woodland: Pinyon pine and juniper are the dominant species in this widespread southwestern plant community. At the Grand Canyon, this community exists on the South Rim and inside the canyon down to the Redwall Limestone. Ecologically, it demarcates the elevation that receives significant snow during the winter months. (For the first 1000 feet below the rim on the Bright Angel Trail, this community is interspersed with the mountain scrub and chaparral community, composed of species including Gambel oak, fernbush, serviceberry, and snowberry.)
Blackbrush scrub: This community is defined by the dominance of blackbrush and covers the Tonto Platform. Unless you head toward Plateau Point, the characteristic monostands of blackbrush are not observed along the Bright Angel Trail, since the trail descends along the Garden Creek drainage, first down the dry wash below Jacobs Ladder and then through riparian (streamside) vegetation below Indian Garden. On the South Kaibab Trail, you are in blackbrush scrub as you cross the Tonto Platform and approach the Tipoff.
Mojave Desert scrub: This community exists below the Tonto Platform down to the Colorado River. The species growing here can withstand extremely high summer temperatures and many require milder winters. Although it is informally termed Mojave Desert scrub, the plant community contains many species not present in the Mojave Desert of southeastern California. For instance, species such as brittlebush and catclaw acacia that are typical of the warmer Sonoran Desert of southern Arizona exist in the Inner Gorge. Farther downstream, species such as honey mesquite, characteristic of the Chihuahuan Desert, enter the mix. Indeed, along the Colorado River corridor a unique combination of species grow, for three of North America’s four deserts merge in this region.
Riparian: This community exists only in locations with permanent water, such as along the banks of the Colorado River and its side canyons. The availability of water means that large trees and dense thickets of shrubs and herbs are able to grow in these locations. The dominant species include Fremont cottonwood, coyote willow, seepwillow, saltcedar, honey mesquite, and catclaw acacia. The riparian vegetation along the Colorado River has changed significantly since the completion of the Glen Canyon Dam in 1964; water flows are now restricted and the river no longer floods as severely, a process that once scoured the vegetation along the banks and deposited vast quantities of sediment. Beginning in 1996 the Bureau of Reclamation has three times released higher flows from Glen Canyon Dam to replicate historic flooding. The largest flow occurred in March 2008, when they released twice the usual flow of water. Preliminary research indicates that this flood effectively mimicked natural floods, creating large sandbanks, reducing the establishment of nonnative plants, and creating backwater environments.
These vegetation zones can be identified by just a few dominant shrubs and trees, but more than 900 species occur below the canyon rims and in spring a diligent botanist might locate several hundred species along this walk. Any attentive hiker will notice many tens of distinctive and, when flowering, colorful species. Such diversity exists because the landscape is complex, creating many unique combinations of physical, chemical, and biological conditions. Each species prefers a certain soil depth, soil made from a specific rock, a specific small-scale climate, and a specific topographic position. Your walk will take you past geographic features that include solid rock, washes, steep slopes, seeps, small dunes, river terraces, the Tonto Platform, stream banks, ridges, and valleys, each home to a different collection of species.
DESERT ADAPTATIONS
Colorful flowers attract your attention—and that of pollinators—but stare also at the leaves and stems of plants, for they you will remind you what life is like for plants in the region. For instance, shrubs and perennial herbs that grow in a desert environment must have traits that minimize water loss: Lack of leaves (e.g., cactus), drought-deciduous leaves (leaves that fall off in summer; e.g., blackbrush), small leaves (e.g., Mormon tea) and hairy leaves (e.g., brittlebush and big sagebrush) are all common desert adaptations. Most water loss occurs because plants need to cool their leaves by transpiring (evaporating) water, a process much like humans sweating. Minimizing leaf area and having leaf hairs to reflect light and keep leaves cooler are two very effective ways to preserve water.
However, plants require leaves, or at least green surfaces; the green color indicates plant parts that can photosynthesize, or turn the sun’s energy into the sugars necessary for the plants to grow. In many species green stems compensate for limited leaf area. Drought-deciduous shrubs and many perennial herbs sprout leaves in spring when water is available and effectively hibernate during the hot summers. Other species are annuals. These plants grow for only a single season, germinating when soils are sufficiently moist and producing flowers and seeds in quick succession to avoid dry soils.
Below are descriptions of 38 species along the Bright Angel and South Kaibab trails, including the most common species, as well as some showy, difficult-to-miss species that occur along a specific stretch of trail. They are organized by growth form (trees and larger shrubs, herbs and small shrubs, and cacti and agaves) and are approximately in order of their appearance from the canyon rim to bottom. A list of all species in the Grand Canyon is available on the Southwest Environmental Information Network: http://swbiodiversity.org/seinet/checklists/checklist.php?cl=94.
