MAP POINT 6
From Labyrinth Canyon the river continues a similar meandering course, cutting into even older rocks, and enters what is now Canyonlands National Park.
The stream is still quiet, and we glide along through a strange, weird, grand region. The landscape everywhere, away from the river, is of rock—cliffs of rock, tables of rock, plateaus of rock, terraces of rock, crags of rock—ten thousand strangely carved forms; rocks everywhere, and no vegetation, no soil, no sand. In long, gentle curves the river winds about these rocks… rapid running brings us to the junction of the Grand [now the Colorado River] and Green, the foot of Stillwater Canyon, as we have named it [6 on map]. These streams unite in solemn depths, more than 1,200 feet below the general surface of the country.
Major John Wesley Powell,
July 17, 1869
The rocks Powell described from Stillwater Canyon consist of Triassic Moenkopi and Chinle Formations that he first observed in Flaming Gorge, and the Permian (280 to 275 Ma) Cutler Group. The chocolate- to brown-red-colored Moenkopi was deposited in a tidal-flat environment, as attested by its abundance of ripple marks on thin slabs of rocks, and the Chinle, famous for its petrified wood, uranium, and beautiful multicolored mudstone and shale, represents a river floodplain. The Permian Cutler Group consists of the red-brown floodplain deposits of the Organ Rock Formation at river level capped by prominent cliffs of the White Rim Sandstone which represents ancient coastal dunes.
Large uplifts and basins developed in the Colorado Plateau during the Laramide orogeny (mountain-building event) between the latest Cretaceous (about 70 Ma) and the Eocene (about 34 Ma). Canyonlands National Park and the surrounding region is on the northern end of the broad Laramide-age Monument uplift, which is responsible for exposing the impressive stratigraphic section of older rocks carved into by both the Green and Colorado Rivers in southern Utah.
MAP POINT 7
Once past the confluence of the two great rivers, the Colorado River flows in a southwesterly direction descending through one of the wildest series of rapids in Utah until it reaches Lake Powell 23 miles downstream.
We come at once to difficult rapids and falls, that in many places are more abrupt than in any of the canyons through which we have passed, and we decide to name this Cataract Canyon [7 on map].
Major John Wesley Powell,
July 23, 1869
The geology along Cataract Canyon is unique and the Colorado River likely has been a factor in the canyon’s structural development. The river follows a relatively straight course down the axis of a large anticline (upwarp) called the Meander anticline. Open-marine limestone beds of the Pennsylvanian (305 to 300 Ma) Honaker Trail Formation dip to the southeast and northwest, respectively, on each side of the river topped by progressively younger Permian formations. The Honaker Trail is underlain in the subsurface by the older Pennsylvanian Paradox Formation that contains evaporite rocks (gypsum and salt) which were deposited in a restricted marine environment. When under pressure, evaporites can flow like toothpaste being squeezed from a tube and push up the overlying rocks or even reach the surface (Powell recognized one such location and named it Gypsum Canyon, a side canyon to Cataract). The Meander anticline was formed this way and is underlain by a mass of mobilized gypsum and salt. As the Colorado River eroded the overlying section of rocks, the pressure on the evaporites below was reduced allowing them to push up even more. The evaporites withdrew from under the rocks adjacent to the canyon which caused collapse, faulting, and slumping towards the river. This process is still active today and contributes to huge rapids like Satan’s Gut and Little Niagara. This area is known as The Grabens in Canyonlands National Park.
MAP POINT 8
Upon leaving Cataract Canyon, the Colorado River turns westerly and enters the upper reaches of Lake Powell, named, of course, for the famous explorer. After passing the Dirty Devil River, so-called by one of Powell’s men because of its muddy water and foul smell, the rocks become younger in age (Permian, Triassic, and finally Jurassic) to the south.
On the walls, and back many miles into the country, numbers of monument-shaped buttes are observed. So we have a curious ensemble of wonderful features—carved walls, royal arches, glens, alcove gulches, mounds, and monuments. From which of these features shall we select a name? We decide to call it Glen Canyon [8 on map].
Major John Wesley Powell,
August 3, 1869
The features Powell used to name Glen Canyon are most prominently displayed in the Jurassic (190 Ma) Navajo Sandstone, famous for its classic cross-bedding and representing ancient dunes of windblown sand.
Glen Canyon and the canyons in the surrounding region, including those that Powell explored, formed within the past 5 million years by vigorous downcutting of the Colorado River and its tributaries, exposing more than 8,000 feet of bedrock that spans a period of about 300 million years. Powell no doubt would be shocked and amazed to see the reservoir that bears his name. All outcrops at river level and in many of the side canyons that Powell explored are covered by water, in many places hundreds of feet deep. Fortunately, the lake level creates an ideal horizontal datum along which large folds (anticlines and synclines) bring most of those outcrops, ranging in age from Triassic to Jurassic in the heart of Glen Canyon, to places where they can be observed from the comfort of a boat.
The 710-foot-high Glen Canyon Dam, located just south of the Utah border near Page, Arizona, was authorized by Congress in 1956 to provide water storage in the upper Colorado River basin, and construction began that same year. Lake Powell is the second largest reservoir in the United States (Lake Mead in Nevada and Arizona is the largest). The lake is 186 miles long, and with 96 major side canyons, it has more than 1,960 miles of shoreline—more than twice the length of the California coastline. The surface area of Lake Powell is 266 square miles and it is 560 feet deep at the dam. Lake Powell holds up to 27 million acre-feet of water, enough to cover the state of Ohio with one foot of water! The hot arid climate causes an average annual evaporation of 2.6 percent of the lake’s volume. Siltation in the lake averages 37,000 acre-feet per year, brought in principally from the San Juan and Colorado Rivers. That is the equivalent of 6 million dump trucks of silt each year! Even at that rate, it will take 730 years to fill the lake with silt.
Although the most famous part of Major John Wesley Powell’s 1869 expedition was the journey of what Powell called “the Great Unknown” of the Grand Canyon, he first spent most of his time exploring the canyons of the Green and Colorado Rivers in Utah. This expedition represents an amazing feat by Powell and his team at that time. Before Powell left the Utah Territory and entered what is now Arizona, he wrote “…we reach[ed] a point which is historic.” Powell was referring to a point along the Colorado River known as El Vado de los Padres or Crossing of the Fathers, a ford (now under 400 feet of water in Padre Bay in Lake Powell) named for Fathers Dominguez and Escalante who discovered it during their 1776 expedition through the region. For those of us who boat around Lake Powell or Flaming Gorge Reservoir, or raft the Colorado or Green Rivers, we too reach points that are truly historic—first named and described by Powell and his colleagues 150 years ago. Major John Wesley Powell’s expedition was truly a major contribution to science and an incredible adventure that still inspires a spirit of curiosity and sense of wonderment today.
Powell quotes from Canyons of the Colorado, by J.W. Powell, 1895.