Colorado River

Long ago, the land here was pushed up high.

The river kept flowing and wore the rock away. Slowly it made deep canyons.

The National Park Service's Grand Canyon geology page sums up the canyon's story as D-U-D-E: deposition, uplift, downcutting and erosion. First, layer upon layer of rock was laid down. Then, between about 70 and 30 million years ago, the whole region was lifted up, forming the high, flat Colorado Plateau.

Beginning about 5 to 6 million years ago, the Colorado River began to cut its way downward, and side streams widened the canyon. At Horseshoe Bend, the National Park Service explains, rivers that wound across the land became trapped in their bends as the plateau rose, and cut down through the sandstone layers.

The National Park Service's geology pages describe deposition of sedimentary layers above rocks about 2 billion years old, uplift of the Colorado Plateau between about 70 and 30 million years ago (probably linked to the Laramide Orogeny and shallow-angle subduction of the Farallon Plate, one of two favored hypotheses), and downcutting by the Colorado River over the past 5 to 6 million years. The opening of the Gulf of California, about 5.5 million years ago, gave the river an outlet that lowered its base level by about 5,000 feet (1,524 m).

How the river came to follow its present course is debated. Its upper part may be at least 7 and perhaps 10 million years old, older than the lower part. Hypotheses include headward erosion capturing an older river (Edwin McKee, 1967; Ivo Lucchitta), the spilling over of ancient lakes, and Carol Hill's newer idea of a collapsing groundwater-karst system.

At Horseshoe Bend, the river makes a wide curve.

It bends around a rock, like the shape of a horseshoe.

Below the rim at Horseshoe Bend, the Colorado River makes a wide sweep around a sandstone wall. The National Park Service calls it an entrenched meander: a winding river bend that got cut deep into the rock.

The canyon there is roughly 1,000 feet (305 meters) deep, and the river turns through 270 degrees, about three quarters of a full circle. Horseshoe Bend is part of Glen Canyon National Recreation Area, and Grand Canyon National Park begins about nine miles downstream.

The National Park Service explains Horseshoe Bend as an entrenched meander. The river once meandered across the landscape, making wide bends and following the path of least resistance; when the Colorado Plateau rose, around 5 million years ago, the meandering rivers were trapped in their beds and cut down through the uplifted sandstone layers.

The result is a roughly 1,000 ft (305 m) deep, 270-degree curve in the canyon. The bend and that stretch of river lie in Glen Canyon National Recreation Area, and Grand Canyon National Park begins about nine miles downstream.

Special fish live in the Colorado River.

One is called the humpback chub. It has a hump behind its head.

The wild Colorado River was hard to live in: huge spring floods, near-freezing winters, warm summers and water full of sand and silt. Only eight kinds of fish are native to the Grand Canyon, and six of them are found only in the Colorado River basin.

Many of these fish are large as adults, have a hump or keel behind the head and can live 40 years or more. One is the humpback chub. Park biologists have moved young chub from the Little Colorado River into other side creeks where they may find a good home.

The National Park Service says the river once hosted one of the most distinctive fish assemblages in North America. Only eight species are native to the Grand Canyon, all minnows or suckers, and six are endemic to the Colorado River basin. Shared traits include large adult size, a hump or keel behind the head, spawning migrations cued by spring runoff and lifespans of 40 years or more.

Today only five of the eight are still found in the park. The humpback chub, adapted to deep, swift water, is listed as threatened after many years as endangered, and the park holds its largest remaining population; the razorback sucker is endangered and rare. UNESCO adds that the canyon's walls hold five of North America's seven life zones. Since 2009, biologists have moved juvenile chub from the Little Colorado River to Shinumo, Havasu and Bright Angel creeks and have removed invasive trout.

The edge of the canyon is very high. Stay close to your grown-up.

Bring lots of water. The desert is hot and sunny.

The trail to the Horseshoe Bend overlook is 1.5 miles (2.4 km) round trip. There are railings at the viewing platform, but not along much of the rim, where the drop is 1,000 feet, so the National Park Service says to watch your footing and keep track of children and pets.

Horseshoe Bend is in the desert. The park advises drinking plenty of water, staying out of the sun when you can, wearing sturdy shoes and sunscreen, and avoiding the hottest part of the day.

The National Park Service describes the Horseshoe Bend trail as a 1.5-mile (2.4 km) round trip on a hardened path, with two shade structures but none at the overlook, no water and no toilets along the way. Railings line the viewing platform, but much of the rim is exposed to the 1,000-foot drop.

Its desert-safety advice: drink plenty of water even when not thirsty, limit direct sun, know the signs of heat exhaustion and heat stroke, move strenuous activity to the coolest time of day, and never leave pets in a vehicle. Flying drones from National Park Service lands in Glen Canyon National Recreation Area is prohibited without the superintendent's written approval.