Coulees and Canyons

"What's the longest canyon on Mars? What is Grand Coulee and how big is Grand Coulee Dam? What are the Channeled Scablands? Are there canyons in the ocean?"

Long, Deep and Wide

Valles Marineris on Mars A huge 2,500 mile long canyon called Valles Marineris stretches across Mars. Up to 400 miles wide and as much as 4 miles deep the canyon would reach from Los Angeles to Chicago if were placed on North America. Many side channels show evidence of erosion and massive landslides. It is unclear how this canyon was formed and it is still being studied by NASA and other scientists.

A coulee is a long winding channel cut through lava formations. This term is primarily used in the northwestern United States. The image at right is the Nanedi Vallis area on Mars. This channel might have been carved by water but it is not certain. An unanswered question is: If this was caused by a river or a flood, where did all the water go? Click on image or here for a larger image.

Here is part of the caption for the photo:

"It shows the canyon of Nanedi Vallis, one of the Martian valley systems cutting through cratered plains in the Xanthe Terra region of Mars. The picture covers an area 9.8 km by 18.5 km (6.1 mi by 11.5 mi), and features as small as 12 m (39 ft) can be seen. The canyon is about 2.5 km (1.6 mi) wide. Rocky outcrops are found along the upper canyon walls; weathered debris is found on the lower canyon slopes and along the canyon floor. The origin of this canyon is enigmatic: some features, such as terraces within the canyon (as seen near the top of the frame) and the small 200 m (660 ft) wide channel (also seen near the top of the frame) suggest continual fluid flow and downcutting. Other features, such as the lack of a contributing pattern of smaller channels on the surface surrounding the canyon, box-headed tributaries, and the size and tightness of the apparent meanders (as seen, for example, in the Viking image 89A32), suggest formation by collapse. It is likely that both continual flow and collapse have been responsible for the canyon as it now appears."
Photo Credit: NASA/JPL/Malin Space Science Systems

Coulee on left of road

Coulee south of Coulee City.

Unlike the Grand Canyon, which was eroded by a river, the coulees of Washington were carved out by Ice Age floods. This part of Washington is known as the Channeled Scablands because of the many flood-carved channels and barren landscape. The Grand Coulee is the largest coulee and is used for irrigation of the fertile Columbia Basin. One feature left by the catastrophic floods is Dry Falls, a four mile long waterfall with no water falling! Grand Coulee Dam is one of the largest concrete structures in the world and not only provides water for farmland but electric power for the area.

As you can see from the coulee pictured above the area is very dry or semi-arid due to the rainshadow effect of the Cascade Mountains. Where did all the water and the sand and gravel carried by the floods go? Find out more on the Ocean Canyons page.

This is a view below and down the channel at Palouse Falls. Click image or here for a larger image. Can you imagine the amount of water it took to carve out this canyon?

For more on Dry Falls and Palouse Falls go to the Waterfalls page.

Geology Terms

Here are some basic terms used on this page. Find more geology terms in the Glossary.

Bar: A mound of gravel and sand deposited by flowing water. Bretz and other geologists identified many large bars in the Channeled Scablands.
Basalt: Volcanic rock caused by partial melting of the Earth's crust.
Channel: The deepest part of a river or bay.
Channeled Scabland: Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications. See also Channeled Scablands.
Coulee: Long winding channel cut through lava formations. A term primarily used in the northwestern United States.
Erosion: Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.
Gravel: Rounded rock fragments larger than sand.
Ice Age: A period in Earth's history when much of the continents are covered with ice sheets and glaciers.
Rain Shadow: A mountain or mountain range that blocks rain clouds just as an object might block sunlight to form a shadow. Areas in the shadow are more dry as a result.
Scabland: A flat region with exposed lava rock and a thin layer of soil and sparse vegetation. Usually cut through with channels.

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