Geology of The Grand Canyon Area - Ongoing Geology and Human Impact

Ongoing Geology and Human Impact

The end of the Pleistocene ice ages and the start of the Holocene began to change the area's climate from a cool, wet pluvial one to dryer semi-arid conditions similar to that of today. With less water to cut, the erosive ability of the Colorado was greatly reduced. Mass wasting processes thus began to become relatively more important than they were before. Steeper cliffs and further widening the Grand Canyon and its tributary canyon system occurred. An average of two debris flows per year reach the Colorado River from tributary canyons to form or expand rapids. This type of mass wasting is the main way the smaller and steeper side canyons transport sediment but it also plays a major role in excavating the larger canyons.

In 1963 Glen Canyon Dam and other dams farther upstream started to regulate the flow of the Colorado River through Grand Canyon. Pre-dam but still historic flows of the Colorado through Grand Canyon ranged from 700 to 100,000 cubic feet (20 to 2,800 m3) per second with at least one late 19th century flood of 300,000 cubic feet (8,500 m3) per second. Discharge from Glen Canyon Dam exceeds 48,200 cubic feet (1,360 m3) per second only when there is danger of overtopping the dam or when the level of Lake Powell otherwise needs to be lowered. An interim conservation measure since 1991 has held maximum flows at 20,000 cubic feet (570 m3) per second even though the dam's power plant can handle 13,200 cubic feet (370 m3) per second more flow.

Controlling river flow by use of dams has diminished the river's ability to scour rocks by substantially reducing the amount of sediment it carries. Dams on the Colorado River have also changed the character of the river water. Once both muddy and warm, the river is now clear and averages a 46 °F (8 °C) temperature year-round. Experimental floods approaching the 48,200 cubic feet (1,360 m3) per second level mentioned above have been carried out in 1996 and 2004 to study the effects on sediment erosion and deposition.

Grand Canyon lies on the southern end of the Intermountain West seismic belt. At least 35 earthquakes larger than 3.0 on the Richter Scale occurred in the Grand Canyon region in the 20th century. Of these, five registered over 5.0 on the Richter Scale and the largest was a 6.2 quake that occurred in January 1906. Major roughly north—south trending faults that cross the canyon are (from west to east), the Grand Wash, Hurricane and Toroweap. Major northeast-trending fracture systems of normal faults that intersect the canyon include the West Kaibab and Bright Angel while northwest-trending systems include the Grandview—Phantom. Most earthquakes in the region occur in a narrow northwest-trending band between the Mesa Butte and West Kaibab fracture systems. These events are likely the result of eastward-migrating crustal stretching that may eventually move past the Grand Canyon area.