Subsidence at Lake Mead's center

Approximately 22 cm of subsidence was measured at the center of Lake Mead in 1950, compared to the ground level recorded in 1935, before the lake was filled. The cause is attributed to the weight of the water retained by the Hoover Dam, located in the Black Canyon of the Colorado River between Nevada and Arizona.

Earth's crust yields under the load

The Earth's crust, typically considered solid, has yielded under the load of a lake with a capacity of 35.5 km³. The ground subsided by approximately 218 mm between 1935 and 1950, with the subsidence continuing for 15 years after the lake was filled. Repeated leveling by surveyors allowed this subsidence to be measured precisely.

Hoover Dam's construction and water retention

The dam began to retain water on February 1, 1935, although construction was not yet completed. The last concrete pouring took place on May 29, 1935, and the inauguration on September 30, 1935. Standing 221 m tall, the structure closed the Black Canyon and forced the river to spread upstream.

Lake Mead's size and water weight

Lake Mead covers 635 km², six times the area of Paris intra-muros, with a total capacity of 35.5 km³. On July 30, 1941, the lake's surface was at an altitude of 1,220.45 feet (approximately 372 m), with a depth of 580 feet (approximately 177 m). Each square meter of the lake bed at this depth supports the equivalent of 177 tonnes of water, the mass of about 30 adult African elephants.

Precise measurements of subsidence

Surveyors measured the ground level very precisely in 1935, before the lake was filled, and repeated this in 1941, 1950, and 1963. At the center of the lake, the ground descended approximately 120 mm in 1941, then approximately 218 mm in 1950. In 1963, the measurement gave 200 mm, indicating the ground had slightly risen.

Subsidence details and variations

The 22 cm in the title is the rounded maximum of 1950, referring to the center of the lake, not the entire region. At the dam itself, about 120 mm is cited. The subsidence stops after 1950. Other works based on leveling conducted between 1935 and 1950 mention subsidence up to 17 cm depending on the measurement point.

Slow subsidence and delayed effect

The most telling aspect is not the magnitude, but the delay. The ground did not sink all at once; it continued to descend for 15 years, from 1935 to 1950. However, the lake already contained 24 million acre-feet (approximately 29.6 km³) in 1938, more than 80% of its total capacity. This delay is the trace of a slow flow deep down, under a rigid part of the crust, 30 ± 3 km thick, where the mantle rock flows slowly.

Satellite confirmation and geological model

Satellite measurements, carried out much later, confirmed this model. The ground still rises and falls when the lake rises and falls. An analysis using satellite radar, published in 2007, revisited the lake's filling history since 1935 to model the deformations around the shores.

Geological consistency and sediment buildup

The two-layer model proposed in 2000, a rigid crust placed on a flowing mantle, accurately reproduces the amplitude and temporal evolution of the data. Between 1935 and 1949, approximately 1.76 km³ of sediments were deposited at the bottom, adding weight to the lake's load. The 35.5 km³ figure is the total capacity, not the measured content at that date.

Later surveys and average subsidence

A geodetic survey from 1963-64 conducted by the Bureau of Reclamation indicates an average subsidence of 118 mm since 1935, consistent with the approximately 120 mm cited for the structure. The maximum of 218 mm, however, is recorded at the center of the lake basin, and it is this figure that is rounded to 22 cm.