The Rio Grande constitutes the major feature through Valencia County

At 1,896 miles long, it is the fourth longest river in the contiguous United States. It originates in Colorado, flows through New Mexico and along the Texas-Mexico international border to the Gulf. The Rio Grande itself, with relatively low average flow, did not carve the valley through which it runs.

Will Rodgers is quoted, “The Rio Grande is the only river I ever saw that needed irrigation.”

The river was not always such a long river through geologic time. The river developed gradually in a wide zone formed by tectonics. The Rio Grande Rift zone opened up as the continent broke apart over the full length of New Mexico and into Texas, starting about 25 million years ago.

The bottom of the rift sunk most between 16 and 9 million years ago, tectonically creating the wide rift zone. As the rift bottom was sinking, the edges of the rift started to erode. The rivers carried sediments to the rift center, piling up to 21,000 feet of sediments.

Until six million years ago, the ancestral Rio Grande flowed south through Valencia County into a lake between Belen and Socorro. That “Lake Socorro” was a shallow lake that occasionally dried up. Rivers and streams had difficulty filling the Albuquerque-Valencia County area with sediment because the bottom of the rift was sinking more rapidly than the rivers could fill the basin with sediments.

Therefore, the Rio Grande could not escape farther south for several millions of years. At around six million years ago, the sinking of the rift decreased, sediments built up and the river broke free, reaching Truth or Consequences around 5 million years ago. The Rio Grande then reached progressively farther south, reaching El Paso about two million years ago when it formed a wide lake. The river finally reached the gulf about one million years ago.

The Rio Grande was a wild river before dams were built upstream from Valencia County, and historically meandered constantly. In a meandering river, finer grain sediments are deposited in the inner bend, where the water flows more slowly. In the outer bend of the river, the water moves around a curve with a higher velocity.

There, the river erodes the banks and any sediments deposited are coarser. An excellent example of geologic deposits from a meandering river was illustrated by the geologist Harold Fisk, who in the 1940s prepared a map of meanders and deposits formed by the Mississippi river over thousands of years of river meandering.

Such flow pattern also applied to the natural Rio Grande flow. In the early Spanish days, the Rio Grande was flowing at the western base of Tomé Hill. In 1899, the Rio Grande changed its main channel near Isleta and cut off the Acequia de Belen’s intake.

However, in the 1920s and 1930s, the Middle Rio Grande Conservancy District constructed levees on both sides of the Rio Grande, along with 400 miles of riverside drainage ditches. These features constrained the area of lateral movement of the Rio Grande.

This resulted in a more reliable location of the riverbed with much narrower meanders but this also created a stable, wide area of water-saturated soil that allowed for the growth of the bosque that beautifies the swath of land on both sides of the river in Valencia County.