River sandbars to be lowered north of Los Lunas bridge; public comment sought

A draft environmental assessment for a proposed maintenance project in the Rio Grande near Los Lunas is in the public comment period. Community members are urged to provide feedback by Friday, Dec. 20.

The project area is about two miles north of the Los Lunas Main Street river bridge, between river miles 162 and 164. The purpose of the project is to improve water conveyance through that section of the river — called the Los Lunas Subreach — by lowering five vegetated sandbars in the river, which will widen the river on both the east and west banks, according to a letter sent to residents by the U.S. Department of the Interior’s Bureau of Reclamation on Nov. 22.

According to the draft assessment, the project area is about 1.6 linear miles of river and 73 acres owned by the Middle Rio Grande Conservancy District.

This particular subreach has what is called a “semi-perched channel,” where the riverbanks are higher than the adjacent floodplain and base of the spoil levees, the assessment reads. The spoil levees are non-engineered river levees built for flood protection and are used to dispose of spoil from river and canal excavations.

Once river flows overbank the main channel banks, water tends to move laterally down to the levee base, with some continuing downstream parallel to the levees. With limited locations for the overbanked water to return to the main river channel, during high flows the base of the levees become saturated — sometimes requiring emergency maintenance to prevent failure — and water standing in the floodplain. In 2019, repairs of spoil levees cost the MRGCD more than $1.2 million, according to the bureau.

The EA notes these conditions can also lead to the endangered Rio Grande Silvery Minnow becoming stranded in those areas. In its analysis of possible environmental consequences, the bureau believes the project would result in indirect, long-term beneficial impacts to the minnow and its habitat by restoring the bankline and creating habitat.

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Field measurements of water surface elevations indicate in recent years the area is overbanking even with progressively lower flowrates in the river. This problem is attributed to the river channel narrowing due to encroaching vegetation and sediment buildup along the riverbank and sandbars.

The EA also reports there has been minor channel bed aggradation — the depositing of material by a river — between the Isleta Diversion Dam and the bridge, which may also play a role. According to data in the assessment, from 1992 to 2012, the river channel has narrowed by 140 feet and had 2/10 of a foot of channel aggradation, which is “well within the margin of error over that period of time and length of river.”

The primary goal of the project is to reduce the water surface elevation by increasing channel capacity to reduce overbanking and keep it within the historic floodplain, which is the floodplain outside the 550-foot-wide channel established by jetty jacks installed in the 1960s and 1970s.

The assessment states in order to keep this project within one construction season, the excavation will be limited to a maximum of about 120,000 cubic yards. However, with more work needed to improve conveyance capacity overall, the project will be a pilot for a future width maintenance program.

The draft environmental assessment can be found on the Bureau of Reclamation’s website, usbr.gov. Go to News & Multimedia in the main menu bar. Scroll down and click “More News Releases,” then scroll down to Nov. 25 and click on “Reclamation seeks public comment on proposed project at Los Lunas River Mile 163.” The public comment period ends on Friday, Dec. 20.

For more information on the project, contact the project manager, Jancoba Dorley, at 505-462-3584 or jdorley@usbr.gov.