Going deep: Arroyo dredging design contract awarded

A $1.6 million contract to design the dredging of roughly 6.3 miles of the Arroyo Colorado, from U.S. Business 77 to Cemetery Road within Harlingen, has been awarded to a Colorado-based company by the U.S. Section of the International Boundary and Water Commission.

The two-year design phase will precede the actual dredging project itself, deemed necessary to increase the Arroyo’s flood-conveyance capacity, which has gradually diminished over the years due to sediment build-up, according to USIBWC. Stantec Inc. of Denver will carry out the design work.

The Arroyo crested at 30.44 feet on March 28, crushing the previous record of 24.22 feet, after historic rains deluged the Rio Grande Valley and especially Harlingen on March 26 to 28. IBWC Commissioner Maria-Elena Giner said she was there to see it.

“During my recent visit to some of the hardest hit areas from last month’s historic rainstorm, I observed first-hand the Arroyo Colorado’s reduced capacity,” Giner said.

The Arroyo Colorado is seen following heavy rainfall and widespread flooding by Hugh Ramsey Park in Harlingen on Friday, March 28, 2025. (Mark Reagan | Valley Morning Star)

“I promised government officials as well as irrigation managers that we would do everything possible to help improve the Lower Rio Grande Valley’s flood control system. This award is a big step in that direction. Removing accumulated sediment from the Arroyo Colorado will reestablish its design capacity and help communities get rid of excess water before it can cause damage.”

The IBWC’s (Lower Rio Grande Valley Flood Control Project) worked as it was meant to during the recent rains despite the loss of capacity due to sediment, according to the agency. During heavy rainfall, the system is designed to divert up to 105,000 cubic feet per second (cfs) into the Main Floodway upstream of Anzalduas Dam, about five miles south of Mission.

From the Main Floodway, 20% of the flow (21,000 cfs) goes into the Arroyo Colorado and the remaining 80% goes into the North Floodway. Flows from the North Floodway and Arroyo Colorado drain into the Laguna Madre, according to IBWC. The Flood Control Project begins in Penitas and extends downstream along the Rio Grande to Brownsville, according to IBWC.

The Project covers 180 miles of the river from Penitas to the Gulf, providing significant protection against Rio Grande floods for the lands and population on both sides of the river, according to IBWC.

Anzalduas Dam, one of two diversion dams on the Rio Grande, is used to divert the U.S. share of floodwaters into the U.S. interior floodway, divert water into Mexico’s main irrigation canal, and release water downstream for users in both countries, IBWC said. The other diversion dam, Retamal Dam, 16 miles southeast of McAllen, is used to divert Mexico’s share of floodwaters into that country’s interior floodway and regulate flood flows downstream.

The Arroyo Colorado is pictured Thursday, Jan. 13, 2022, in Harlingen. (Denise Cathey/The Brownsville Herald)

IBWC spokesman Frank Fisher said the agency has specified a “nominal three feet” to be dredged from the Arroyo Colorado in the Scope of Work for the design phase.

“This was determined as required to convey the design flow of 21,000 cfs from the hydraulic modeling performed in house,” he said. “The depth at different locations will be based on the existing cross section, and subsurface condition without impacting the existing banks.”

Any estimate on how much material will be removed along the 6.3-mile stretch will depend on “final depths of the dredging design,” Fisher said, adding that the contractor eventually selected for the construction phase will estimate the volume to be dredged. Also, the project does not involve widening the Arroyo, only deepening, he said.

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