USIBWC Spill Events and Transboundary Flow Events from Mexico into the San Diego Region

The U.S. Section of the International Boundary and Water Commission (USIBWC) owns five canyon collectors that are intended to capture dry weather transboundary flows for treatment at the South Bay International Wastewater Treatment Plant (SBIWTP) located in the U.S. near the U.S.-Mexico border. The canyon collectors are referred to as Stewart’s Drain, Canyon del Sol, Silva Drain, Smuggler’s Gulch, and Goat Canyon. USIBWC contracts with Veolia North America to operate and maintain the SBIWTP, the canyon collectors, and other associated infrastructure (collectively referred to as the Facility).



According to the 1944 Water Treaty for the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande and stipulations established in International Boundary and Water Commission (IBWC) Minute No. 283, Mexico has the primary responsibility for preventing the discharge of wastewater to receiving waters in the Tijuana River Valley. However, the U.S. Federal Government also has a role. This includes assisting with equipment, maintenance, and resources in the containment of wastewater discharges through utilization of the canyon collectors, which are intended to act as a failsafe to collect and divert untreated sewage and other dry weather transboundary flows to the SBIWTP for treatment. Uncaptured flows can enter waters of the U.S. and/or waters of the State of California, potentially polluting waters within the Tijuana River Valley and south San Diego coastal waters.

USIBWC Facility Spill Events, Tijuana River Transboundary Flow Events, and dry weather Canyon Collector Transboundary Flow Events are defined as follows:

Spill Event
A Spill Event is a discharge, or any other type of emission or release, of waste from any portion of USIBWC’s Facility due to system overflow, flow stoppage, system leaks and breaks, operational failure and/or infrastructure failure.

Dry Weather Canyon Collector Transboundary Flow Event
A dry weather Canyon Collector Transboundary Flow Event is any flow across the U.S.-Mexico international border under dry weather conditions to any one of the five canyons equipped with a canyon collector system, including Stewart’s Drain, Canyon del Sol, Silva Drain, Smuggler’s Gulch, and Goat Canyon, that is not captured by the canyon collector system for treatment at the SBIWTP. Dry weather is any period that is not categorized as wet weather. Wet weather is defined as the period of time when a storm event produces 0.1 inch or greater within a 24-hour period plus the following 72 hours. A dry weather Canyon Collector Transboundary Flow Event also constitutes a Spill Event when a transboundary flow less than or equal to the canyon collector’s maximum design capacity is not captured by the canyon collector for treatment at the SBIWTP.

Tijuana River Transboundary Flow Event
A Tijuana River Transboundary Flow Event is any flow across the U.S.-Mexico international border, under dry or wet weather conditions, in the Tijuana River. USIBWC flow gauge data for the Tijuana River is available on the USIBWC Water Data Portal..

USIBWC reports Spill Events, dry weather Canyon Collector Transboundary Flow Events, and Tijuana River Transboundary Flow Events pursuant to Order No. R9-2021-0001 as amended, National Pollutant Discharge Elimination System No. CA0108928, Waste Discharge Requirements for the United States Section of the International Boundary and Water Commission South Bay International Wastewater Treatment Plant Discharge to the Pacific Ocean Through the South Bay Ocean Outfall (NPDES Permit).

USIBWC submits preliminary Spill Event and dry weather Canyon Collector Transboundary Flow Event reports within three days of the event and submits monthly reports with information on Tijuana River Transboundary Flow Events. The following table summarizes the information that USIBWC has reported to the San Diego Water Board since July 1, 2024. The table is updated, if needed, approximately once a week.

Information on previous Spill Events, dry weather Canyon Collector Transboundary Flow Events, and Tijuana River Transboundary Flow Events is available in the San Diego Water Board’s November 2021-September 2024 Executive Officer’s Reports.