The Tijuana River Valley

Between the border and the beach is a protected wetland, a regional park, and neighborhoods like Nestor and San Ysidro. When the river crosses the border, this is where it goes first.

A wetland worth protecting

The Tijuana Estuary, where the river meets the ocean at the south end of Imperial Beach, is the largest coastal wetland in Southern California, according to California State Parks.¹ More than 90% of the region's wetlands have been lost to development, which makes this one especially important.¹

It's part of the Tijuana River National Estuarine Research Reserve, one of 30 reserves in a national system managed with NOAA,² and it's recognized under the Ramsar Convention as a wetland of international importance.⁴ More than 370 bird species have been recorded here, including endangered species like the light-footed Ridgway's rail and the California least tern.¹³ The Tijuana Slough National Wildlife Refuge was established here in 1980 largely to protect that rail.³

About three-quarters of the river's watershed is in Mexico,² so what happens upstream in Tijuana decides what reaches the estuary.

What the sewage does here

In storms, and whenever pumps or pipes fail in Tijuana, sewage-laden water flows through the valley and into the estuary. The reserve names wastewater from Mexico, trash, and sediment among its most critical management problems.⁵ Sediment washing down from Tijuana's eroding canyons fills in channels and marsh, and trash collects in the vegetation and along the banks.

The Saturn Boulevard hotspot

The valley's worst air comes from one place. Where the river passes through old culverts at Saturn Boulevard, the water churns and releases hydrogen sulfide, the gas that smells like rotten eggs. Readings nearby have reached 4,500 parts per billion, against a California standard of 30.¹⁴⁸

After the Parallel Gravity Line in Tijuana broke in late May 2026, county air monitors recorded hydrogen sulfide above EPA's four-hour Acute Exposure Guideline Level of 360 parts per billion from about 10 p.m. to 4 a.m. on June 2, with a one-hour average of 915 parts per billion.⁶ A $2.5 million interim pipe extension, designed to carry low flows under the road and release them below the water surface so they stop churning, is underway, with construction expected in 2027.⁷¹³ In September 2026 the county approved $4 million to design and permit the permanent fix, estimated at about $25 million. Construction of the permanent fix is not yet funded.¹¹¹²

What catches the trash and sediment

• Sediment basins in Goat Canyon and Smuggler's Gulch slow canyon flows so heavy debris settles out. They're dredged periodically. A state-funded upgrade at Smuggler's Gulch, now in design and environmental review, will add sediment basins, larger culverts and trash capture devices.¹⁵¹⁶
• The trash boom across the main river channel, just north of the border, has stopped about 2,000 tons of trash and debris over two storm seasons. It only catches floating debris; sewage, dissolved contaminants and sediment pass through.⁹¹⁰

See Technology trials for how the boom has held up.

Visiting the valley

The Tijuana Estuary Visitor Center, the regional park trails, and Border Field State Park are some of the best places in the South Bay to walk and watch birds. If you go:

• Stay out of the river channel, standing water, and the estuary's water.
• Check hydrogen sulfide levels before you go, and skip days after storms or spills. The county air district will send email or text alerts when levels pass 30 parts per billion.⁶
• Check with County Parks and State Parks for current trail and area closures.

Is it safe today? has the current status and warning signs.