A team of hydrologists at the University of Chicago published findings Thursday in the journal Nature Fluid Dynamics confirming that the Chicago River, already famous for being one of the only rivers in the world whose flow was permanently reversed by engineers in 1900, has begun flowing in a third direction that researchers are calling “lateral-diagonal-up.”

“We don’t have a word for it,” said Dr. Miriam Vasquez, the study’s lead author, during a press conference at the university’s Hinds Geophysical Laboratory. “It’s not upstream. It’s not downstream. It’s not even sideways, exactly. It’s as if the river has invented a new spatial axis and decided to go that way instead.”

The phenomenon was first observed in November 2025 by a graduate student conducting routine flow measurements near the Riverwalk. Initial readings were dismissed as equipment error, then as “some kind of weird vortex thing,” and finally, after four months of repeated measurements using increasingly expensive equipment, as “genuinely unexplainable by any existing model of fluid dynamics.”

The river’s original reversal in 1900 was an engineering marvel—a massive infrastructure project that redirected the flow away from Lake Michigan to prevent contaminated water from reaching the city’s drinking supply. Hydrologists have long studied the river as a case study in human intervention in natural systems. This third directional flow, however, appears to have no human cause.

“We’ve ruled out construction, seismic activity, climate patterns, and a surprisingly persistent theory involving magnets,” Dr. Vasquez said. “The water is simply choosing to go in a direction we didn’t know existed. It’s humbling, honestly.”

Measurements indicate that the third flow is intermittent, appearing most strongly between 2 and 4 AM on weeknights, which Dr. Vasquez noted is “the exact window when no one is looking at the river, which feels deliberate.” The flow has been detected at multiple points along the Main Branch and South Branch, though it appears to be strongest near the confluence at Wolf Point.

The city’s Department of Water Management issued a statement saying it is “aware of the research” and is “monitoring the situation,” while emphasizing that the municipal water supply remains unaffected. “The river can flow in as many directions as it wants,” the statement read. “Our filtration systems are direction-agnostic.”

Other scientists have responded to the findings with a mixture of fascination and skepticism. Dr. Harold Pratt, a fluid dynamics professor at MIT, called the paper “either the most important hydrological discovery in a century or the most elaborate prank in academic publishing history.” He added that he plans to visit Chicago in the spring to conduct independent measurements, “assuming the river hasn’t figured out how to flow through time by then.”

The research team says further study is needed, and has applied for a National Science Foundation grant to install permanent monitoring equipment along the Riverwalk. In the meantime, Dr. Vasquez urged Chicagoans not to be alarmed. “The river isn’t dangerous,” she said. “It’s just… creative.”