CHICAGO (CBS) — If you took a rainy stroll along the Chicago Riverwalk this weekend, your eyes weren’t playing tricks on you: the Chicago River took on two vastly distinct colors — one of them seemingly unnatural.

At Wolf Point, the normally dark, muddy color of the north and south branches of the river was starkly met by a vibrant teal more reminiscent of the green hue pumped in for the city’s annual St. Patrick’s Day river dyeing, or the waters of Canada’s Lake Louise.

The strange sight started to become apparent on Friday and was lingering into Monday; people in downtown high-rises saw it more clearly than at street level.

At Wolf Point, the normally dark, muddy color of the north and south branches of the Chicago River was met by a vibrant teal on the first weekend of November 2017. (Credit: CBS)

Officials with the city’s Department of Water Management and the Metropolitan Water Reclamation District of Greater Chicago have been investigating.

“The DWM and the MWRD went out to investigate this area this morning,” water department spokeswoman Megan Vidis told the Sun-Times on Sunday. “The discoloration is not due to outfill, but caused by a combination of the storm, seasonal vegetation changes and other naturally occurring circumstances.

“We will continue running tests on the water in the impacted areas to monitor the situation.”

The head of one environmental group was skeptical that the flow of sewage into the river wasn’t at least partly to blame. But she said that she’d defer to City Hall’s explanation for now.

“We’ll have to see what the tests reveal,” said Margaret Frisbie, director of Friends of the Chicago River.

According to Frisbie, heavy rainfall such as the kind that fell on Saturday morning can overwhelm sewers, with the overflow escaping through “outfalls” — giant tubes that carry the excess stormwater and sewage into the river.

“Sadly, when it rains we get a lot of sewer drainage in the river,” Frisbie said. “It’s not good, it’s harmful to the fish and to people’s health.”

An MWRD spokesperson said “HAZMAT responders did not find any indication of illegal discharges or contaminants….”

Their explanation for the two tones?

Water from the north and south branches of the river are decidedly dark, filled with sediment churned up from muddy banks in recent storms.

Whereas Lake Michigan water is greenish blue and stays that way downtown where it’s in a concrete channel, sort of like a tub.

About a half-inch of rain was recorded at O’Hare Airport on Saturday, with more falling Sunday, according to the National Weather Service.

The river’s quirky coloring capped off a troubling week for the city’s iconic waterway. Crews are still trying to locate the source of an oil spill that slicked the Bubbly Creek portion of the river on the South Side on Oct. 26.

Heavy rainfall can cause a lot of trouble for the river. Just last month, the river overflowed onto its scenic walkways after a persistent rainstorm, causing police to tape off several entryways from Franklin Street east to the lake.

(TM and © Copyright 2017 CBS Radio Inc. and its relevant subsidiaries. CBS Radio and EYE Logo TM and Copyright 2017 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed. The Sun-Times Media Wire contributed to this report.)