Starting July 31, 2018, the US Army Corps of Engineers replaced the dam with a series of man made
riffle pools so that the maximum drop is one foot. This allows fish to go upstream and canoes and kayaks to go downstream during normal river flows. The USACE also restored "14 acres of riverbank and five acres of aquatic beds by removing invasive species and planting native vegetation." [
USACE,
ChicagoRiver]
"Friends has advocated for four Chicago River system dams to be removed to reconnect an additional 55 miles and provide enormous environmental benefit. The North Branch Dam is the second of the four to be removed." [ChicagoRiver]
MWRD
posted three photos with the comment:
The confluence of the North Branch of the Chicago River and the North Shore Channel on May 9, 1911, viewed looking north from the Argyle Street bridge showing the dam and Foster Avenue bridge. The dam featured in the historical photo was removed in 2018 as part of a series of restoration efforts by the U.S. Army Corps of Engineers, the Metropolitan Water Reclamation District of Greater Chicago, and the Chicago Park District to allow upstream fish migration and improve navigation and surroundings for boaters.
"To reverse the flow of the North Branch, the water level was lowered four feet where the new North Shore Channel met it. So this dam was built in 1910 as part of that project to compensate for the difference in water elevations."
ReplyDeleteWhat? This doesn't make sense. The north branch didn't need reversal, and the flow was already increased/augmented by the completion of the North Shore Channel. So, then, what was the purpose of the dam?
Wow, I flunked citing a reference for that statement. They dredged the river so that it would be four feet lower here so that the North Shore Channel would have enough slope to achieve the flow they wanted.
DeleteI got some info back from a group involved with the dam removal and they pointed me to an old book (by Libby Hill) to explain the purpose. The North Shore Channel bed was apparently dug 4 feet below that of the North Branch at this location. In that case, you'd totally need a dam to stop erosion and silting of the channel at the junction since the river's velocity would be increase so much from the fall. My error was assuming that the NSC was built at the same level as the river. The book says that the dam was a problem from the beginning, though, as the function of a dam itself meant that the branch above it was now more prone to block sewer overflows/outfalls into the river. And then, of course, this water level difference meant that there was still bank erosion immediately behind the dam they now had to contend with. It then appeared that from the 1930's to 1940's the government worked to mitigate all of this by reengineering the North Branch.
DeleteMakes me kind of curious what the elevation difference is between the bed of the channel at Wilmette, and then when the combined channel/river meets the main stem & south branch of the river?
Anyway, yeah, you'll want to find another quote or come up with your own explanation for the building of the dam, because the current quote is incorrect. It was definitely not built to reverse the flow of the North Branch; in fact, it was quite the opposite since they were trying to slow down the increase flow caused by it being higher than the bottom of the North Shore Channel.
DeleteLink to the Libby Hill book explaining this:
https://books.google.com/books?id=FueLDwAAQBAJ&lpg=PA130&dq=North%20Branch%20Dam%20Chicago%20built%20in%201910%20erosion%20and%20flooding&pg=PR6#v=onepage&q&f=false
"To reverse the flow of the North Branch..."
DeleteAgain, please remove this. This is incorrect. Nothing in the north part of the CAWS was built to reverse the direction of any waterway. In fact, the dam was built for nearly the opposite purpose: to prevent the North Branch was speeding up and eroding its banks, since the North Branch's water surface elevation would be lower than that of the river's.
Deleted. Today it is obvious to me that it was wrong. The North Branch always flowed south. It was the difference between flowing south from this bend to flowing south from the Wilmette harbor.
DeleteNot the whole thing! lol There needs to still be an explanation of why it was built. I've cited the source for the purpose of its construction in one of my above posts. The grade of the NSC bottom was designed at a lower height than that of the natural North Branch, so the dam was built on the North Branch to compensate for what would be a difference in water level, which would otherwise erode the banks and bottom of the North Branch. The dam also helped to mitigate the sediments created that erosion from filling up the channel were the two intersected.
Delete