There were no "big" rivers, ever. More like springs. We have lots of subterranean water, so out of the 18 rivers we have in the city, 16 have their sources here [0]. They were used to power mills in the 19-20th century during the industrialization. Many of the rivers that used to go through the city center flow underground.

I live close to the river Olechówka [1], which flows into a regulated reservoir that used to feed a mill - so the area is called Młynek, "Little Mill" :)

[0] https://podwodnalodz.blogspot.com/2013/09/o-wodzie-po-ktorej... [1] https://i.imgur.com/SIp8CxN.jpeg