If you don't know how to access many of the rivers in northern Illinois, it means you've been looking in all the wrong places or haven't been looking at all. I will be adding more to this set of links and organizing them better, as I keep finding more information. Maps are key to river access, so be prepared to print them all out.
First off, you have to have a license:
Illinois Department of Natural Resources
The funny thing about the IDNR site is that for fishing info it's pretty useless. You can get all the basic info from their fishing section, but if you actually want to know where to go fishing, you're better off going to their camping and hunting sections. You get maps. The hunting section is for the explorers out there. You should look through as many hunter fact sheets as you can find for the areas you like to fish. Each fact sheet has an attached map. You can figure it out from there.
IDNR Hunter Fact Sheets
Fox River
For the Fox River and its tributaries, what you want is access. This river is probably the most accessible river in the Chicago region with park districts and forest preserve districts buying up more land along the river every year. All park and forest preserve districts have maps. You can usually get them off their website. But sometimes you have to talk to someone or just be out wandering. Some park districts don't have maps. Here's some that I rely on for all things Fox River.
Fox Valley Park District
Kane County Forest Preserve District
Kendall County Forest Preserve District
Oswegoland Park District
East, West and Main stem, DuPage River
For the DuPage River, it's branches and main stem, you'll be surprised at how much public access there is. You'll hear individuals and groups talk about how spots on the Dupe for fishing shouldn't be given away so freely. I think the heads of the Forest Preserve Districts would be disappointed in that attitude. The FPD's WANT people to come out. That's why they exist. Here's a couple of links to maps of public land in DuPage and Will Counties.
DuPage County Forest Preserve District
Forest Preserve District of Will County
Then there's a link to the Plainfield Park District. Kind of hard to keep a body of water a secret when so much of it is accessible via public land.
Plainfield Park District
From the Bolingbrook Park District you want to get the Bike Path Guide and the Park Map. Since I initially set this up, Bolingbrook changed their site. Good luck finding what you want or getting anything to download. What a piece of crap. Note to webmasters (see Naperville below too) simpler is better. Bells and whistles are junk if they bog thing down and aren't very well organized.
Bolingbrook Park District
The Naperville Park District easily has the worst park info I've seen to date, but it will give you a very general idea of land along the DuPages that is public. Under their park amenities, they don't even list fishing.
Naperville Park District
The Shorewood Park District has a couple of pdf maps to download when you get to their site. Look for their Existing Trail System Map and the DuPage River Trail Map which incorporates all the towns up and down the river.
Village of Shorewood Parks and Recreation
I checked out the DuPage River where it flows through Channahon about 10 years ago and never got back to fish there. Their parks map show a couple of interesting access areas.
Channahon Park District
Stay tuned for more.