Before: Willow copse on rubble island, January 2010.
There are seven islands in McKee marsh which were created with rubble from the Woolly Mammoth dig that took place here in the 1970's.

This new activity (I've not seen beaver activity in DuPage County since 2002, alongside the West branch of the DuPage River in Winfield Illinois. That individual cut down a half-dozen trees, left and never returned (as of 2010). That beaver or beavers (or, at least, their descendants)  may be responsible for this cutting as well - the small creek that drains this marsh runs into the same river, about 1/4 mile to the west. This is about 6 miles downstream from Winfield.

 I believe these are transient populations, as the marsh this summer dwindled in fall to barely 3-4 feet of water in a very limited area. I have never seen any damming nor lodge building - just these trees cut down - and left lying in place (below).

Same island after: Christmas Eve, 2010.
 Almost all the willows are cut down. Note - the beavers won't touch the invasive Buckthorn also growing there.

In Chicago, several beavers have returned and made a home near the Lincoln Park's North Pond. The "Lincoln Park Beaver" has not been as well received by the Chicago Park District and the Lincoln Park Conservancy, which was concerned over damage to trees in the area. In March 2009, they hired an exterminator to remove a beaver family using live traps, and accidentally killed the mother when she got caught in a snare and drowned. Relocation costs $4,000-$4,500 per animal. Scott Garrow, District Wildlife Biologist with the Illinois Department of Natural Resources, opined that relocating the beavers may be "a waste of time", as there are records of beaver recolonizing North Pond in Lincoln Park in 1994, 2003, 2004, 2008 and 2009. As of fall 2009 a new beaver lodge has appeared on North Pond's northwest bank.

There's a reason for the phrase "Busy as a beaver."