- explore the lake just upstream of their dams...  particularly in order to figure out what the
  apartment complex's concerns regarding water level might be

- try to locate where their den is...  possibly on one of the islands?  Certainly it's underground
  somewhere now, since the dams were destroyed.

- once one of their dams becomes built up again...  allegedly it's strong enough to walk on.  Use
  this to explore a bit.  (though don't do it too frequently, as obviously it probably bothers the
  beavers)

- locate (either on foot, or on Google Maps) where the water comes from upstream...  is it actually
  a distributary of the Des Plaines river?  It almost appears to originate at the golf course
  there...  which would mean that it's the combined output of the wetlands surrounding the river
  upstream.