One thing I noted from the coverage this morning was that the canal has a fairly good current.
I also listened to the audio between Greta and Cory, and if there's four wrecked cars down there, with that current, it's very possible that anything dumped in there could drift downward and get caught in the wreckage on the bottom. I think that's why they want to pull those four cars out - to see what's tangled up with the cars.
The area looks like it a commercial area, and on a Sunday night there wouldn't be anyone around to see someone dumping something into that canal.
My theory...........when DP learned that Cassandra had filed a missing person report with the state police, he knew he had to hurry up and dispose of the body. He figured the odds of anyone searching and finding her in the canal were on his side. Cory told Greta that canal was 45 miles long. He figured no one would search that canal, especially since there were other bodies of water in and around Bolingbrook to search. And even if they did search that canal, he figured it would be a needle in the haystack proposition for LE.
But, I think LE has something very specific that makes them believe that this is where DP dumped her body. Going back to my original theory.............I've felt that the reason for DP's disguise with the bandana, sunglasses, and hat, was so he wouldn't be recognized by someone who might have seen him that night. I think he WAS seen, and DP knows he was seen, and was initially worried that when Stacy's disappearance made the local news, that someone would say "hey- that's the guy I saw in the wee hours of the morning out by the canal", and call LE.