Rivers are by far the hardest areas to search and the lowest success rate in finding people.
How far downstream they are would depend on the speed of the current.
You need a current in excess of 1.5 knots to move a body once it is on the bottom, and even then they tend to get caught up in stuff on the bottom eventually, usually within the first mile or so. (My personal record for people still on the bottom was 2.5 miles) in navigable waters, much farther in white water.) Bodies sink at a rate of 2 feet per second so knowing the depth and the speed of the current a rough calculation of the location of first contact with the bottom can be made. Checking the current in that area will tell them if the body will possible move or not.
Bodies found 100 miles downstream are found after the putrefaction process creates gases and the body floats and then travels down stream on the surface.
The search area under the bridge would be brutal. Along with all the twisted metal and debris from the collapse, there would be a bunch other junk in the water as well. The bridge was built in the 70s and a lot of bridges built at that time got rid of construction scraps by dropping them in the water. Some divers are earning their paychecks there this week