Yes Wit there is silt in the backwaters of dams. There is also gravel and even larger forms of debris that one normaly associates with river valleys.
Undoubtdly the fines from these deposits will migrate at a faster rate than the largest and a great deal will depend on the frequency and size of the natural occuring floods that will cause this migration of gravel that has been stopped in the rivers that have dams.
Part of the cost in removal estimates deals with control of or planning for dealing with the possibility of containment of excessive siltation
Maybe you should look at it akin to a surgical procedeure when thy cut into you they cut up a bit of good stuff and get rid of the bad and leave you in pretty poor condition for awhile. But the end result is (usually) the patient is saved to resume a healthy life once again.
In the case of the Elwha it is so gravel poor that the only place you will find spawning fish is in little braids that recently opened and the gravel that is available has not been carried away the main river bed established for years is virtually void of gravel and needs some recruitment from behind those walls.