When they built Hoover Dam they made a big deal about the intakes for the power plant being at the bottom of the lake, so that the sediment would be passed through and the lake would not fill up with sediment. Over time with the lake level constant there would probably be a sediment delta most of the way to the dam, with an underwater angle of repose slope down to the level of the intakes at the dam. As a wild guess I would think that the slope would be 10 miles in length to drop 500 feet. (My wild guess was way off, see below for better numbers.) The sediment gets transported pretty far by underwater turbidity currents, and I've heard stories of muddy water boiling up at the face of Hoover Dam many hours after a flash flood dumps a load of mud and sand directly into the reservoir. I don't know of any massive reservoirs that have silted in, but the flood control reservoirs below the San Gabriel Mountains have this issue on a smaller scale.
(A little more Internet research later...)
The 1986 Lake Powell Survey (sedimentation rates)
https://www.usbr.gov/tsc/techreferences/reservoir/1986 Lake Powell Survey.pdf has a chart showing the slope of the sediment delta on page 30. In 1986 the slope extended from about Dark Canyon (mile 163, about 3,680 elevation) to mile 140 and about 3,430 elevation. That's a drop of about 250 feet in 23 miles, or about 10 feet per mile as an easy approximation. If we figure the elevation of minimum power pool at 3,490, that's 210 feet below full pool, so the final sediment delta at full pool with power generation draining the silt from the face of the dam would be 21 miles from the dam, which I think is between Warm Creek and Gunsight Bays. That still leaves about 100 miles of main channel from where the sediment delta is now.