I have one of the many dry waterways on my place, lots of run off after 6"+ rain of course, but dry looking most all the time; thing is there is water moving under the gravel on most of these ditches and streams all the time. There is some substrate that holds water from going deeper into the earth and it moves the gravel ever so slightly all the time, I know a branch that flows at both ends and has a half mile of dry gravel between. Creeks I'm familiar with seem to all be underlayed with stone with the gravel over it; often the gravel is almost floating on water that moves through it. In effect the gravel flows. Digging/dredging accelerates the flow, as an example, a neighbor excavated a 1/4 mile of the dry ditch and straightened it some what many years ago; over the next several months my portion of the ditch became noticeably deeper even with no rains that caused runoff. I saw a mining operation set in one spot for about 40 years that never ran short of new gravel. I'm sure it would still be there if the divorce had not ended in selling off the equipment and land. Dams tend to stabilize the stream of gravel above them, I think, by stopping or slowing the flow of gravel as well as by slowing the runoff during floods.
An observation on the erosion of the hills causing the gravel in the creek, My hill gravel is all sharp and is mostly chert of some sort and all most creek gravel is well rounded soft rock. I have never found a sea shell fossil in any of the hill rock, but, see many "scallop" fossils in the ditch rocks. All the dry ditches hereabouts are several feet deeper than they were even 30 years ago. The one at my house is no wider than it was 50 years ago but is at least 4' deeper than it was then.
These bottoms are also loaded with 'sharp gravel' to the extent that some fields would look like solid gravel after being plowed, back in the days when they still farmed it.