Dredging the San Jac is non-starter. It's got one primary channel that wanders depending on rainfall run-off. It silts in every time there's a large rain event due to all the tributaries that run off into it dragging debris, sand and mud. Would cost millions to dredge it and it would fill back in pretty quickly. And as Emp said, where you going to put all that dredge offal? Some of that stuff is presumably toxic.
ALL waterways should be properly maintained including dredging if it is not already cemented in. An amount to dredge the San Jac has already been published. It's not as much as you think. (I'll try to find that post) The San Jac has been a source of sand mining in this area for decades. Someone will make money off the sand.