Originally posted by dlh But what about the Venturi Effect???
Pressure is not a concern in stream dynamics. As I said, I don't want to type the entirety of several hours of lecturing. After all, there's also the gouging of the channel caused by increased velocity, and sedimentation upstream because of impoundment, and if a levee is breached, the levee then acts as a dam to prevent the water from getting back into main channel, and enrichment of "river bottom" land by sediment is reduced or eliminated on the "dry side" of levees (except when a levee is breached), and when discharge declines more sediment is dumped within the area bounded by the levees, including in the channel, which raises the entire level of the river in that zone, which increases the risk and potential severity of flooding in upstream, downstream, and levee proximal areas, and - - well - - it is not nice to fool with Mother Nature.
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Another quip I would tell students: building levees is like trying to put a stream into a pipe, except it's always only a half-pipe, open at the top down its entire length.