Read it through once

Of course it would be illogical to conclude that the constrained river can ever lower its flood slope so as to make levees unnecessary, but it is believed that, by this lateral constraint, the river as a conduit may be so improved in form that even those rare floods which result from the coincident rising of many tributaries will find vent without destroying levees of ordinary height. That the actual capacity of a channel through alluvium depends upon its service during floods has been often shown, but this capacity does not include anomalous, but recurrent, floods.