Rivrguy,

In the stream channel, as the water gets deeper due to increased flow, the velocity gets faster. Faster water has higher energy, and the higher the energy the heavier the substrate that can be moved. This is a major reason why sections of rivers that are lower downstream, and more likely to be riprapped and channelized, have less effective spawning areas. The river cannot spread out as flows come up, so velocity increases and scours out redds. Where rivers can spread out over wide flood plains, spawning success is higher. Some eggs are lost to sedimentation there, but the whole redd doesn't get blown away.

Hydrology is fascinating. I wish I had studied it more.

Sg