After reading a previous post that asked whether it is possible to encounter a hydraulic control without a pool, it sparked the opposite question: Can you have a pool and no hydraulic control?
We have encountered situations where a pool is located downstream of the crossing, but there is no obvious control (boulders or cobble barrier, or similar), only a gentle upward slope of either sand or gravel that represents the end of the pool. In this instance, is it appropriate to indicate no hydraulic control is present? Or is the slope at the end of the pool considered a control?
Photo depicts a downstream pool in a wetland area, with the end of the pool where the main flow path appears to narrow (red line shown).
RPC,
In purely 'hydraulic speak', with a focus on the protocol and the reasons behind trying to capture pool information, it technically isn't possible to have a pool without something controlling the water surface profile.
That said, it is often hard to distinguish between pools and just naturally varying sections of streambed depth - and pools themselves are part of the natural variation in stream channel characteristics (pool -> riffle -> run, repeat). The characteristics of pools, and the elements of the channel that stand out and make it easier or harder to observe a pool, can change at different flows. Think back to our training site - it has a pool on the downstream side with a hydraulic control consisting of boulders and cobble. But, at lower flows, the pool water surface is lower, and the hydraulic control actually moves upstream, back towards the crossing, and the water surface profile becomes more controlled by a gravel/sand deposit that is in front of the line of boulders and cobble and normally hidden from view.
So, back to the question, in this photo I can't tell if I would consider that a pool or not, or just a deeper section of the stream without a hydraulic control (a run section that happens to be deep). If the gradual slope of sand and gravel is very gradual, it might not make sense to indicate a pool. If it is more drastic, and at low flows you can picture it maintain the water surface profile (one indication being there would be a riffle downstream of the pool) then it might be appropriate to record a pool and the hydraulic control.
Thanks,
Tom