That's a fantastic illustration of how Nicepix
Thank for putting them up.
On the weir Matt refers to it's more simple than all that effort. The weir has a drop of about 2 M, less in the middle as it's a slow V shape. As the water rises on a flood, and 2 M is achieved quite frequently on God's own river the Ribble
because it's a Spate river. Further aided by the fact that below it is tidal and subjected to small tidal rises. More importantly though than the small tidal rise, is tidal holdup of escaping river water.
These factors facilitate a flattening out of the river as it goes over the weir. That's to say both sides of the weir become of equal depths of water.
When it's in this state and you were seeing it for the first time, the only indication you'd get that there was a weir there is the hump of turbulent water rising up. The greatest turbulence appears in the middle of the river.
Again in this state is where the Fluid Dynamics come into play. The Law of FD states there are 3 points in an open top channel where the flow will be slowest and less turbid. The bed and both sides. The reason for this is frictional drag. What the returning silvers do during the spring is cross the weir barrier by sticking as close to the the banks where the flow is slowest. It is here the effort needed to cross the barrier is where they need only to expend moderate energy to pass. Easy peasy really!
The other question that is likely to be asked from this is, "How do they know when the time is right to start the upward migration?" Well it your turn now to answer this, as I'm not doing all the work for you
