Judging from the comments I’ve been receiving, I guess a little more explanation, in the form of illustrations, is needed. The illustration to the left shows, crudely, sort of what the stream looks like when all the boards are in place in the weir. The depth in front of the weir is really probably only about 45cm or so. I’m not at the site right now, so I can’t measure it.
When someone wants to go across the stream and it’s looking a bit hazardous, or just annoying, they pull up a plank or two in the weir and lower the water level a little. The water continues to flow, of course, just a little shallower. The flow rate at the crossing is fairly slow as it’s comparatively wide and perhaps deep (I haven’t waded across it because you can walk across the weir). I’m surprised they just haven’t filled it in with something, but I presume rocks and such would make it a bumpy ride on a motorbike. I plan to keep the water at the weir at a fixed height.
This is roughly what I’m contemplating, but it is completely out of proportion and, hopefully, much easier to do in reality. Everything is really much more subtle. I’ll have to confirm, of course, that I can raise the level of the water behind this ford as much as I’d like, but I’m pretty sure I can.