The Itchen weir at Woodmill is the head of the tidal section of the river, and is located a couple of hundred yards upstream from the Boathard. It's an ideal place to practice those flashy paddling moves!
This Guide is all very subjective, and completely depends on river flow / tide height / sluice gate. If you have any doubt, ask someone with more experience, or come back when it has dropped a bit.
As you look at the Weir from downstream, the water level will be below both concrete platforms, and the hanging bars aren't being pushed by the water. This is the best level for novices.
As you look at the Weir from downstream, the water may be just lapping over the concrete platform on river left. Slightly higher and the hanging bars will just start to be pushed by the river flow. This can be a great level for flashy moves, such as the paddle twirl. The water is also likely to look slightly brown.
If it's been raining hard, you will see the water almost up to (or over, as pictured) the river right concrete platform. The weir is potentially dangerous at this level. The hanging bars will be being pushed pretty hard by the water. The water will be brown and smelly.
Due to a troll living upstream playing with a magic sluice gate, you may find levels can rise and fall dramatically in minutes. The weir can get dodgy so it is always worth bringing throw lines, two rescues have occurred where swimmers were retrieved with throw lines in higher water and a swimmer was caught holding on to the corrugated iron retaining wall throw lines would have made the situation easier.
The weir at a VERY high level:
What UKRGB has this to say about the mighty weir