Generally it's what you can't see is the problem, as Adrian said local knowledge is the key. We paddled in Oxford where there are two weirs close to each other although they look identical on the surface one is a killer and one isn't.

There is a feature at Llangollen that has characteristics of a small weir. While I was there and to see what would happen I poked the nose of my canoe into the flow; from then onwards it sucked me in and I couldn't escape, had this been a "nasty weir" I would have been doomed!

[video=youtube;xc36yLeBWMQ][/video]

Yeah, weirs are a law unto themselves and take years of experience to be able to judge them, and even then, they still catch people out. Local knowledge is really useful.

As a beginner, I'd stay clear unless with experienced paddlers who know the weir.

The upstream face of anything with fixed ironmongery is always dangerous.

The downstream side needs local knowledge.

Mal Grey wrote: As a beginner, I'd stay clear unless with experienced paddlers who know the weir.

++1

Note that some major rivers have canoe trails on where canoe passes or chutes have been built on/around some of the weirs - always worth doing an online search for rivers you paddle to see if there is already a canoe trail or guide which will highlight any canoe passes, or portage points. The vast majority of smaller rivers don't have anything like this and you will need to gain local information.

Even after looking at a weir at a lot of different water levels to assess the power and escapability of the hydraulic and check for bits of rebar sticking out where concrete has crumbled over the years, or corners of sheet piling etc, it is still possible to miss clues, like anti-scour sills under water which create an inescapable hydraulic completely invisible from the surface, or that the face is undercut for some 20 feet back forming an underwater cave you could end up in.