Deep canyons and gorges in the wilderness are places that entail some sort of atavistic attractiveness for any trekker or explorer, but they can be hazardous because of flash floods.
A canyon, wadi or deep cut in the ground exist for physical reasons: It means that water runs periodically there, even if it appears dry, that gravity forces the liquid to move thorough that area, and that water, with all its might, finds it difficult to cut thorough the soil around the place and so it is funnelled thorough a line that concentrates its energy.
All this means that even a dry canyon can ill itself with water up to the brim, quite suddenly and with extraordinary force. That is a flash flood. Funnelling is a very important concept here: Water that falls in a wide area, or ice or snow melting tends to move following the existing terrain and goes down, cutting.
If the soil is soft, then the cut tends to be wide. If it is not, canyons are formed. So, if in essence, if it stars raining even tens of kilometres away from you, if the terrain funnels the water in your direction, all the little drops that fall so romantically elsewhere add up like piranhas to form a collective monster and the place in which you are standing fills up like a swimming pool, sometimes in seconds and with no warning.
Canyoning, caveing and mountain climbing share some common grounds and techniques, and so it is pretty usual to climb down a cave or canyon to enter and explore it a little. Once you are down in a place that required you to climb down you have to face the facts: Getting out will likely be more laborious than just walking uphill. So, if you go down into a canyon, respect these two rules:
Always keep your eye on a very proximate way to get out and up the canyon, even if it is just a few metres high - preferably, of course, all the way out and to the top, but for survival reasons, anything that puts you out of the way of the water will do, at least for a while.
Never underestimate the weather: Meteorology changes in minutes in some regions, especially in mountain ranges. You need to be absolutely certain that the weather will remain good while you enter the canyon, and remember, not just within your visual range but beyond.At the slightest sign of trouble, get out and up as soon as possible.
Flash floods come in seconds; sometimes the only warning sign is a rumble moments before impact, and generally are difficult to survive. The rule of thumb must be "If in doubt, pull back to your camp." You can, after all, return to visit the place the next day, but only if you are alive.