What causes flash floods?


Flash floods are one of the most significant dangers in a canyon. A rise in water levels can turn benign features into impassable sections.



The most common cause of flash flooding is precipitation, Fortunately rain is relatively simple to predict using multiple forecasting platforms. Factors such as convective rain rather than frontal rain will increase the likelihood of a flash flood as it is likely going to fall in sporadic heavy showers.


Canyons with high altitude mountainous catchments will be affected by snowmelt. As air temperature increase, snow will begin to melt causing the water level in the canyon to rise. Often peak flow will correspond to peak temperature, for example; if the peak temperature is 20 degrees at 3pm then peak flow will shortly follow. Snow melt is especially dangerous as it can easily be overlooked on warm sunny days.

Glacial melt

Some of the best canyons are glacial fed, the fine rock flour found in the water accelerates the process of erosion. Glacial fed rivers will often have a 'pulse' effect during warmers months, with peak melt water levels rising at mid day and dropping when its cooler at night.

Hydro Dams

The UK is full of hydroelectric stations, although there are many different types of designs, you only need to watch out for dams that are able to release high volumes of water, such as tipper gates that could send a wall of water rushing down the canyon with no prior warning.

Natural Dams

Natural dams are formed by fallen trees or stone that has created a pool of water that would otherwise not be there. A natural dams strength is unreliable, teams should make a careful assessment of natural dams when release could cause dangers below.

False Floors

Mostly found in narrow canyons, false floors are formed when logs get jammed at the top of a drop, over time, stones build up behind the logs, forcing them to collapse, when this happens pools can suddenly empty forcing a surge of water down the canyon below.


Loose ground above the canyon may result in a random slip, they range in size but have the potential to fill pools in seconds, resulting in sudden high flows further down the canyon.

Accelerants / Hinderance


The permeability of the ground will decide how much of the water is absorbed back into the ground and how much will make it into the rivers.


The gradient of the catchment will directly effect the speed the water will drain, the steeper the ground the faster the water will find its way to the rivers.


Plants are effective at reducing the severity of the flash flood, Catchment areas with large areas of tree cover often see slower reactions to precipitation.


Air temperature on larger catchment areas will provide a noticeable amount evaporation, reducing the levels of the rivers.

How a flood risk can become increased


If the canyon is difficult to escape, the rise in levels will make the trip extremely committing. If levels become unmanageable mid descent, rescue may be required.

Water Volume

Often, low flowing canyons are still possible to descend in high flow as the water hazards are easier to manage. Higher flow canyons are often much more difficult to manage.


Some canyons do not have suitable rigging for high flow, this means water hazards will be difficult to avoid without building transient anchors.