Assessing a river
No two rivers are the same and all are subject to constant change due to differing flow rates, so it is important that a leader has the skill and knowledge to distinguish between safe and dangerous places to cross. Before this is achieved, a leader must have a basic understanding of what occurs to water when it is forced over, through and around different obstacles. River reading is an art which cannot be learned from books alone, but the factors below are a good start in knowing what to look for.
Current is the most basic feature of a river. In a perfectly symmetrical river the current will be strongest in the deeper centre section. The strongest current will sweep around the outside of a river bend. When constricted, water is forced through the narrow point, gaining speed, and then slows again where the river broadens. As the river gradient steepens (the rate of fall downhill of the riverbed) the current will be swifter. Where there is a constriction combined with a drop, the section is known as a shoot.
A stopper wave occurs where a very strong current of water flows over an obstruction, creating a wave or current below the obstruction which appears to flow back upstream. Naturally occurring stopper-waves caused by boulders, logs or other debris are dangerous because objects (including people) can be trapped in the stopper, and escaping from it is difficult. Escape might be possible by swimming to one side of the stopper, or, for larger stoppers, diving deep and swimming downstream.
Strainers are extremely dangerous. They are fixed objects that allow the current to pass through them, such as a fallen tree with many branches in the water. They are often the most dangerous hazards that have to be dealt with. The danger is that while water can move through and around, solid objects are pinned against the strainer on the upstream side.
Eddies are sections of river which swirl around, and can result in people or objects being trapped in the whirlpool, and unable to get to shore or into the main stream flow.