Rapids

In geography the term rapids refers to a stretch of a river or creek where there is significant drop in elevation, causing the water to speed up, get more shallow, and turbulent.

Some rapids are so shallow, so fast-flowing, and so turbulent, that boats can not be propelled past them. Other rapids can e transitted by vessels going downstream, but vessels going upstream have to be towed. This is called "shooting the rapids".

Places where travelers have to unload their cargo, and carry it around an unnavigable part of a river are called portages. When the volume of cargo is large enough navigational aids, like canal locks, can be installed, to enable vessels to circumvent unnavigable parts of the river.