Where do rivers usually begin? at the ocean
at groundwater outlets
in mountainous regions
in lakes and larger rivers

Headwater
All rivers have a starting point where water begins its flow. This source is called a headwater. The headwater can come from rainfall or snowmelt in mountains, but it can also bubble up from groundwater or form at the edge of a lake or large pond.Jul 30, 2019

Most rivers begin life as a tiny stream running down a mountain slope. They are fed by melting snow and ice, or by rainwater running off the land. The water follows cracks and folds in the land as it flows downhill. Small streams meet and join together, growing larger and larger until the flow can be called a river.

It must be measured in k.hope my answer is to you : )