What does River Washing mean?

A process of washing denim to soften it and give it an aged look. The process involves including pumice stones and cellulose enzymes into the wash along with the denim.

Heddels explains River Washing

River washing refers to one of many techniques used to soften denim and give it an aged, vintage look. As with stone washing, the first step of the process involves laundering the fabric in a washing machine with pumice stones. Cellulase enzymes are then added to the washer and washing continues until the desired degree of aging is achieved. Like stone washing, this technique is not as popular today as in the past due to the environmental impact as well as the bits of stone often remaining in the fabric once the process is complete.