The geological feature formed by a river flowing through a canyon is called a "gorge" or "valley." As a river erodes the surrounding rock over time, it carves deep into the landscape, creating steep walls that form the canyon's distinct shape. The process of erosion involves both hydraulic action, where the force of the water dislodges material, and abrasion, where sediment and rocks carried by the river grind away at the canyon walls. Over thousands to millions of years, this process can lead to the formation of dramatic features, attracting visitors to observe their natural beauty.