1. Hydraulic Action: The sheer force of the water itself can dislodge and carry away rocks and sediment. This is especially powerful in fast-flowing rivers.

2. Abrasion: This is the process of rocks and sediment carried by the river grinding against the riverbed and banks. It's like sandpaper wearing down a surface. This is how rivers create smooth, rounded rocks and deepen their channels.

3. Attrition: This is the process of rocks and sediment colliding with each other, causing them to break down into smaller pieces. This process reduces the size of the material being transported by the river.

4. Solution: This is the process where slightly acidic water dissolves soluble minerals in the rocks and sediment. This is especially important for eroding limestone and other carbonate rocks.

While technically there are four ways, the most common and significant forms of river erosion are hydraulic action, abrasion, and attrition.