Here's why:

* Steep Sides: Canyons are known for their dramatic, often vertical or near-vertical sides.

* River Erosion: The primary force behind canyon formation is river erosion. Over millions of years, the constant flow of a river cuts through rock, gradually deepening and widening the channel.

* Time: Canyon formation is a slow process, requiring immense amounts of time for rivers to carve out such impressive features.

Other landforms with steep sides might include:

* Cuesta: A ridge with a gentle slope on one side and a steep slope on the other.

* Escarpment: A long, steep cliff, often formed by faulting or erosion.

However, the combination of steep sides and river erosion strongly suggests a canyon.