Rocky terrain, water volume high and steep would the erosion happen fast or slow?

If terrain is rocky, steep and with significant amount of water, thus will lead to fast erosion.

Explanation:

Erosion is process in which soil and rock particles are removed from the surface, usually by the force of water, sun or/and wind.

If some terrain is steep and without vegetation, these are one of the main conditions for potential erosion to happen. Steeper the terrain bigger the chanse for erosion. Also, less vegetation more 'space' for erosion.

It is important to outline that plants with their root systems tied soil and rocks and prevent them from becoming erosive. This way reforestation is important in term of preventing formation of landslides, rockfalls and erosion landforms.

Water (or more precisely precipitation, such as rain) is the big factor in formation of any erosion. If there is steep, rocky terrain, intensive rain and outflow will eventually initiate erosional process.