A waterfall is formed with hard and soft rock, the sof rock erodes quicker than the hard rock. This eroison is caused by hydraulic action which is the fast power of the water, eroding the bank. Also abrasion which is rocks carried by the river wearing down its bed and bank. Over time this cauese a plunge pool to form and eventually forms a over hang. The overhang will fall back into the river due to gravity.
A waterfall forms when a river flows over a relatively tough bands of rock or when there is a change in gradient of the river channel
The river falls over the rock edge into a plunge pool and underlying weaker rocks are erdoded more quickly than through tougher rock