Simplify the following text for a Grade 5 native English-speaking person.
Original: Heavy rainfall on August 7 and 8 , 1708 , the year following the HÅ ei eruption , caused an avalanche of volcanic ash and mud and broke the dams , flooding the Ashigara plain .
Simplified: Heavy rainfall on August 7 and 8 , 1708 , caused an avalanche of volcanic ash and mud and broke the dams . The river flooded the Ashigara plain .