Flood is the catastrophic event would have the biggest impact on a watershed.
Step-by-step explanation:
Watershed is basically an area where common rivers and streams will eventually empty into larger water bodies like the ocean. Floods will cause an excessive amount of water to flow through the watershed and this excess water cannot be absorbed.
This will eventually cause runoffs and also accelerates the land erosion. Floods can change the direction of the streams and new channels can be formed and there will be redistribution of the water sources. The flora and fauna too is disturbed. Homes that are built on watersheds are affected the most when floods occur.