Quick Answer
A flat Earth without mountains would have different rainfall patterns, river systems and habitats. Mountains force air upward, create rain shadows and store water as snow and ice. Removing them would therefore affect climate and freshwater far beyond the mountains themselves.
Mountains are more than dramatic scenery. They influence atmospheric circulation, water storage, erosion, habitats and where people can live.
Rainfall changes
Air forced over mountains cools and can produce precipitation. Removing high terrain would eliminate many rain-shadow effects and redistribute rainfall across regions.
Rivers lose their sources
Many major rivers begin in mountainous regions where snow, glaciers and high-altitude rainfall feed drainage systems. A flatter planet would need a different network of watersheds.
Glaciers shrink or disappear
High elevations allow snow to persist where temperatures are low. Without mountains, many present glaciers would have nowhere to exist.
Biodiversity changes
Mountain ranges isolate populations and create many climate zones over short distances. Their removal would erase some specialized habitats while connecting others.
Established scienceMountains strongly influence regional precipitation and river drainage. The exact global effect of removing them would depend on how the flat surface was created.
The bottom line
A mountain-free Earth would still have continents and oceans, but its climate, rivers and ecosystems would be fundamentally reorganized.
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.