Problem: Fluvial erosion cuts into mountains and reduces their height. Why haven't streams worn all of Earth's mountains flat? Need Assignment Help? Superposed streams preserve surrounding mountain topography. Stream channels become entrenched, which limits topography changes. Persistent geothermal energy continually creates new topography. Fluvial erosion deepens valleys, effectively creating new topography.: Fluvial erosion cuts into mountains and reduces their height. Why haven't streams worn all of Earth's mountains flat? Superposed streams preserve surrounding mountain topography. Stream channels become entrenched, which limits topography changes. Persistent geothermal energy continually creates new topography. Fluvial erosion deepens valleys, effectively creating new topography.