An Everest trek involves a tremendous amount of up-and-down walking. All the rivers in this part of Nepal flow south form Himalayan glaciers, but the trek route proceeds east. Therefore the trail must climb to the ridge that separates two rivers, descended to the river itself, and ascend the next ridge. Even though the trek begins at an elevation of 1860m, on the sixth day it crosses the Dudh Kosi at only 1500m – after considerable uphill climbing, it will come to almost 9000m of elevation to Everest base camp.