When delineating a watershed using a Digital Elevation Model (DEM), what specific characteristic of the terrain defines the boundary lines, and why is ignoring subtle changes in this characteristic problematic?

A watershed, also known as a catchment area or drainage basin, is a geographical area where all precipitation and surface water runoff ultimately drains to a common outlet point, such as a river, lake, or ocean. When delineating the boundaries of such an area using a Digital Elevation Model (DEM), which is a raster-based representation of terrain elevation where each cell contains an elevation value, the specific characteristic of the terrain that defines these boundary lines is the drainage divide. These are also commonly referred to as ridge lines or local topographic high points. A drainage divide is the line of highest elevation that separates two adjacent watersheds, effectively dictating the direction of surface water fl....