How does a topographic divide separate neighboring catchments?

In hydrologic sciences, a topographic divide—often referred to simply as a divide—serves as the boundary line that separates adjacent catchments (also known as drainage basins or watersheds).1

The function of a topographic divide is based on the direction of surface water flow: all precipitation that falls on opposite sides of the divide is directed into different drainage basins. Because a catchment is defined as the area that drains to a specific measurement point on a stream channel, the divide acts as the physical limit of that area. Consequently, the divide ensures that water, sediment, and other hydrologic inputs are partitioned into distinct systems, allowing scientists to apply the water balance equation—which accounts for precipitation, discharge, evaporation, and storage—to a clearly defined geographic region.21