A tributary is a stream or river that flows into a larger body of water, typically a larger river or lake. A divide, on the other hand, is a line or ridge that separates two drainage basins, or areas where water drains into different bodies of water.
A tributary does not cross over a divide because the divide acts as a barrier that separates the drainage basins. Tributaries flow into the larger body of water on one side of the divide, and do not flow into the larger body of water on the other side. So, water from a tributary will flow towards the divide but will not cross it, instead will flow into the closest side of the divide and continue its flow.