Lessons from two rivers that never mix

The Meeting of Waters is the confluence between the Rio Negro, a river with dark (almost black colored) water, and the sandy-colored Amazon River or Rio Solimões, as it is known the upper section of the Amazon in Brazil. For 6 km (3.7 mi) the river's waters run side by side without mixing. It is one of the main tourist attractions of Manaus, Brazil. The same also happens near Santarém, Pará with the Amazon and Tapajós rivers.

This phenomenon is due to the differences in temperature, speed and water density of the two rivers. The Rio Negro flows at near 2 km per hour at a temperature of 28°C, while the Rio Solimões flows between 4 to 6 km per hour a temperature of 22°C.

As I read and looked at this amazing picture, I drew an important truth: The difference in temperature, speed and water density made two rivers impossible to mix.

So I wondered, What can represent the "TEMPERATURE", "SPEED", and "WATER DENSITY" of human relationship that determine if a guy and a girl can flow together into a godly relationship and marital bliss?

Any suggestions?