The Legend of the Cagayan River

When Amang Bathala created the world and everything in it, he wanted the mountains to be triangular, the oceans oblong, and the rivers and streams rectangular. It was Diwata Nayades', together with her helpers, Duwende Diling and Nuno Kagat, task to create the rectangular rivers and streams.

Will Duwende Diling and Nuno Kagat obey the rule of creating rivers and streams? Why do they have to obey this?