Story of why yamuna's water is darker than that of ganga.

Story 1

After losing his first consort Sati who killed herself by jumping in the fire-pit of the priest-king Daksha, Shiva lost all interest in worldly life. He held his wife’s corpse and wandered aimlessly in the world until Vishnu cut the corpse into tiny pieces.
To rid himself of the sorrow, the gods requested Shiva to bathe in the river Yamuna. His sorrow scorched the river black.

Annexure

Later Shiva took another consort, Parvati, princess of the mountains. Once a demon called Raktabija terrorized the world. Every drop of his blood gave rise to his clone making it impossible for the gods to kill him. To help the gods, Parvati drank every drop of Raktabija’s blood. The blood darkened Parvati’s skin and she became known as Kali, the dark one. This form so terrified Shiva that he looked away. To regain his interest, the goddess bathed in the Yamuna and emerged as Gauri, the radiant one. Yamuna let the dark colour percolate in her being. Yamuna has thus been associated with accepting the sorrows and pollutions of the world. That is why her mood is always forlorn.

(courtesy :http://devdutt.com)