Srimad Bhagvata Mahapurana: Book 10: Chapter 50: Verses 26-35
On the field of battle were set flowing in hundreds streams of blood issued from the bodies of foot-soldiers, elephants and horses that were being mowed down-streams in which arms (of fallen soldiers) floated like (aquatic) serpents, the heads of men like tortoises, their hands and thighs like fishes, the locks of men like duck-weeds and their excellent and big gems and ornaments like gravel and stones. The streams were thick with shoals in the shape of killed elephants and alligators in the shape of slain horses; (nay) they were full of waves in the shape of bows and bushes in the shape of (other) weapons, and looked terrible with eddies in the shape of shields (or wheels). They brought terror to cowards and encouragement to the valiant of one army to give battle to the other. By Sankarsana (Balarama) of immeasurable prowess, even as He was (busy) annihilating the vainglorious enemies with His pestle,was brought to destruction (the remnant of) that force, led by Jarasandha (the king of the Magadhas), (even) though it was hard to cross and fearful like the ocean and unfathomable (in prowess) and endless (in extent) 0 dear Pariksit! That was (however) a mere amusing play on the part of Sri Krsna and Balarama (the two sons of Vasudeva), the Rulers of the universe. The (aforesaid) discomfiture of the enemy's forces is no wonder on the part of the Lord who carries on the (processes of) creation, preservation and annihilation of (all) the three worlds by way of His (divine) sport, endowed as He is with endless attributes.