kālindyāḥ salile śive
It appears from this statement that Dhruva Mahārāja had already been instructed how to practice the eightfold yoga system, which is known as aṣṭāṅga-yoga. This system is explained in our Bhagavad-gītā As It Is, in the chapter entitled “Dhyāna-yoga.” It is understood that in aṣṭāṅga-yoga one practices settling the mind and then concentrating it on the form of Lord Viṣṇu, as will be described in the following verses. It is clearly stated here that aṣṭāṅga-yoga is not a bodily gymnastic exercise, but a practice to concentrate the mind on the form of Viṣṇu. Before sitting on his āsana, which is also described in Bhagavad-gītā, one has to cleanse himself very nicely in clear or sacred water thrice daily. The water of the Yamunā is naturally very clear and pure, and thus if anyone bathes there three times, undoubtedly he will be very greatly purified externally. Nārada Muni, therefore, instructed Dhruva Mahārāja to go to the bank of the Yamunā and thus become externally purified. This is part of the gradual process of practicing mystic yoga.