By the arrangement of yogamaya, Krsnas pastimes with mother Yasoda were all regarded as ordinary. So here was an opportunity for Krsna to show His mother that the whole universe is situated within Him. In His small form, Krsna was kind enough to show His mother the virat-rupa, the universal form, so that she could enjoy seeing what kind of child she had on her lap. The rivers have been mentioned here as the daughters of the mountains (nagams tad-duhitrh). It is the flowing of the rivers that makes big forests possible. There are living entities everywhere, some of them moving and some of them not moving. No place is vacant. This is a special feature of Gods creation.