Only once one can bathe in the same 'river'.Most of us love taking bath in rivers. In our country rivers like Ganga, Yamuna, Godavari, Kaveri, etc.are generally well known and are given certain holistic importance also. 'Ganga snanam' is much preferred by Indians, especially those who belong to Hindu religion.
However, most of the people used to take bath in a river in the neighbourhood. Those who belong to Hindu religion, earlier, started the day with a dip in the river. River is given much importance in the life of elderly people. Every day they prefer this routine.
Taking bath in a particular river every day was part of their belief. But if look at the nature of any river, the water is always flowing. That means if a bath is taken now, just after a few seconds or minutes, that river is no more there. 'River' here means the physical one, it is the water content, the neighbouring atmosphere, the animal contents in the water, etc.. Evidently all these put together give the real river concept. That means the river in its physical sense has changed she he takes his second dip. This when generalised implies that one can have a dip in a river only once.