There are 3 major rivers that originates from the GHNP : Parvati, Sainj and Tirthan. Most of the tourist infrastructures for your visit to the Great Himalayan National Park are located in the Tirthan valley between Banjar and Gushaini.
The number of days mentionned on the map are the required number of days to reach the place in the GHNP and to come back. There are many other treks that we can offer you in the ecozone of the Park.
There are main way to reach the GHNP overland.
The most common is the way followed by the Volvo buses coming from Delhi and going to Manali. It goes via Chandigarh, Rupnagar, Bilaspur, Mandi and then Banjar to reach the Tirthan valley or Sainj to reach the Sain valley.
The alternate route is via Shimla and Jalori pass. It has more scenic streches compared to the first one but it is also longer in time distance.
Check this page for more information.