Haldi (Turmeric) being a tuber which grows below the surface of earth, the waves of Pruthvitattva (Absolute Earth Principle) in it are in far greater quantity than in stems growing above the surface of earth.
The seven rivers referred to, are the Holy rivers Ganga, Godavari, Narmada, Kaveri, Krushna, Brahmaputra and Yamuna. Water collected from these seven rivers is put in the ritualistic pot, which is used during ritualistic worship.
It is beneficial to offer the patri (Specific leaves offered during worship) of those flower plants which are favourite of the Deity. Patri is related to the colour particles, whereas the flowers are related to the fragrant particles.
Drawing a rangoli in a sattvik pattern around a wooden platform or altar before commencing any puja (Ritualistic worship) helps to form a protective sheath, of rapidly moving waves emanating from that pattern, around wooden platform or altar.