rippling, gurgling, whisper, rustling, and hushed.
Winding, mellow
He became more mellow as he aged.
Mellowed is a verb. It's the past tense of mellow.
River Severn (sounds like seven)
Are you thinking of the River Tigris.
The Amazon river dolphin, or pink river dolphin, Inia geoffrensis, does make sounds and calls.
The Mississippi River
Water flowing through a river can make a variety of sounds, such as splashing, gurgling, bubbling, and rushing. These sounds are created by the movement of the water over rocks and obstacles in the river, as well as the force of the current.
Indus river
The river Wye (it runs along the Welsh/English border)
The letter s.
"Red River."