Elder crocodile god
Nobles traveling on a barge, forcing everyone to yeild right-of-way
Wedding procession, either on the river, or on the bank. Funeral/holy day, etc... can easily replace.
Cataracts with fey/lesser deity guardians - the Nile had several sets of cataracts that divided Egypt into upper, middle, and lower sections, so they were very important.
Depending on your geography, where a lesser river joins your "nile" you could have a fortress where passers-by must present documentation/pay bribes/sneak past