I'm land locked with a little river/lake in my city and Dratini don't spawn here. Maybe 1-2 totally random spawns for our whole city in a day.
I'm almost level 34 and no Dragonite. I've been walking my dratini forever.
Likely a similar situation for you. Probably very random spawns by your river. Best you can do is keep walking the river and try to get some 10km eggs from stops down by the water and hope for the best.