"Este río no tiene corrientes."
Translation:This river does not have currents.
Ah, the letter thorn (or þorn).
If you use Windows and have the United States-International keyboard enabled, you can type "þ" with right-alt t (hold down right-alt while hitting t).
Otherwise, you can use alt 0254 (hold down alt while entering 0254 on the numeric keypad). If you've got United States-International turned on, then it has to be the left-alt; but, since they numeric keypad is almost always to the right, that's not too big of a deal.
Thanks for the encouragement to think about it, Iago.
I guess you are not a fisherman or a canoeist (for example). Of course rivers have currents - main current, back currents, cross currents, turbulence. The only "river" without a current isn't un río but un arroyo .
But wow! So many people expect logic from Duolingo; for that matter, so many expectations of only logic from other people! If we are expecting to learn the language as it is used in the real world, then I think we would be entitled to be disappointed if we weren't given the opportunity to recognise illogical statements as well.
PS I suppose the river could be frozen solid. But even ice flows --- just very slowly!
Currents are flows of water within another body of water. Streams are usually considered to be a smaller form of rivers, with their own internal currents. Saying a river has no currents doesn't make a lot of sense, but it might be trying to say there are no eddy currents or cross currents.
In American English, we say, "this river doesn't have any current" or "this river has no current at all" or "this river has no current whatsoever." I could understand someone technical, not a common person, but an aquarist, saying currents of they were studying types of current flows or something.
You actually can use "hasn't." I don't think it's very common though. http://www.merriam-webster.com/dictionary/hasn%27t
Edit: And by "can use," I mean, can be proper English. No comment on its use here or on Duo in general.