Is it possible to actually end the civil war? I've finished the Imperial side on one character. They talk about stormcloaks in the hills, and you can see, in certain places (like the Legion castle in Solitude, the upper room in Dragonreach, the side room in the Hall of Kings, the imperial camps, etc) where there are "conflict maps" (the maps of skyrim with the little red and blue flags indicating who controls what forts), who controls what.

If I go to the "blue" locations and clear them, will the Imperials take control of them. If the Imperials control all of the forts, does the war "end"? What about Riften - Riften still seems to be controlled by the Stormcloaks. Is there some way (e.g. clearing out some "key" fort) to give Imperials control of Riften?

No, there is no real 'end'. The enemy camps each contain an essential general, so you cant ever clear them.

You can't clear the camps, the camp leaders are essential

Well, if you get to the end of the Civil War, and actually listen to the dialog you'll hear something about that there are still pockets of resistance and that after a while they will eventually give up and go home. I expect this stuff to be taken care of in the first DLC.