Waterfall by Lisa T. Bergren

Very teen angsty, but not bad. It kept me reading - if only to see what crisis was going to be so compelling that our heroine would give up her medieval sweetie and return to her proper century. So, that should have telegraphed the basic plot. Teenage sisters trapped for the summer on an archaeological site in Italy accidentally discover a "portal" which transports them back to the 14th century.

In the manner of stories of this type, main character sister falls for a local princeling, who also falls for her and all the subsequent implausibilities are quite readable and fun.

There was one major problem with it - an issue which I have addressed at length on other books in this list. Where do these people get their titles? The title of this book has absolutely nothing to do with the story, nothing! It is clearly intended to be the first in a potentially endless series which is titled River of Time (nothing like telegraphing the plot of the entire series), but there is no allusion to time as a river or a current, and even then one might expect some connection between the plot and the title. It is as if the writer had this "concept" and then couldn't quite make the story fit the concept, but was so enamored of it that she couldn't turn loose of it and chose a more appropriate title.

Oh, well.