It will be low. It's pretty much at summer level. But, not so low that you can't enjoy yourself. Here are three locations(showing depth and flow), Franklin, Bellevue, and Kingston Springs.
http://waterdata.usgs.gov/tn/nwis/uv...cd=00065,00060
http://waterdata.usgs.gov/tn/nwis/uv...cd=00065,00060
http://waterdata.usgs.gov/tn/nwis/uv...cd=00065,00060
At this level there should be nice water clarity. Also, the Harpeth is full of squeezes followed by long deep pools. You may have to hop out for a moment to get over some stubble but, overall it should be an enjoyable trip. FYI, I plan to get out this weekend on the Harpeth as well.