You can check the current and historical flow rates here:
http://scitech.pyr.ec.gc.ca/waterweb/hydroPortfolio.asp
You'll need to drill down, I can't provide a direct link to the Missinaibie (@ Mattice) gauge 04LJ001.
You can see that September is historically the low point for this river, this is more of a problem downstream from Mattice than on the upper river. From my personal experience the upper river is runnable even at quite low levels while the lower river (especially below Thunderhouse) can be a real "drag".