My money says no significant rise has occurred. I'm not a party pooper; this is the Spring, the reason we love it is because it is always floatable!
There appear to be two gages at Mammoth Springs causing this confusion. Gage 07069190 says the river came up 0.4' on August 9, then another 0.6' on August 10, several days before it rained. And then it held steady for at least 4 more days, instead of beginning to fall gradually back towards its original "summertime" level. And while this was going on, the river down at Hardy continued to fall. A one foot rise at Mammoth springs would be significant, and we'd see SOMETHING down at Hardy by the next day unless the Hardy gage was broken.
Gage 07069220 says it rained 0.4" on August 13, but it wasn't enough, the river just held steady. That's consistent with what I would expect for a summer shower, and consistent with the Hardy gage.
The two gages have different information about who is "partnered" with USGS to fund/maintain the gage. So I'm thinking one of these gages is in the process of being abandoned. Since the 07069220 has more data (like rainfall, water temp, etc) AND is giving me believable data, my guess is that gage 07069190 doesn't have long to live, or is already dead and just doesn't know it, like the proverbial chicken with no head.
If you want a better answer than that, like maybe some facts instead of just an educated guess, then you're going to have to go to the gage, click the comments link, and ask USGS what the "real" story is. (But guessing was more fun.)
PS: Jan's Plan was even better than guessing. Go float the river and report back!
If anyone else wants to go, give a shout or better yet, text your phone number to 501-589-2055 & I'll call you.. 
