If you know all this, I apologize. Many people new to the area dont.
The ERL is a good hike, although well used. The Little Missouri River level is critical to its accessibility, and not getting stranded by a rain storm. the Little Missouri River guage at Langley is what we watch to see if its crossable. The hazard here cannot be overstated, anything under 4' is considered passable on the ERL (about thigh deep). The lower the better. 22 people at the Albert Pike campground died during the night a few years ago when the river level came up 23 ft quickly from a thunderstorm several miles away. In minutes, flash flood. If it rains hard while you are out there, you can be cut off and stranded till water level drops.
Im not sure how well a dog would do crossing fast moving knee deep or deeper water. I would not take one personally only due to this.
You will have several good water crossings, and innumerable small ones. Ok, maybe the small ones arent innumerable, I think they total 30 something. Your feet will stay wet for 2 days. Not to worry, when its 95F, soaking your feet in cold water cools your body off fast. Its great and keeps your socks clean too.
I suggest you will benefit from the website, www.backpackingarkansas.com. Register with the forum, and then you will see sub-forums including one on ERL, where to park safely, strategies, some maps, etc.
*** http://www.duprephotography.com/ERL/index.html*** See this link, one of the members at backpackingarkansas setup a ERL website with much of the needed information on it.
Also, a useful thing is to download a gps track from someone with all the water crossings, campsites, etc marked. Helps to know where the next campsite is sometimes. There are about 65 pre-established campsites on the loop, so no need to make any new ones. That is 2+ per mile.
Arkansas prime hiking season is Oct - April. The rest of the year, is hot, and the trail may begin to get overgrown in places. In summer, the bugs and ticks can carry you away.
Its a great hike though. The Athens-Big Fork trail is no slouch. The hills may only be 400-800ft, but they are unrelenting, and theres 6 or 7 in a row. Clockwise is harder direction because you climb the steeper side of the ridges,and descend the gentler side.
In spite of this, the way I like to do it is to park at FS106 near the Little Missouri, and get the 2 wide river crossings out of the way going clockwise immediately. It lessens chance of being cut off by a rain storm later if the forecast is iffy. Although others have seen other creeks on the loop like Blaylock that is normally ankle deep, be chest deep and impassable as well.
Have fun, it is a good hike. There is also an article on it in Trail Groove http://www.trailgroove.com/ if you havent seen it.
Also, on parts of the trail, the entire floor of the forest on both sides of the trail will be nothing but a carpet of poision ivy. Getting off trail to pee, can be challenging in spots. May not be something good for a dog to be running thru either, just something to look out for.