you can get some pretty big long burly nails and then nail them in at an angle from one log to the other. You won't get the nail straight through both logs, but if you get 2 inches of nail into each log it should hold together pretty well.
From that picture though it looks like you've already got a pretty good build up.
look at the way logs are layed when constructing log cabins. An axe is used to remove a rounded wedge from each log. Then you put them together and nail or screw, it removes material and makes it more structurally sound. Axes and chainsaws come in really handy when making shore style stunts out of logs. I usually cut the end of the log in half lengthwise about 6 inches down, which makes a flat surface. If you do that to both logs at the appropriate angles it makes for a strong joint. You need to know something about old school carpentry to make it work. Its like Amish barnbuilding and log cabin construction. These styles make the strongest structures.