As the volume in a given stretch of water increases the river undergoes a transformation. What may have once been a channelized river bed with loads of eddies and islands can turn into a single rapid that can go on for miles. Islands can turn into holes, eddies can sit feet above the current, friendly holes can turn into killer death pits and floating debris can be a real danger, as can the edges of the river which can now be flowing through a forest of trees.
That said; big water boating is great fun and tremendously rewarding, requiring its own unique skill set. However big water in the wrong river, with the wrong group or at the wrong time can be extremely dangerous. Some rivers get easier with higher water. Others become much harder and some become unpaddleable.
Make sure you always check the river levels, guidebooks and weather forecast before heading out on a trip.