Husband and Wife where paddling down the right side of the river following a small set of waves. As the bow paddler cleared the last wave she noticed an almost submerged catwalk extending aprox 20' from the bank made of stone slab rock being propped up by columns of rock. the canoe was traveling at a speed of aprox 10mph as it T-boned the catwalk turning the boat broadside and immediately flipped and ejected occupants. As the paddlers surfaced they where immediately washed into and then under the submerged rock. the appeared 10' downstream un injured. This is an extremely dangerous obstruction that needs to be avoided.