These are located in Monrovia Canyon Park along the trail up to the waterfall. They are stamped with the year 1964 and were built by the L. A. County Flood Control District. Their purpose is to slow the water down, or break its momentum by making it drop over the edge, during times of heavy rains. Without them, the water can flow powerfully enough to take out everything in its path.
When they were put in, a road was built up to the waterfall for the trucks and equipment. As they worked their way back down the trail, the road was torn out behind them. The water has changed things so much in the canyon, it's hard now to believe it was possible for a road to be there.
I haven't seen it myself, but I hear that the fire department has a photo of one of their fire trucks parked at the waterfall.