sentences:  During floods in the 1990s, an almost sheer waterfall appeared on Arroyo Trabuco in northern San Juan Capistrano, threatening the foundations of a railroad bridge.The drop required quick reinforcement with grouted riprap.With an average gradient of 29 percent it has become a major barrier to migrating fish and other riverine organisms, and thus isolates aquatic environments in upper Arroyo Trabuco from the rest of the San Juan watershed.