The Low-Head Dam on the San Gabriel River presents a critical hazard for paddlers, creating a dangerous hydraulic recirculation zone that can trap and potentially drown boats and swimmers. This man-made structure forms a uniform, powerful backwash that creates a uniform, inescapable reverse current, making it essentially unrunnable and requiring a mandatory portage on river right or left. Extreme caution is advised, as low-head dams are considered among the most lethal obstacles in whitewater environments, with no safe route through the structure.