A river strainer is a common and deceptive hazard in waterways, representing any obstruction that allows water to flow through its gaps but effectively traps larger solid objects like boats, gear, or people. This hazard is frequently compared to a kitchen colander, which filters liquid while retaining solids. These permeable barriers pose one of the greatest dangers to those navigating rivers in canoes, kayaks, or rafts. Strainers are particularly common after floods or periods of high water, which move large debris and increase the force of the current against them.
Understanding the Physical Danger
The danger associated with a strainer comes from the immense, sustained hydraulic force of the river current. Unlike a solid rock that deflects water, a strainer permits the water to pass through its structure, but the gaps are too small for a person or a boat to follow. When a solid object is pushed against the obstruction, the water continues to flow through the strainer, but the object is held in place. This creates a significant pressure differential, where the full force of the upstream current is focused on pinning the object against the branches, roots, or debris. The resulting pressure can be thousands of pounds, making self-rescue virtually impossible once a person or craft is trapped.
A boat caught in this situation will often be folded, rapidly filled with water, or capsized, pinning the occupant underwater. Even on relatively calm rivers, the pressure exerted by the current can turn a small, partially submerged branch into a life-threatening entanglement. The water passing through the obstruction creates a “sieve” effect, which can drag a swimmer down and hold them beneath the surface. This entrapment, often occurring in fast-moving water, dramatically reduces the time available for a rescue. A swimmer’s buoyancy vest becomes useless when pinned underwater by this relentless force.
Identifying Common Types of Strainers
Strainers can be broadly categorized into natural and man-made structures, all sharing the characteristic of being permeable obstacles. Natural strainers are the most common and often consist of fallen trees, known as “sweepers,” or full log jams that span a significant portion of the river. Root wads from eroded banks, dense shoreline brush, and accumulated vegetative debris can also form strainers, especially on the outside bends where the current is strongest.
Man-made strainers frequently involve submerged infrastructure or debris washed into the river. Examples include collapsed fencing, discarded vehicles, metal rebar, or bridge pilings that accumulate branches and trash. Visual cues that suggest a strainer include patches of turbulent, bubbling water where the current is forcing itself through small gaps, or bouncing twigs and debris caught on a seemingly invisible object beneath the surface.
Safety Protocols for River Navigation
The most effective strategy for managing strainers is constant vigilance and avoidance. Before embarking, scout the river and visually assess unfamiliar or challenging sections of the waterway from the shore to identify potential hazards and create a safe passage plan.
Maintaining a safe distance from the riverbanks is an effective way to avoid most strainers, which often form along the eroded edges of the river. If a strainer is spotted and cannot be avoided, the safest course of action is to “portage,” or carry the boat and gear around the obstruction on land. Never attempt to paddle over or through a strainer, even if it appears to be partially submerged.
If a collision with a strainer is unavoidable, especially if swimming after a capsize, the individual should swim aggressively toward the obstruction. The goal is to climb immediately onto the strainer or get over the top of it, keeping the body high and out of the main force of the current. Attempting to push away or swim downstream will increase the chance of being pinned and pulled under the water.