Swift Water Rescue and Tension Diagonals: Mastering the Current

By Jimmy Osborne Fire Chief and Swift Water Rescue technician

Published on welikefirefighting.com

The Power and Danger of Moving Water

When people think of dangerous environments, they often think of fire, hazardous materials, or high-angle cliffs. But water — fast, cold, and relentless — is one of the most underestimated threats in emergency services. Swift water can move with enough force to sweep away vehicles, uproot trees, and overpower even the strongest swimmers.

For firefighters, technical rescuers, and SAR teams, understanding how to work with — not against — moving water is critical. And among the most versatile and life-saving techniques we have in our toolkit is the tension diagonal.

This article will take you deep into the world of swift water rescue, how tension diagonals are used to solve challenging problems, and how to master them in your team’s operations.

Understanding Swift Water Rescue

Characteristics of Swift Water

“Swift water” refers to any water moving at a speed greater than 1 knot (1.15 mph). It can include rivers, flooded urban streets, storm drains, and spillways. What makes swift water dangerous isn’t just speed — it’s:

• Volume: Even shallow moving water exerts tremendous force.

• Obstacles: Rocks, trees, rebar, vehicles.

• Turbulence: Eddies, hydraulics, and boils that trap or flip victims.

• Temperature: Hypothermia is a major risk, even in “warm” weather.

• Debris: Floating or submerged hazards.

Risks and Hazards

The hazards in swift water are numerous:

• Entrapment: Feet-first swimming is important to avoid foot entrapments.

• Strainers: Trees or fences that allow water through but trap people.

• Hydraulics: Recirculating currents at dams and drops.

• Pinned Victims: Victims pinned against obstacles or submerged.

• Cold Shock: A sudden immersion can cause unconsciousness within minutes.

Rescue Priorities and Strategy

In swift water, rescues must prioritize:

1. Self-rescue (your safety first)

2. Reach and throw rescues (low risk)

3. Row and go rescues (higher risk)

4. Helicopter rescues (specialized)

Every move must be calculated, rehearsed, and performed with team communication at the forefront.

Equipment in Swift Water Rescue

Key pieces of gear include:

• PFDs (personal flotation devices) rated for swift water

• Helmets (not climbing helmets — water rescue helmets)

• Throw bags with floating ropes

• Dry suits or wet suits

• Fins (for swimming)

• Knives (accessible, preferably blunt-tipped)

• Carabiners, pulleys, prusiks, and webbing for rigging systems

• Inflatable rescue boats or rafts

• Mechanical advantage devices (for tension systems)

Principles of Swift Water Rescue

Before we get into tension diagonals, it’s critical to review the foundation of water rescue:

Self-Rescue First

You can’t help anyone if you become the second victim. All rescuers must practice:

• Defensive swimming

• Aggressive swimming

• Recognizing when to abort and reattempt

Reach, Throw, Row, Go

This classic rescue sequence reminds rescuers to start with the least dangerous techniques and escalate only if necessary:

1. Reach: Extend a pole, stick, or throw a rope.

2. Throw: Use a throw bag to toss a line to the victim.

3. Row: Use a boat to reach the victim.

4. Go: Enter the water yourself (last resort).

Advanced Techniques: Tension Diagonals

What is a Tension Diagonal?

A tension diagonal is a rope system set across a moving body of water at an angle. It’s held taut and allows rescuers or victims to move from one bank to another by using the force of the current against the tensioned rope.

Picture it like this:

Instead of fighting the current head-on, you “surf” along the angled rope, letting the water push you across.

How it Works

Physics does the work. The diagonal line resists the current, and the moving water naturally pushes the rescuer downstream — but the rope’s tension directs that energy across the river.

It’s efficient, it minimizes time in the water, and it requires no motorized support.

When and Why to Use It

Tension diagonals are ideal when:

• Victims are stranded on rocks or islands.

• Direct swimming is too risky.

• Boats are unavailable or too dangerous to deploy.

• Rapid movement of gear or rescuers is needed across a wide stretch of water.

Setting up a Tension Diagonal

Equipment Needed

• Static or semi-static rope (suitable diameter and length)

• Two strong anchor points (trees, vehicles, or rock bolts)

• Pulleys

• Prusik minding pulleys (for adjusting tension)

• Carabiners

• Prusiks for progress capture

• Load release systems (like a load-releasing hitch or MPD)

• Throw bags

Basic Setup Steps

1. Identify anchor points on both sides of the river, slightly upstream of the crossing point.

2. Establish communications between both banks (radios, whistles, or visual signals).

3. Set up a tagline or messenger line to pass the main rope across.

4. Rig the main rope using mechanical advantage to tension it.

5. Install a load-release system for emergency adjustments.

6. Tension the line, but not so much that it lifts too high out of the water.

7. Test the system with a weighted object before sending rescuers or gear across.

Anchor Considerations

• Must withstand significant force, factoring in water load and potential shock loading.

• Redundancy is critical.

• Natural anchors like trees or rocks are ideal; artificial anchors may be used if necessary.

Communicating During Setup

• Assign a clear Incident Commander (IC) for rigging.

• Use hand signals if radios fail.

• Confirm tension level visually and verbally.

Types of Tension Diagonals

Single Rope

• Most basic setup.

• One rope, one tensioned line.

• Good for small teams or quick access.

Twin Rope Systems

• Two tensioned ropes: one for access, one for safety.

• Redundancy if one line fails.

Tagline Variations

• Victim or rescuer attached to a secondary, sliding tagline.

• Helps control downstream drift.

Case Studies: Tension Diagonals in Action

1. Flooded River Crossing (2020, Midwest USA)

A fire department deployed a twin-line tension diagonal to rescue a family stranded on an island during a flood. Two lines increased stability in high current, and a swift evacuation was completed in under 10 minutes.

2. Urban Flood Event (2023, California)

During a sudden flash flood, rescuers set up a makeshift tension diagonal between two concrete barriers, allowing rescuers to ferry victims over a flooded freeway section.

Common Mistakes and How to Avoid Them

1. Poor Anchors: Always double-check and load test.

2. Over-Tensioning: Can snap ropes or uproot anchors.

3. Communication Breakdowns: Always establish signals in advance.

4. Improper Load Release: Ensure you can dump load quickly if the system fails.

5. Forgetting Backups: Use backup prusiks and secondary lines when possible.

Training and Drills for Swift Water and Tension Diagonals

• Practice setting up both basic and twin-line diagonals quarterly.

• Conduct timed drills with full PPE and real tension loading.

• Simulate victim ferries across diagonals.

• Include nighttime and low-visibility drills.

• Integrate drone use for scouting and monitoring the system.

Conclusion: Mastering the Current

Swift water rescue is one of the most demanding and dangerous technical disciplines a firefighter or rescue technician will ever face. Mastery comes through training, preparation, and humility in the face of nature’s power.

Tension diagonals are one of the greatest tools available when the water fights back. When rigged properly, they turn a deadly environment into a manageable challenge, allowing rescuers to work with the river instead of against it.

Train hard, stay safe, and trust your rigging. The river doesn’t care if you make a mistake — but your team does.