Understanding River Dynamics

Swiftwater Rescue Training – Reading the River

Flowing water carries powerful forces. Swiftwater rescuers must understand how rivers move to ensure safety and effective rescue operations. This guide introduces key concepts in river dynamics, including river flow patterns, common obstacles, and major hazards.

River Banks – Left and Right

“Left” and “right” banks of a river are based on the direction you are facing while traveling downstream.

When facing downstream:
• The right bank is on your right.
• The left bank is on your left.
When facing upstream, these directions are reversed. This means the actual physical left side becomes the “right bank,” and vice versa.

Primary Current – Laminar Flow

The main current refers to the natural direction water flows in a river, typically through the deepest, unobstructed channel. This current forms laminar flow – a layered flow where water moves in horizontal sheets.

Slower water exists near the riverbed.
Faster water is found just below the surface, because surface tension with the air slightly slows down the topmost layer.

Imagine a stack of plywood boards with round wooden dowels between them. As you push the bottom board, each board moves slightly faster than the one below it. The top board (representing the river’s surface) moves slower due to friction with air.

Laminar Flow and Swimming

This flow concept matters for rescuers. When performing a defensive swim, swimmers often struggle to keep their feet up. That’s because slower, deeper currents naturally pull their legs downward.

Since rivers are deeper in the center, flow in the middle is faster than near the banks. This is especially true in natural rivers, but manmade channels like canals or under bridges may have more uniform flows across the surface.

Rocks or submerged objects can disturb this flow. Deep rocks might cause vertical upwellings, known as boils, which appear as bubbling areas on the surface.

Upstream and Downstream “V”s

When two rocks or obstacles narrow the river channel, they create unique current shapes:

Upstream V: Water piles up against rocks, forming a “V” pointing upstream.
Downstream V: Water rushes between obstacles, forming a “V” pointing downstream through a chute or rapid.

These V-shapes indicate differences in water height. Water stacks up against rocks (forming cushion-like areas), then drops rapidly between them. Swimmers and paddlers should avoid upstream Vs and aim for downstream Vs – these mark safer passages.

River Bends

Rivers naturally curve. Due to inertia, the main current shifts toward the outside of the bend, where:

Water flows deeper and faster.
Erosion is more aggressive.
Debris and fallen trees may collect, forming hazardous strainers (obstacles that trap people but let water flow through).

The inside of bends is shallower and calmer.
Rescuers swimming through bends should:

Stay near the inside.
Point their heads slightly toward the inside to counter fast outside currents that might spin their bodies around or push them into obstacles.

Chutes and Standing Waves

When river water is forced through a narrow gap (called a chute), its speed increases. This creates a smooth tongue of fast water.

Once the chute opens up into deeper or slower-moving water, it generates a series of scalloped, evenly spaced standing waves.

Narrow chutes may only form small drops and light waves.
Wide chutes (spanning the river) can form large waves several feet high.

Key point for swimmers: Never try to breathe while passing through waves — inhale only between waves.

Obstacles: Rocks and River Features

Rocks are the most common river obstacle. Their depth and size affect how they change the flow.

Fully submerged rocks may have little surface effect.
Shallow rocks push water upward, forming pillows (rounded surface bulges).
When water can’t flow directly behind a rock, it creates a void or cavity. This results in three features:

Pillows – Upstream bulges formed by water pushing against a rock.
Eddies – Circular flows behind obstacles where water swirls and may even reverse direction.
Hydraulics or Holes – Recirculating water formed as water drops and churns downstream of an obstacle.

Eddies – Anatomy and Rescue Use

Eddies form behind rocks or obstacles as water flows around and then fills the empty space.

There are three parts to an eddy:

Upstream Return Flow
• Water flows back upstream into the void.
• Creates a boundary with the main current, called the eddy line or eddy wall.
• Strong eddy walls can feel like hitting a wall — turbulent and unpredictable.
Neutral Flow Zone
• The calm interface between up- and downstream flows.
• Ideal for rescuers to rest without being pulled in or pushed out.
Downstream-Flowing Eddy
• Water that appears still, but actually flows slowly downstream.
• Beginners often enter here (it feels safe), but may exit the eddy unintentionally.

Reminder: Always swim upstream within an eddy to stay in position.

Conceptually, an eddy is like a “sideways hydraulic” — both are caused by the river trying to fill a void. Eddies are often helpful for rest or rescue, but strong ones can be hostile and dangerous.

Hydraulics (a.k.a. Holes)

Hydraulics form when water flows over a large obstacle (like a rock or ledge), drops, and then churns back upstream. Three key flow directions occur:

Recirculating flow – Water flows backward upstream to refill the void.
Neutral flow – The center zone (boil), where water appears to bubble.
Downstream flow – Water escapes the hydraulic and rejoins the main current.

Friendly vs. Keeper Holes

The shape of a hole affects its danger level:

Smiling Hole
• Center is upstream of the sides.
• Easier to escape sideways.
• Safer for swimmers and paddlers.
Frowning Hole
• Center is downstream of the sides.
• Tends to trap people in the strongest part.
• Called “keepers” — they hold and drown victims.

Swimmers and rescuers should know how to read these features and stay out of keeper holes. Rescuers can use throw ropes from downstream to reach victims trapped in holes, but must avoid crossing the boil zone — doing so may turn them into the next victim.

Strainers – The Most Dangerous Obstacle

A strainer is any object that lets water flow through but traps solid objects — like a pasta strainer.

Common strainers include trees, branches, or rocks.
Water flows through, but people and boats get pinned.
Especially dangerous on outside bends, where fast water pushes victims into the obstacle.

Strategy: Always avoid strainers. If contact is unavoidable, actively swim toward it and climb on top to avoid being pushed under.

Undercut Rocks

These are rocks where water flows under instead of around.

Victims can be sucked beneath and trapped.
Often hard to spot — eddies may look unusual, lines may be missing, or boils may form downstream.
Key signs: • No pillow on the upstream side
• Weak or missing eddy lines
• Strange flow behavior behind the rock

When in doubt, stay away. Undercuts are deadly and difficult to escape from once caught.

Low-Head Dams – The “Drowning Machines”

Low-head dams are man-made structures where water flows over a ledge, forming a perfect hydraulic across the entire river width.

There are no escape routes — water circulates from bank to bank.
The smooth surface above gives no warning (called a horizon line).
Often causes multiple drownings — including rescuers who misjudge the hazard.

Key rescue notes:

Never enter a low-head dam’s hydraulic.
Use special tools (inflated fire hose, throw ropes, boats).
Enter only from slack water downstream, and only with full understanding of the hydraulic structure.

Artificial Hazards and Debris

Old dams, rebar, bridge piers, and debris piles can create serious hazards:

Sharp objects like riprap spikes or reinforcing bars can injure or trap.
Always scout new rivers at low water levels to check for hidden dangers.

Drowning Traps – The Hidden Threat

Most people avoid rivers during obvious floods. But many drownings happen at medium water levels, when rivers look normal.

Key elements:

Depth + Speed = Dangerous force
• Waist-deep water with strong flow can knock a person down.
• Add 1 foot of depth and you’re swept away.
Deceptive Appearance
• Water may look normal.
• Visitors often can’t judge if the river is higher than usual.
• Vegetation (like flooded gravel bar plants) is a good clue — if underwater, the river may be in “drowning trap” mode.

Important: Swiftwater rescuers must recognize this danger and educate the public. People drown not only during floods, but also during “normal-looking” river flows that hide deadly force.

Conclusion

Understanding river dynamics is essential for swiftwater rescuers:

It helps prevent the rescuer from becoming a second victim.
It supports faster, safer, and more effective rescue operations.
It builds familiarity with the environment rescuers operate in.

References

Kauffman, R. (2015). Swiftwater Rescue Packet. McHenry, Maryland: Garrett College. (Unpublished training packet).

Bennett, J. (1996). The Complete Whitewater Rafter. Camden, Maine: Ragged Mountain Press.

Dillon, P., & Oyen, J. (Eds.). (2009). Building Your Canoe Basics (Chapter 6), in Outdoor Adventures: Canoeing. Champaign, Illinois: Human Kinetics, March.

Dillon, P., & Oyen, J. (Eds.). (2009). Water Safety and Survival Skills (Chapter 5), in Outdoor Adventures: Kayaking. Champaign, Illinois: Human Kinetics, March.

Kauffman, R., Taylor, S., & Price, R. (1991). A Recreational Gauging and Information System to Alert Potomac River Users of Dangerous Water Levels. Annapolis, Maryland: Department of Natural Resources, Boating Administration, Planning and Policy Program. 305 pages.

Reference page: This guide explains swiftwater rescue search techniques, including how to use the Point Last Seen (PLS) and Last Known Position (LKP) to narrow search areas, plan hasty searches, and manage river hazards safely.

References　National Fire Protection Association (NFPA)