Backyard Nature: How water striders defy the creek currents

On a hike in Soapstone Park over the Fourth of July weekend, I was surprised to see a water strider zigzagging in the stream as I took my first step at the crossing closest to Broad Branch Road.

I had never seen striders there, but maybe I had not looked hard enough.  Certainly there are not the numbers that draw my attention in Rock Creek Park off the Valley Trail. There, the strider trails look like rain drops.

At the Soapstone Creek crossing, I was mesmerized as I noticed another strider glide to where the stream funnels between the rocks.  The strider stopped itself and easily evaded the pull of the current to maneuver upstream. I wondered, how does it do that?

After some research, I learned that the strider is able to stand on the surface of the water due to water tension. Then, to propel itself forward, the creature pushes the water backward. But how?

No one knew until a study published in 2003 determined that striders create vortices with their middle legs.

The leg works similarly to a paddle for a boat, only it does not not break the surface of the water. Instead, it pushes a small volume downward and backward. That action creates a backward rotating ball of water that pushes the strider forward.

Somehow, this small force is strong enough to thwart the pull of the stream current. And it seemed as if the strider I watched in Soapstone Creek enjoyed playing with the stream’s flow, first floating for a time, then sprinting off in the other direction.

The following Tuesday I was hiking again, this time on the Valley Trail north of Military Road. I was looking for the usual bevy of water striders, but Rock Creek was running high and fast after about an inch of rain fell the night before, so I did not expect to see many water striders fighting the current.

I was wrong.  Crossing the Rolling Meadow Bridge, I found a mass of water striders moving up the creek against the swiftly moving water.

Even seeing it for myself, I found it hard to wrap my mind around the phenomenon.