Vermont. May 03, 2010
Being an incurable over the side of bridges watcher for 40 years can lead to some entertaining events that unfold while peering into the water. This bridge was a small wood and steel bridge 15 feet above the water that I had looked over the side of on many occasions but this time the unexpected events made me smile.
When a river bank washes out people get nervous and try to stop the river from being a river. The conventional wisdom up until 20 years ago was to address erosion by putting in place large boulders or quarried stone along the bank as rip rap to deflect the flow of the river away from that eroding spot. Of course all that approach does is to export the erosion problem down river to another spot.
As a matter of physics water will dissipate its energy and will do so in several ways. One of those is to carry sediment in its flow. Think of it like you carrying a bag full of gravel, it takes more of your energy to walk from here to there carrying that bag. The same is true for rivers but the gravel is suspended in the water. When a river has energy that has not been dissipated it will erode sediment from wherever it can including the banks and bottom of its own channel.
When you place stone rip rap on a river bank all of the energy in the flow is deflected and the river cannot pick up any sediment from the stone surface, so nothing has happened to lessen the energy of the river. If a river cannot erode sediment the energy in high flows just continues on down river until the river finds a surface that it can erode thereby picking up sediment and lowering its energy level.
In this case someone had tried to rip rap the shore up stream from the bridge with quarried stone. The river had made short work of washing out the rip rap and tumbled some of the stones down river. One that was almost a perfectly cube with sides of about 2 feet in length, width and height had ended up under the bridge and it was sitting upright so looking down in to the water the top was a square flat surface, like a granite table top about two feet under the surface.
Sitting right smack in the middle of that table top was a crayfish (Cambarus bartonii), one of those tiny lobster looking critters that live in all of our healthy lakes, streams and rivers. This one wasn’t all that tiny either, with a 3 inch long body and large pincers (chelipeds) at the end of its front two appendages. Circling around the rock like a shark was a small mouth bass (Micropterus dolomieu) about 16 inches long. What ensued next was fun to watch; doubtless the crayfish did not enjoy this encounter.
The bass would dart toward the crayfish. The crayfish would put up its pincers in a defensive posture and actually scuttle toward the bass. This confused the bass to no end and exercising the better part of valor, discretion; it would turn away from those raised and open pincers and resume swimming around the rock. The bass tried to communicate that it was not interested at all in the crayfish. The bass actions seemed to say, “I am just out for a lazy afternoon swim and see I am not even watching you Mr. Crayfish, just out here enjoying the water.” Despite this show of studied indifference eventually the bass would dart again for the crayfish.
The crayfish all this time kept pivoting in the center of the rock keeping the bass in front of it at all times so it was not surprised when the next attack came. It was ready with its pincers held high with their menacing jaws wide open.
The crayfish parried attack after attack, pincers wide open scurrying toward the fish each time the bass darted toward the crayfish. The crayfish would then reset itself backing up to the center of the rock. It was a standoff. No matter how disinterested the bass acted the crayfish knew that if it dropped of its defenses it would be dinner.
This went on for about 30 minutes. The bass finally got it, this crayfish was not going to let down its guard and the bass had no desire to take on those pincers. As soon as the bass swam off the crayfish backed to the far side of the rock away from the departing bass and jumped off sinking quickly to the bottom.
Although that particular crayfish was never spotted again, hurrying down to the river while tying a crayfish pattern streamer on to my fly line and casting down river in the direction of the bass did hook one 16 inch bass that thought for an instant until the line tightened it had finally outsmarted that crayfish.
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David Deen is River Steward for the Connecticut River Watershed Council. CRC has been a protector of the Connecticut River for more than half a century.