River Currents: Table top standoff
If one is an incurable over-the-side-of-bridges looker for 40-plus years, one can see some entertaining events unfold while peering down into the water. This bridge was a small, wood and steel bridge 10 feet above the water. On many occasions there were fish spotted but this time there was an unexpected show of predator and prey.
When a riverbank washes out, people get nervous and try to stop the river from being a river and moving around. 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 riprap 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 except the river carries the sediment in the water column. 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 riprap on a riverbank, 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 downstream 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 riprap the shore upstream from the bridge with quarried stone. The river had made short work of washing out the riprap and tumbled some of the stones down river. One that was almost a perfect cube with sides of about 2 feet in length, width and height had ended up just downstream of the bridge and it was sitting upright. Looking down in to the water, the top of the stone was a square flat surface like a granite table top about two feet under the surface.
Standing on its four pair of walking legs, 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 streams and rivers. This one was not all that tiny either, with a 3-inch long body and large pincers (chelipeds) at the end of its front two appendages. While wondering how that crayfish ever got on top of the rock, a 16 inch small mouth bass (Micropterus dolomieu) came into view and began circling around the rock like a shark. The encounter that ensued was fun to watch, although the crayfish likely did not enjoy it.
The bass would dart toward the crayfish. The crayfish would scurry to face the fish and 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. After each retreat, the actions of the bass indicated it was not interested at all in that crayfish. Its actions seemed to say, "I am just out for an 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 its defenses it would be dinner.
This went on for about 15 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, casting a crayfish imitation pattern down river in the direction of the departing bass did hook one 16-inch bass that may have thought for an instant, until the fishing line tightened, it had finally outsmarted that crayfish.
David Deen is River Steward for the Connecticut River Watershed Council. CRWC has been a protector of the Connecticut River for more than 60 years.
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